Extreme Weather

Weather CSI: Reconstructing Hurricane Winds, Storm Surge, and Rainfall

Using Forensic Meteorology to Verify Hurricane-Related Insurance Claims

2017 was a record-setting year for global insured (and un-insured) losses due to natural disasters, driven in no small part by the costliest hurricane season in U.S. history. Most of the estimated $200 billion in U.S. damages resulted from the wraths of hurricanes Harvey, Irma, and Marie during August and September 2017; and while nature required only a few weeks to wreak such havoc, assessing, tallying, and ultimately rebuilding from the damage will take far longer.

Although assessing hurricane-related claims may seem fairly straightforward to those unfamiliar with the process – “Hey, the building was either hit by a hurricane or it wasn’t, am-I-right?” – meteorologists and experienced claims adjusters know that often isn’t the case, especially as one moves farther from the eye of the storm and therefore farther from the most extreme and obvious impacts.

The boat next to the swimming pool may be a dead giveaway, but can you tell which other damages were due to last year’s hurricanes?

It is with these more ambiguous claims that forensic meteorology offers valuable insight – reconstructing conditions at the loss location to identify and quantify hurricane-related hazards.

Almost without exception, damage due to tropical cyclones (hurricanes and tropical storms) can be attributed to high winds, extreme rainfall, and/or storm surge. The magnitude of these impacts at any given location depends on numerous factors beyond simply the closest approach of the eye of the storm. The strength, size, and speed of the tropical cyclone; coastal topography; orientation of the coast relative to the storm’s wind field; distance from the coast; and the relative location of landfall all play determining roles.

Failure to understand and account for these factors can lead to under- or over-estimation of impacts, and ultimately to poor coverage decisions.

What follows is a discussion of each major impact (wind, rain, and storm surge) and the data and analytical tools available to reconstruct conditions at a given location. The following discussion specifically addresses tropical cyclone-related hazards, but many of the same analytical methods can be applied to other types of weather events, including severe thunderstorms.

High Winds
In the 45 years since the Saffir-Simpson scale was introduced, hurricane wind speeds have become nearly synonymous with hurricane intensity and damage potential. A Category 1 storm with sustained surface winds of 74-95 mph is described as producing “some damage,” mainly to roofs, fences, trees, and telephone poles; while a Category 5 storm with sustained winds above 157 mph is described as producing “catastrophic damage,” completely destroying a large percentage of framed structures and leaving the impacted area uninhabitable for weeks to months.

Hurricane Irma comes ashore in Florida.

While there is value to the Saffir-Simpson scale – increasing winds do indeed produce increasing damage – the maximum sustained surface wind speed near the eye of a hurricane does not capture the whole story of that storm’s destructive potential.

Two storms with similar maximum wind speeds can produce vastly different amounts of damage – the size of the storm, and therefore the size of its wind field, as well as the forward speed of the storm greatly influence its destructive potential, as Category 3 hurricanes Ivan ($18.8 billion in damages) and Dennis ($2.5 billion in damages) demonstrated when they impacted the same areas along the Gulf Coast just 10 months apart in 2004 and 2005. Despite similar maximum wind speeds, Ivan – a larger storm – produced seven and a half times more damage. In other words, size matters with hurricanes.

Recognizing that maximum wind speed isn’t the best measure of a hurricane’s destructive potential, the tropical meteorology community has been developing new, more comprehensive indices. Such indices include the Cyclone Damage Potential (CDP) index as well as Integrated Kinetic Energy, both of which account for a tropical cyclone’s size as well as its maximum sustained winds.

Although these metrics have not yet seen widespread use outside the meteorology community, expansion to the emergency management community and ultimately to the media and the general public is likely in coming years as these groups seek a more accurate understanding of – and more effective way to communicate – hurricane-related dangers.

Comparison of the wind fields associated with Hurricane Andrew (1992) and Hurricane Irma (2017). Hurricane-force winds extended outward more than twice as far in Hurricane Irma. Credit: Bob Al-Greene/Mashable

Straight-Line Winds
Winds within a hurricane – both sustained winds and gusts – are generally strongest in and near the eye wall, with wind speeds decreasing with distance from the center of the storm. The smaller the hurricane, the more quickly winds decrease with distance.

For example, Hurricane Andrew was a very compact storm whose sustained hurricane-force winds extended outward only 30-45 miles, while Hurricane Irma – a much larger storm – had hurricane-force winds extending outward nearly 100 miles. Despite similar maximum wind speeds near the eye of the storm, if a property were 50 miles from the center of Hurricane Irma, it would have experienced significantly higher wind speeds than a property 50 miles from the center of Hurricane Andrew.

Tornadoes

The locations of the 23 tornadoes (red triangles) reported in association with Hurricane Irma. Irma’s track through Florida is also marked, with cooler colors representing weaker wind speeds as the storm moved over land. All tornadoes occurred within the right-front quadrant of the storm, and all but 4 occurred in coastal locations. Credit: Blue Skies Meteorological Services

Tornadoes can and most often do occur far removed from the center of a hurricane, in the outer bands of discrete thunderstorms that rake counterclockwise away from the eye of the storm. The vast majority of these tornadoes develop in the right-front quadrant of the hurricane, relative to its direction of motion. During Hurricane Irma, 23 tornadoes were identified across the state of Florida. All occurred in areas impacted by Irma’s right-front quadrant.

Tornadoes that form in association with hurricanes are generally short-lived and relatively weak, occurring most often near the coast where wind shear is strongest as the discrete thunderstorms move ashore. However, tornadoes in tropical cyclones can occur further inland: more than two dozen tornadoes were reported in and around the Houston metro area during Hurricane Harvey, some nearly 80 miles from the coast.

If significant wind damage is observed at a property located 50-200 miles from the center of a tropical cyclone, tornadic impact should be investigated as a possible explanation.

Data
Straight-line wind speeds are assessed through both in-situ (weather station) observations as well as post-storm damage surveys. For locations near a weather radar site, radar velocity data of near-surface winds can also be extrapolated to estimate surface winds (this method is less useful for locations far from the radar, where extrapolation becomes less accurate).

Hurricane wind gusts can and often do cause even the most robust weather station anemometers to stop working. During Hurricane Irma, many of the weather stations located at Miami-area airports stopped reporting wind speeds during the peak of the storm.

Therefore, to obtain the most comprehensive and rigorous reconstruction of wind conditions, all three data sources should be used. Weather station data provide on-the-ground observations, while damage surveys assess the aftermath to determine the wind speeds necessary to produce the observed damage. In areas removed from weather stations or population centers, weather radar offers data on winds aloft and insight into winds near the surface.

Tornadoes, which are small-scale phenomena that rarely happen to impact a weather station, are assessed through damage surveys and radar data. The storm cells that produce tornadoes in tropical cyclones are generally shallow, so radar data is most useful for locations within 60-70 miles of the radar site. Beyond that distance, the radar beam may overshoot the top of the storm.

Rainfall-induced flooding in Simonton, TX, 40 miles west of downtown Houston. The image to the left shows Simonton before Hurricane Harvey while the image to the right shows the town immediately after Hurricane Harvey. Figure: Blue Skies Meteorological Services. Satellite Imagery: Digital Globe.

Extreme Rainfall
As Hurricane Harvey demonstrated to devastating effect in August of last year, rainfall-induced flooding can be at least as destructive as extreme winds during tropical cyclones. Harvey is estimated to have caused more than $100 billion in damages, primarily due to widespread flooding in and around the Houston metropolitan area – far more damage than was wrought by the storm’s winds.

Although the more-than-four-feet of rain that Harvey squeezed out of the skies over Texas were truly unprecedented, most tropical cyclones produce intense, heavy rainfall that can lead to flooding both during and after the storm, often far inland of the landfall location. Over the last 30 years, such inland flooding has been responsible for more than a quarter of the deaths associated with tropical cyclones in the United States.

Data
In-situ rain gauge data provide measured rainfall totals at specific locations. In urban areas rain gauges may be less than 5 miles apart, while in rural areas, they can be 50 or more miles apart. Radar data can fill in the gaps between rain gauges by providing rainfall estimates anywhere within the radar’s coverage area.

Cause of death in the United States directly attributable to Atlantic tropical cyclones, 1963–2012. Storm surge was responsible for nearly half of all deaths, while rain-induced flooding was responsible for more than a quarter. Together, wind and tornadoes were responsible for only 11% of such deaths. Water really is the far deadlier threat during hurricanes. Credit: Rappaport 2014

Radars estimate rainfall using an algorithm that associates radar reflectivity values with rainfall rates. The exact relationship between reflectivity and rainfall rate can differ between storm events; so to determine whether the radar rainfall estimates are accurate for a given event, in-situ measurements should be compared to the radar estimate of rainfall at those locations. Confidence in the radar estimate increases when the radar-estimated rainfall total is similar to what was measured at a rain gauge at the same location.

Rain gauges and weather radar can tell us how much it rained but not whether that rainfall actually caused flooding. To determine the locations and extent of flooding, we turn to storm reports and damage surveys. The National Weather Service collects storm reports for all tropical cyclones, and the US Geological Survey conducts damage surveys for most major flooding events. For prolonged, widespread flooding events like Hurricane Harvey, satellite data can also reveal which areas were impacted.

Storm Surge
Along the coast, storm surge is often the greatest threat to life and property when a hurricane strikes. As with hurricane winds, the most severe storm surge typically accompanies the right-front quadrant of the storm, where the storm’s wind field and forward motion act to push water onshore. But unlike hurricane winds, which tend to be similar in nearby areas and gradually decrease with distance from the eye, storm surge can vary greatly even within the span of a few miles.

The reason for this variability is that storm surge is a complex phenomenon dependent on characteristics of both the hurricane and the local landscape.

Among the local variables that impact storm surge are the width and slope of the continental shelf as well as the geometry of the coast, bays and estuaries. All else being equal, storm surge along a coast fronted by a wide, shallow continental shelf – as exists along much of the Gulf Coast – will be significantly greater than along a coast where the continental shelf drops away quickly.

Potential Storm Surge Flooding Map for the Jacksonville, FL area. This map was created by the National Hurricane Center as Hurricane Matthew churned northward just off the Florida Atlantic Coast in 2016. Notice how the greatest potential storm surge (reds and oranges) follows the streams, rivers, and estuaries of low-lying coastal regions along and north of Jacksonville. Locations tens of miles inland along one of these features could experience storm surge greater than a location just a few miles inland of the coast – coastal geometry plays a crucial role in determining storm surge. Credit: NOAA NWS and National Hurricane Center (NHC)

Among the storm-specific variables that impact storm surge are hurricane intensity, size, forward speed, and angle of approach to the coastline. The slightest change to any one of these characteristics can significantly increase or decrease storm surge potential. Despite popular belief, it is the winds of the hurricane that generate the vast majority of its storm surge – less than 5% of the surge is due to the low-pressure effect of the storm “sucking up” the ocean toward its center.

In addition to storm surge – formally defined as the abnormal rise of water generated by a storm – one must also consider the local astronomical tide. Storm surge rides atop the normal tidal flow, creating a combined “storm tide” that ultimately determines the depth and extent of inundation. A storm that strikes at high tide can result in inundation several feet deeper than if that same storm strikes at low tide.

A final factor that influences damage along the immediate coast is wave action. Neither the storm surge nor the storm tide take into account the effects of large, wind-drive waves that batter the coast during a hurricane. Wave action can significantly increase the impact of storm tide along the immediate coast, overtopping seawalls and sandbags where the storm tide alone would not. Often, the height of waves riding atop the storm tide can exceed the height of the storm tide itself.

As a recent example, during Hurricane Irma, wave wash marks in the lower Florida Keys were observed 10 – 15 feet above the storm tide. Water has a weight of 1,700 pounds per cubic yard, so prolonged pounding by large waves can cause substantial structural damage.

The three causes of water damage and inundation along the coast: storm surge, astronomical tide, and wave action. Storm surge combines with the normal astronomical tide to form the storm tide. Large wind-driven waves ride atop the storm tide and batter structures nearest the coast. Credit: NOAA.

Data
Ahead of a hurricane, the United States Geological Survey (USGS) typically deploys a network of temporary storm-tide sensors along the immediate coast in the projected path of the storm. These sensors record the depth of the storm tide throughout the event, and comparison of the maximum storm tide at nearby sensors provides insight into the range of water levels experienced along a particular stretch of coastline.

In those areas not covered by the storm tide sensor network, the USGS often performs surveys of visible high-water marks in the immediate aftermath of the storm. High-water marks are created when small, light debris carried along the top of the water is deposited on vertical surfaces like walls and doorways, and as with storm tide sensors, they provide data about the maximum water height at a given location.

The National Weather Service also performs post-storm damage surveys that include findings regarding storm surge, maximum inundation, and wave height, if available.

The Bottom Line
Tropical cyclones tend to be well-documented extreme weather events. Data from weather stations and radar sites generally become available within a few days of the event, while post-tropical cyclone reports and summary storm reports require weeks to months of processing before they are released. Forensic meteorological analysis of site-specific storm impacts can therefore begin almost immediately and can be updated as new data becomes available.

If you are involved in a weather-related investigation and would like to discuss how a forensic meteorologist could support that work, please reach out to Blue Skies Meteorological Services for a free, no-obligation consultation.

Foreseeability of Weather Events

In forensic meteorology, we are often asked whether a given weather event was “foreseeable” – in other words, whether those impacted by or involved in the event should have or could have seen it coming.

Foreseeability generally has three components when considering weather events – climatology (what is “normal”), forecast (what is expected) and communication (whether those making decisions have access to crucial information).

Climatology

In many ways, climatological expectation is about timeline. Over a long enough timeline, even extreme weather events can be expected to occur.

That may sound counterintuitive, so let’s take a familiar example. If you flip a coin 20 times, what are the odds that you’ll get a run of 15 tails in a row? It doesn’t take a statistician to tell you those odds are pretty low.

Flooding in Houston, TX due to Hurricane Harvey

But what if you flip the coin 200 times? 200 thousand times? As the number of total coin flips grows so too do the odds of seeing a run of 15 tails in a row, for no other reason than that there are more opportunities for it to occur. While a run of 15 tails would be extremely unusual within a trial of only 20 flips, it would normal and expected within a trial of 200 thousand flips. In other words, if you flip a coin 200 thousand times, you should expect to see a run of 15 tails somewhere within that trial. An event that is unusual within a short trial is actually expected within a longer one.

The same is true of extreme weather events. Take the “100-year flood”. What a meteorologist or hydrologist means when they say a “100-year flood” is an event that has a 1-in-100 chance of occurring in any given year. Over a long period of time, we would therefore expect to see such a flood occur, on average, about once every hundred years.

What “100-year flood” doesn’t mean is that a given location is “due” for such an event if the last one occurred more than 100 years ago or that it is “safe” if the last one occurred less than 100 years ago. Every year, the odds are the same: 1-in-100.

The expectation of a given event (from a statistical, climatological perspective) does not change based on past weather. A “100-year flood” is just as likely to occur next year as it was last year.

Go back to the coin flip example: If you have a correctly balanced coin, each flip carries 1-in-2 odds of showing tails. Even if you flip 10 tails in a row, the 11th flip carries the exact same odds of coming up tails: 1-in-2. It may feel like you’re “due” for heads after 10 tails in a row, but that’s psychology speaking not statistics.

What does change after an extreme weather event is knowledge among the affected population. Once an extreme event occurs, people know 1) that it can happen, and 2) what the impacts are. So while the occurrence of one extreme event does not shift the climatological expectation for when a similar event will recur (e.g. suffering through a “100-year flood” this year doesn’t mean you’re safe next year), it can impact the foreseeability of a similar event. After all, people know that it can happen because they’ve already seen it happen.

Forecast

The fact that a given weather event is within climatological expectations doesn’t necessarily make it foreseeable for the affected population. The weather-related impacts that population could reasonably have anticipated depend on a number of factors, including the weather forecast and impacts statements, as well as how that information was disseminated to the public and/or decision makers.

A heat wave grips the US desert Southwest

Weather forecasts, especially for major events, are generally quite good and have improved significantly over the past several decades. Anyone not living under a rock is unlikely to get surprised by a hurricane or a heat wave. Even the warning lead-time for tornadoes is up to an average of 13 minutes – plenty of time to take shelter if you get the warning. (We’ll return to that “if” in a moment.)

That said, sometimes meteorologists get it wrong. Small changes to atmospheric conditions can have major impacts on how a weather event evolves (that proverbial butterfly flapping its wings in Brazil). When there is significant uncertainty in a forecast, meteorologists try to express it. However, scientific uncertainty is a nuanced thing, and often the uncertainty statements are the first thing to get stripped for the headlines. So while meteorologists and decision makers with direct access to them may understand the forecast uncertainty and its implications, the general public may not.

News headline during Hurricane Irma’s approach of Florida

Remember when Hurricane Irma was definitely going to wipe Miami off the map? Neither do I, because at no point during Irma’s evolution was an impact on the southeast FL coast a sure and immanent thing. But “Miami About to be Obliterated!” makes a far more clickable headline than “landfall near Miami looking possible, but storm could still swing further west or east”.

Which brings us to the final crucial piece in foreseeability: information access.

Communication

Even if a given weather event is within the climatological norms; even if the forecast is close to perfect, with uncertainty and potential impacts clearly and understandably communicated – even then a weather event can be “unforeseeable”.

If the people impacted by the weather event do not have access to or should not be reasonably expected to seek out forecast information, the event may not have been foreseeable. For example, every few years, hikers are injured or killed in the desert southwest when flash floods turn dusty arroyos into raging rivers.

  • Are afternoon thunderstorms during monsoon season climatologically normal? Yes.
  • Do forecasters generally recognize and issue warnings when it appears such storms may cause flash flooding? Yes.
  • Are the resulting flash floods foreseeable to the hikers 10 miles away from the storm and 30 miles from the nearest town? Maybe not.

That last question is arguably more difficult and is where forensic meteorology largely bows out of the discussion.

A flash flood in the desert – foreseeable?

A forensic meteorologist can tell you whether a given event was climatologically unusual, whether the forecasts leading up to the event were accurate, and how that forecast information was disseminated.

But whether those who were impacted should have ultimately “seen it coming” is a more complex issue.

What if different forecasts disagreed? If a truly extreme event was forecasted, is it reasonable for people to think the forecast was exaggerated? What is reasonable ignorance or incredulity in this age of information-overload and often hyperbole? If someone has access to forecast information, should they be expected to actively seek it out? Should they be expected to act on it?

These are often the types of questions that arise in weather-related investigations and disputes. What starts out as a series of relatively straightforward meteorological questions ultimately winds into a complex web of human psychology and societal and cultural expectations.

The meteorological science is a crucial component of such investigations, but developing a rigorous and comprehensive understanding of the situation requires cross-disciplinary collaboration and communication. The weather is where the questions start, but not necessarily where they end.

If you are involved in a weather-related investigation and would like to discuss how a forensic meteorologist could support that work, please reach out to Blue Skies Meteorological Services for a free, no-obligation consultation.

New NWS storm surge flooding maps help you run from the water and hide from the wind

Hurricane Dennis brought strong winds and dangerous storm surge inundation to the Florida panhandle in 2005. Credit: USGS

Hurricane Dennis brought strong winds and dangerous storm surge inundation to the Florida panhandle in 2005. Credit: USGS


“Run from the water. Hide from the wind.” This adage speaks to a truth long known among meteorologists and emergency management officials: it’s generally not the wind that’ll kill you – it’s the water.

Between 1963 and 2012, only 11% of tropical cyclone (hurricane and tropical storm) deaths were due directly to the wind. Yes, hurricane-force winds can be frighteningly destructive and frankly awe-inspiring, but when the numbers are tallied, water is undeniably the deadlier threat. Combined, storm surge, inland flooding due to heavy rains, and high surf are responsible for 88% of tropical cyclone deaths. The storm surge alone kills nearly half of all people who lose their lives during these events.

The reasons for this are simple but not necessarily obvious.

Cause  of  death  in  the  United  States  directly   attributable to Atlantic tropical cyclones, 1963–2012. Credit: Rappaport 2014

Cause of death in the United States directly attributable to Atlantic tropical cyclones, 1963–2012. Credit: Rappaport 2014

Why Water Not Wind

Especially in areas with modern building codes and construction, structures can withstand long duration strong winds. Even when structures are significantly damaged by debris or the wind itself, they continue to offer some shelter and protection from the wind. This is why people in the path of a tornado are advised not to jump in their car and drive away but rather to shelter in place in an interior room on the lowest floor. The roof can be ripped off a building and the exterior walls toppled, yet a tiny bathroom or closet often remains intact and offers a pocket of safety amidst the chaos.

Water is different. It rises inexorably, under doors and through windows. No room is safe. Entire neighborhoods are engulfed at once – there is no accessible safe haven once the water begins to rise. Storm surge water seethes with debris, from toxic waste to power lines to floating vehicles. And unlike the wind, which weakens quickly as a storm departs, storm surge and inland floodwaters can engulf a region for days to weeks. Those who don’t drown during the storm are often trapped without water or food until rescue personnel arrive, and when the impacted area is large and damage is extensive, rescue can be slow coming.

Hurricane Matthew

If you were watching news coverage as Hurricane Matthew marched menacingly up the east coast of Florida earlier this month, you probably noticed that meteorologists and local officials were focusing far more on the potential for unprecedented storm surge than on the storm’s maximum winds, despite Matthew being a Category 3 or 4 storm with winds up to 145 mph at times. Major Florida cities along the Atlantic coast and along the St. John’s River (which empties into the Atlantic Ocean just east of Jacksonville, FL) were threatened by a storm surge unseen for at least a century.

Small closets remain intact after a tornado ravaged this neighborhood. Credit: Alabama Wx Blog

Small closets remained intact after a tornado ravaged this neighborhood. Credit: Alabama Wx Blog

Matthew was a major hurricane with destructive winds, but it was the water, not the wind, that led governors, mayors, meteorologists, and emergency managers alike to issue impassioned pleas for those in the path of the storm to evacuate. Like hurricanes Sandy, Ike, and Katrina before it, Matthew testified to the fact wind tells only part of the story of a cyclone’s destructive power.

The rest of that story, and the far deadlier chapters, are told by water.

And starting this year, emergency managers, public officials, and the general public have another tool to evaluate the impact of that water on their communities. Developed by the National Weather Service and made operational for the 2016 hurricane season, Potential Storm Surge Flooding Maps are specific to each tropical cyclone and show a reasonable worst-case scenario for storm surge inundation at the neighborhood level. In other words, it shows the storm surge heights that a person should prepare for before a storm, given the uncertainties in the meteorological forecast.

Potential Storm Surge Flooding Maps

The Potential Storm Surge Flooding Maps are based on the existing National Weather Service (NWS) Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model. (Modeling involves a dizzying and inescapable array of acronyms.) The SLOSH model takes into account forecast uncertainty and, when run as an ensemble, provides an envelope of possible storm surge outcomes based on the current forecast.

An entire neighborhood is engulfed by water due to inland flooding associated with Hurricane Matthew in early October 2016. Credit: Quavas Hart.

An entire neighborhood is engulfed by water due to inland flooding associated with Hurricane Matthew in early October 2016. Credit: Quavas Hart.

Ensemble modeling involves running a model (or group of models) many different times, each time with small differences in the initial conditions or model assumptions. For the SLOSH model, these differences represent plausible futures for the track and intensity of the hurricane given current observations and historical forecast errors. The result is a set of possible storm surge scenarios, with each member of the set representing a slightly different hurricane track and intensity.

Once this set – or “ensemble” – is assembled, it is analyzed statistically to determine a reasonable worst-case storm surge scenario at each location, and the depth of that storm surge is displayed on the Potential Storm Surge Flooding Maps. In this instance, “reasonable worst-case” is defined as that storm surge depth which has a 1-in-10 chance of being exceeded at each location. (In other words, at any given location, there is a 90% chance that the actual storm surge will be less than or equal to what is displayed on the map, given the current forecast.)

Official National Hurricane Center forecast for Hurricane Matthew as of October 6, 2016, with the storm's wind field marked. Credit: NWS Jacksonville

Official National Hurricane Center forecast for Hurricane Matthew as of October 6, 2016, with the storm’s wind field marked. Credit: NWS Jacksonville

Utility and Limitations

The Potential Storm Surge Flooding Maps are a valuable visual tool to assess storm surge risk as a tropical cyclone approaches land. The SLOSH model that forms the basis of the maps takes into account the factors most critical in determining storm surge:

  • The hurricane’s forward speed, angle of approach to the coast, and landfall location
  • Forecast uncertainty in the hurricane’s track, intensity, and wind field
  • Astronomical tides
  • The specific topography of the coast
  • Artificial features like roads and levees

However, the potential storm surge flooding map does not take into account a number of factors that do not impact storm surge directly but that nonetheless greatly impact overall flooding and water damage. The maps do not account for:

  • Wave action (i.e. waves that occur on top of the storm surge)
  • Freshwater flooding from rainfall
  • Riverine discharge (i.e. river flooding)
  • Flooding resulting from levee failures
  • For mapped leveed areas – flooding inside levees, overtopping of levees

Wave action
Wave action can significantly increase the impact of storm surge along the immediate coast, overtopping seawalls and sandbags where the storm surge alone would not. The potential storm surge flooding maps show only the potential storm surge – they do not provide any information about expected wave height. In many cases, the height of waves riding atop the storm surge can exceed the height of the storm surge itself.

The anatomy of the storm surge. NWS Potential Storm Surge Flooding Maps display the storm tide (storm surge + astronomical tide), but do not address the impact of waves on top of the storm tide.

The anatomy of the storm surge. NWS Potential Storm Surge Flooding Maps display the storm tide (storm surge + astronomical tide), but do not address the impact of waves on top of the storm tide.

Information about expected wave height and wave action impacts can be found in hurricane-related text products from the National Weather Service, including the local area forecast discussions and hazards and impacts statements. Interests along the coast and in areas protected by levees should pay particular attention to the impacts of wave action and weigh those impacts in addition to the direct impacts of the storm surge when making hurricane preparations.

Freshwater and Inland Flooding
Flash flooding, areal flooding, and river flooding due to excessive rainfall are responsible for over one quarter of all tropical cyclone-related deaths and can impact areas far inland and outside of the storm surge risk area. Hurricanes and tropical storms can dump tremendous amounts of rain in a short period of time. Even weak tropical systems can produce devastating amounts of rain. For example, the remnants of Tropical Storm Amelia in 1978 flooded central Texas with four feet of rain. Interests in low-lying areas prone to flooding and along creeks and rivers expected to be impacted by a tropical system should pay careful attention to rainfall forecasts and expected impacts.

Levees
Forecasting the protection offered by levees during a tropical cyclone is complex and difficult, as Hurricane Katrina tragically demonstrated in 2005. Wave action, the depth and speed of the storm surge, and the strength and construction of the levees all influence the amount of protection they provide. Interests living in leveed areas need to remain especially vigilant as a tropical cyclone approaches, and consider all relevant risks, not just those posed by the storm surge.

Application during Hurricane Matthew

The potential storm surge flooding maps issued during Hurricane Matthew’s approach of Florida’s east coast offered a clear and dire assessment of the risk for unprecedentedly severe storm surge inundation.

A briefing from the NWS Jacksonville forecast office on the expected local impacts from Hurricane Matthew. Note that storm surge was the primary concern. Credit: NWS Jacksonville

An October 6th briefing from the NWS Jacksonville forecast office on the expected local impacts from Hurricane Matthew. Note the emphasis on storm surge-related dangers. Credit: NWS Jacksonville

The maps for Hurricane Matthew were so dire because the worst-case scenario was squarely within the cone of uncertainty. It wasn’t just possible. It was likely that locations along the east coast and the St. John’s River would be under several feet or more of water if the hurricane continued to hug the coast or made landfall in northeast Florida or southeast Georgia.

More than just the beaches were under threat. The potential storm surge flooding maps showed areas along the St. Johns River — inland regions that don’t typically think of themselves as vulnerable to storm surge –- under up to 6 feet of water as Matthew pushed a wall of water at the coast and up the mouth of the St. Johns. Although the worst-case scenario fortunately did not materialize, reports were still received of the St. Johns River flowing backwards on the morning of Saturday, October 8th, as storm surge and hurricane-force onshore winds pushed the sea inland.

“Fortunately” is the correct word in this situation, because it was nothing more or less than luck that prevented a truly worst-case scenario from actualizing. Had Matthew drifted 20 miles farther the west, the Atlantic coast of Florida, and inland river cities like Jacksonville and Palatka would have experienced truly devastating flooding.

The Bottom Line

The National Weather Service’s new potential storm surge flooding maps provide a graphical, easy-to-understand, quantitative assessment of storm surge risk along the U.S. Gulf and Atlantic coasts. By displaying a reasonable worst-case scenario at each location, they show both local officials and the general public the storm surge depth for which they should prepare themselves, enabling well-informed preparation and evacuation decisions.

However, these maps are not intended to offer a complete picture of water-related risks associated with tropical cyclones. They do not take into account wave action (i.e. waves riding atop the storm surge), rainfall-induced flooding, or potential levee failures. These impacts must be considered separately when assessing the impact of a hurricane or tropical storm in a given area.

And, as always, the output of any model is only as good as its input.

Ultimately, the accuracy of the potential storm surge flooding maps depends on the accuracy of the hurricane track and intensity forecast. Those forecasts – and the storm surge inundation maps derived from them – are continually refined as new data from in-situ and remote sensing platforms (like hurricane hunter aircraft measurements and satellite imagery, respectively) as well as new model guidance become available.

During Hurricane Isaac, a storm surge causes tides to quickly rise while rough waves pound the concrete seawall along the shores of Lake Pontchartrain.

During Hurricane Isaac, a storm surge causes tides to quickly rise while rough waves pound the concrete seawall along the shores of Lake Pontchartrain.

It’s That Time of Year Again: Get Storm Ready

tornado approaching homestead

Ah, springtime – the season when life, greenery, and sandals return to the land and towering storms return to the sky.

Severe weather is already visiting parts of the southern Plains this year (including Oklahoma and Texas), and it will soon begin its annual march northward across the country as winter gives way to spring and summer.

Severe Weather Season

tornado season by month

The progression of “tornado season” northward across the country from early spring to mid-summer. Original source: NWS SPC

Severe storms are fueled by the tremendous energy of an unstable atmosphere, where cold air vies with warm in a perennial battle for territory. Storms fire up and ride along the front lines – sentinels flashing, booming, blowing, and occasionally swirling their way across the landscape. For this reason, there really is no true, nationwide severe storm season – that clash of warm and cold air masses occurs at different times of year across different regions of the country, resulting in regionally variable storm climatologies.

Typically, storm season ramps up first in the South, where warm air returns earliest (or never really leaves): the Gulf Coast states experience severe storms, including hail and tornadoes, every month of the year. During late winter and early spring, severe storms in the South are usually triggered by strong cold fronts that also bring heavy snow and ice to more northern latitudes.

The structure of a classic supercell. Source: NWS

The structure of a classic supercell. Source: NWS

As March slides into April and May, the southern Plains see the annual resurgence of supercell severe storms, fed by warm Gulf moisture pulled northward on southerly spring breezes and triggered by strong midlatitude storm systems that sweep out of the Rockies. Supercells – a specific type of thunderstorm that is highly organized, long-lived, and characterized by a persistent region of rotation (the mesocyclone) – are responsible for some of the world’s most damaging severe weather, including almost all significant tornadoes (EF2 or greater) and most giant hail.

During June and July, the jet stream shifts northward, steering storm systems (and their accompanying entourage of thunderstorms) into the northern Plains and upper Midwest, where storm season peaks in mid-summer.

But don’t get too attached to the timeline – severe weather doesn’t respect the notion of a “season” and can strike well outside the boundaries of what’s considered the climatological norm, as winter tornado outbreaks so destructively illustrate.

Preparing for Severe Weather

Because of that variability in when severe weather strikes, it’s an excellent idea to remain storm ready throughout the year. However, that first window-rattling rumble of thunder in the spring can certainly provide a little extra motivation and a helpful sense of urgency. Below are some essential storm preparations to ensure that you and your family are ready to weather the storm (pun intended).

Tree Trimming

source: The Davey Tree Expert Company

source: The Davey Tree Expert Company

Trees are beautiful, stately, and attract a wonderful diversity of wildlife. Deciduous trees are also great for your utility bill: providing leafy shade in the summer and allowing the sun to warm your home unimpeded during winter. But if not properly cared for, they can become highly efficient house- and car-crushers during wind storms.

Before the howling winds of spring start blowing across your newly leafed-out trees, call in a reputable arborist or tree trimming company to remove unhealthy limbs and trees and to shape healthy trees to be more wind-resistant. Services typically cost a couple hundred dollars (for basic limb removal) to a couple thousand dollars (for removal of entire trees), and are ideally performed every 3-5 years. A few hundred bucks in preventative maintenance now can save you not only your insurance deductible but also the hassle of major car and home repair.

Insurance Policy

Speaking of insurance, now is the perfect time to make sure you have enough and the right type of insurance. Dust off your insurance policy documentation – or better yet, call your insurance agent for a comprehensive review of your policy. Is your deductible affordable in your current financial situation? The purpose of having insurance is to use it when you need it, so make sure your deductible is an amount you can afford to pay should your home or vehicle sustain damage.

Review your coverage amounts for the structure and its contents – if the worst happens, will your insurance policy pay enough for you to repair or replace your house and belongings? Know whether you have replacement coverage (i.e. the insurance policy will pay for a brand new item equivalent to what was lost) or whether your policy covers only the depreciated value (i.e. the value of a used item equivalent in age, wear and tear to what was lost). Is the type of coverage you have what you need? Have you recently purchased any high-value items that might need to be insured separately as valuable personal property?

Photo by John Williams, courtesy of The Valley Reporter

Photo by John Williams, courtesy of The Valley Reporter

Consider purchasing flood insurance if you haven’t already. Standard homeowners’ policies do not cover flooding, whether from severe storms (flash flooding), prolonged rain (areal flooding), snowmelt, hurricanes, or even sewer back-ups. With an average flood insurance claim of $42,000, that’s not a check you’re going to want to write.

You don’t have to live in a designated floodplain to experience flooding – a third of all federal disaster assistance for flooding goes to people outside of mapped high-risk areas. In fact, if you don’t live in a flood plain, you’re likely to get a preferred rate on your flood insurance policy ($35/mo is the average premium for low-to-moderate risk properties). Flood insurance is offered through the federal National Flood Insurance Program (NFIP) and can be purchased through your insurance agent.

Storm Alert System

What you don’t know can, in fact, hurt you. This is especially true when it comes to severe weather. As technology becomes ever more sophisticated, the channels through which we can receive about information about wicked weather headed our way has rapidly expanded.

Despite the myriad new options, a NOAA weather radio is still an easy, relatively inexpensive, and extremely reliable means of receiving weather alerts even when you’re not by your TV, phone, or computer. There isn’t one brand of weather radio – instead, NOAA maintains a network of radio stations broadcasting information directly from the nearest National Weather Service office, including forecasts and weather warnings. These stations broadcast on a fixed set of frequency channels, and any commercially-available emergency radio worth buying will come pre-programmed with them.

source: Alabama Wx Weather Blog

source: Alabama Wx Weather Blog

When comparing weather radios, look for those that:

  • play a loud, obnoxious tone/alarm when an alert is issued, even if the radio sound is turned off – remember, you want it to wake you out of your soundest slumber if a tornado is barreling toward your house.
  • allow you to specify the particular area for which you want to receive alerts – called SAME (specific alert message encoding) technology, radios with this feature will alarm only if the county or region you’ve pre-selected is included in the warning. You probably don’t want to be woken up from your soundest slumber if the tornado is barreling toward somebody else’s house three counties away.
  • allow you to choose which types of weather alerts will trigger an alarm. If you live in a coastal county, for instance, but not near the beach, you probably don’t want to receive coastal flood warnings. Or you may want to be alerted only to the most extreme events, like tornado warnings, but not all severe thunderstorm warnings.
  • have a battery back-up to ensure you’re able to continue using it if/when the electricity goes out
  • receive AM and FM radio bands so you can also listen to weather information from your local radio station
  • if you live in a rural area, you may also want to consider a radio with an external antenna jack so you can attach a larger antenna for better reception

cell phone weather alerts

For those whose cell phones have become a fifth appendage, Wireless Emergency Alerts (WEA) put critical information right in the palm of your hand. WEA messages are overseen by the FCC and include Amber alerts, presidential messages, and weather warnings. Many if not most newer cell phones are already capable of receiving them. Participation in the WEA program is voluntary but also widespread among mobile carriers. If your phone is WEA-capable, you can choose which WEA messages you receive (with the exception of presidential messages, which are mandatory), so check for instructions specific to your phone make/model to ensure that WEA weather alerts are enabled.

Another mobile device solution is the use of apps. A wide variety of apps will send you a text or alert you through the app itself if hazardous weather is headed your way. The National Weather Service maintains a list of apps, and you can check around for other options. Be aware, though, that cell phone service can be interrupted during and after a major storm – it’s wise to have a back-up means of receiving weather information in case cell service is disrupted.

Storm Plan

Once you know a storm is coming, what are you going to do? The National Weather Service’s Weather-Ready Nation campaign and The Department of Homeland Security’s ready.gov have resources for a number of specific weather hazards, including thunderstorms, tornadoes, floods, and hurricanes. Read through the information for the weather hazards that can impact your area, and formalize a storm plan for each. Make sure everyone in your family knows what the plans are, and practice the plans, especially if you have small children. Some things to consider: where will you go during a severe thunderstorm or tornado, when a hurricane threatens, when floodwaters rise? How are you going to get information as the weather unfolds? Where do you keep your storm kit?

Basic Storm Preparedness Kit

source: Weather Underground

source: Weather Underground

Speaking of storm kits – it’s a really good idea to have one and to keep it in or near your storm shelter (i.e. basement; lowest-level interior, windowless room; or safe room). If you already have a storm kit: excellent! Time to check and refresh the supplies. If you don’t already have a storm kit, it’s time to put one together. Ready.gov and Weather Underground both have thorough checklists for building a basic storm/disaster kit.

At a minimum, storm kits should include food and water, a first aid kit, medicine, sanitation products, supplies to shelter in place (tarps, duct tape, etc), gloves, basic tools for turning off utilities, flashlights, a portable radio, extra batteries, signals for help (whistle, flare, etc), and infant and pet supplies if applicable. You can expand your kit to include other useful but not necessarily critical supplies based on your own needs.

 

We know it’s a lot to think about, but preparing now — before severe weather strikes — will save time, money, and heartache down the line. At a minimum, you’ll have peace of mind knowing you’ve mitigated against preventable damage, planned how you’ll ride out the storm and its aftermath, and insured your property against loss. And if storm season happens not to spare you this year, you’ll have much more than just peace of mind. You’ll have the tools needed to weather the storm.

Wind Damage: Reconstructing Chaos and Variability

Chaos

A bolt from the blue. A rogue wave.

As anyone who has lived on planet Earth for more than a few seasons can attest, nature is full of surprises.

Lightning usually strikes near the core of a thunderstorm, but occasionally it will strike more than 20 miles away, arcing across an otherwise peaceful sky. Ocean swells on a placid sea tend to be of a similar height, but rarely, a rogue wave many times the average height can appear suddenly, damaging or devouring any ship unlucky enough to be in its path.

A rare rogue wave caught on camera as it passes a large ship on otherwise calm seas. Credit: NOAA.

A rare rogue wave caught on camera as it passes a large ship on otherwise calm seas. Credit: NOAA.

Bolts from the blue and freak waves are just two examples of variability and chaos in nature. Here chaos describes a sensitive dependence on initial conditions – the proverbial butterfly flapping its wings in Africa that leads to a hurricane over the Bahamas. If the surface temperature had been just slightly cooler, that bolt from the blue would have sliced the air directly under the storm. If winds over the ocean had been just a tad bit different, that rogue wave would never have formed.

Variability

It is due to this chaos that atmospheric scientists so often speak in terms of probabilities and distributions (as frustrating as that can be to folks who just want to know whether a rain shower will or will not do the work of watering their lawn this weekend – I promise if we knew, we’d tell you).

A bolt from the blue.

A bolt from the blue.

The chaotic nature of the weather leads to variability both between events and within a single event. Thunderstorms are a perfect example: no two thunderstorms are alike, and each storm changes from moment to moment.

To describe such variability, we typically speak of averages and deviations from that average. For thunderstorms, we can describe the average peak thunderstorm wind for an area – say 40 mph for summer afternoon thunderstorms in north-central Florida. However, peak winds in exceptionally strong thunderstorms have been recorded at over 70 mph in this region, while winds in weaker storms may not exceed 25 mph. And within a storm that produces a 70 mph gust, winds are not sustained at that speed throughout the event. The sustained wind speed within such a storm averaged over a 2-minute period may only be 45 mph.

Building codes typically ensure that structures and infrastructure are built to withstand expected conditions for a given area – roofs in snowy areas must be able to withstand a higher load than in areas without snow; structures in hurricane-prone regions must be able to withstand higher winds; structures in seismically active areas must be able to withstand earthquakes. It’s generally not average conditions that cause damage – it’s the exceptions, those events that vary greatly from what is normal and expected.

An urban dweller struggles against strong, gusty winds. Credit: The Guardian.

An urban dweller struggles with strong, gusty winds. Credit: The Guardian.

In the case of thunderstorm wind damage, it’s the gusts. How gusty winds are during a given event depends on a number of factors, but mostly on the surrounding terrain: the rougher the terrain, the gustier the winds. Winds are far gustier in the center of a city, surrounded by skyscrapers and densely packed buildings of many sizes than in a smooth, open farm field.

To illustrate, given a weather station measurement of the 1-minute average wind speed, we would expect peak 3-second wind gusts in the heart of a major city to be nearly 250% higher than that average speed, while peak wind gusts over open farmland would be less than 50% higher.

The orientation of landscape features to the wind direction also impacts wind speed and gustiness. For coastal areas, this is particularly noticeable. Onshore winds – those that have blown unimpeded over a lake or ocean – tend to be stronger and steadier, all else being equal, than winds that have blown over a rough landscape (say, across a large metropolitan area). Similarly, winds blowing parallel to city streets tend to be felt more strongly than those blowing perpendicular.

Reconstructing Damaging Conditions – It’s More Than Just Reading the Station Data

When evaluating weather station data to determine the peak wind speed associated with a given event, we must consider the location of the weather station. Is it a rural site or an urban site? What is the terrain like surrounding the station? Are there certain directions from which the wind would be more impeded or would be blowing over rougher terrain?

How does the weather station site compare to the site at which damage was reported? Is the surrounding terrain similar? Was damage reported at an elevated location – for example, the roof of a high-rise building, where winds tend to be stronger? Differences in both roughness and height must be considered.

A microburst is caught on camera near the airport. Microbursts affect a small area but can produce winds greater than 100 mph. Credit: Las Vegas Review Journal

A microburst is caught on camera near the airport. Microbursts affect a small area but can produce winds greater than 100 mph. Credit: Las Vegas Review Journal

Also of obvious importance in the evaluation of winds associated with tropical cyclones and thunderstorms is estimation of the relative strength of the system when it impacted the weather station versus the damage site. If the damage site was directly impacted by the core of a severe thunderstorm while the nearest weather station experienced only a glancing blow, we would expect the winds experienced at the damage site to be stronger than those recorded at the weather station.

Evaluation of these considerations – location, orientation, height, and roughness – allows forensic meteorologists to estimate peak wind speeds from measured average wind speeds and to evaluate the extent to which a given weather station is an accurate proxy for the site at which damage was reported. The weather station data is just the beginning.

“The devil is in the details.” Nowhere is this truer than in the beautifully chaotic weather, where details determine outcomes.

If you or a client experienced wind damage and need an estimate of the wind speed associated with that weather event, call or email Blue Skies Meteorological Services for a free consultation.