Extreme Weather

Just as every region of the U.S. has its own unique culture and landscape, so too does every region have a unique severe weather profile, or climatology. Some parts of the country are particularly prone to large hail, while others are plagued by rain-wrapped tornadoes, and still others dodge lightning just about every afternoon during the summer.

Below we’ll discuss the four phenomena responsible for most weather-related damage across the U.S. – lightning, severe thunderstorm winds, hail, and tornadoes – and the climatological “hotspots” for each.

Thunderstorms Generally

Florida often claims to be the thunderstorm and lightning capital of the country – and justifiably so, according the meteorological data – but the somewhat ironically named “Sunshine State” isn’t the only region of the United States that’s especially prone to thunderstorms.

According to 25-years of cloud-to-ground lightning strike data from the National Lightning Detection Network, the bull’s-eye for the most lightning strikes and the most thunderstorm days per year is indeed found in southern Florida, where on average, storms rumble across the sky on more than 110 days per year (that’s almost one in every three days!). However, an arc of 70+ thunderstorm days per year extends across the entire northern and eastern Gulf Coast from Houston, TX through all of Florida. In these regions rimming the warm and moist Gulf of Mexico, thunderstorms can be expected to occur nearby on at least one out of every 5 days per year.

A perhaps somewhat less expected stormy region is found in the southeastern Rocky Mountains, where 55 – 80 thunderstorm days per year are typical near the highest elevations (although this probably isn’t too surprising to anyone who’s spent a few summer days hiking in Colorado or New Mexico and watched the clear blue morning skies turn gray and growling by mid-afternoon).

All thunderstorms require three ingredients to develop:
1) moisture
2) an unstable atmosphere
3) a triggering/lifting mechanism

For both of these thunderstorm maxima (and indeed for most of the United States), the primary source of moisture is warm, moist air blowing in from the Gulf of Mexico. From spring through early fall, prevailing southerly winds advect warm, humid air from the Gulf into the southeastern two-thirds of the country. Those steamy summer evenings you spend sitting on the porch sipping a cool beverage and listening to a chorus of crickets as lightning flashes in the distance? That’s likely courtesy of the Gulf of Mexico.

Atmospheric instability is also fairly seasonal, with unstable conditions – an atmosphere that cools quickly with height – most common during spring, summer, and early fall.

However, the thunderstorm triggering mechanism is quite different in the mountains versus along the coast. Anything that forces warm, moist air near the surface to rise into cooler air aloft can act as a thunderstorm trigger.

In the mountains, this trigger is the interplay between the strong high-altitude sunshine that heats the side of the mountain and the mountain itself. As the air near the surface of the mountain warms, it expands and becomes less dense, causing it to rise and follow the slope of the mountain toward the summit. In weak prevailing winds, this upslope flow occurs on both sides of the mountain, and when these flows collide at the summit, they have nowhere to go but straight up, forming a nascent thunderstorm updraft.

In the southeastern Rockies, these upslope flows often tap into the moist air that pools in the Great Plains as a result of southerly winds off the Gulf of Mexico. The Gulf provides the moisture, a relatively steep atmospheric lapse rate provides the instability, and the high-altitude sun heating up the mountainside provides the trigger.

In Florida and along the immediate Gulf Coast, the trigger is a direct result of the proximity of the ocean: the sea breeze. Sea breezes form because land heats up (and cools down) more quickly than water. On a sunny day, the land warms quickly and heats the air directly above the surface. This warming air expands and rises, and as it does so, it is replaced by cooler, denser air from over the ocean. This boundary between warm, unstable air inland and the cool, stable air moving in from the water acts like a shallow cold front that follows the contours of the coast.

During the day, as the temperature difference between the land and the sea increases (the land continues to warm, but the water remains at nearly the same temperature), the sea breeze strengthens and advances inland. Along the sea breeze, warm humid air is forced to rise by the advancing wedge of cooler ocean air that hugs the surface. If the atmosphere is even marginally unstable (and it often is during the summer in the coastal South), this buoyant warm, humid air will trigger thunderstorms along the sea breeze.

In Florida, sea breezes frequently form along both the east and west coasts and march inland as the day progresses. By afternoon, they meet near the middle of the state and the squeeze-play between the two sea breezes causes an expansion and intensification of thunderstorms along the spine of the peninsula. The result? Florida is the repeat winner of the “stormiest state” prize.

Lightning

Unlike most of the other thunderstorm hazards that present a risk to life and property, a storm does not have to be severe to produce damaging lightning. Any storm worthy of the name produces lightning. But not all lightning is equally damaging.

A lightning strike is often composed of more than one (sometimes dozens) of individual strokes. Each stroke carries a different amount of current. About 90% – 95% of lightning strikes are “negative flashes” that initiate from the negatively-charged lower- and mid-levels of the storm. These flashes typically strike the ground near the parent storm, although not always.

That leaves the 5% – 10% of lightning strikes that are “positive flashes”. These originate in the positively-charged upper levels of the storm. Because these strikes arc from the top of the storm, ionizing tens of thousands of feet of air, they must pack a very potent electric punch, with a peak current and electric potential up to ten times greater than negative flashes. Because many positive strikes originate near the edge of a thunderstorm cloud or from the anvil, they also tend to strike the ground farther away from the parent storm – sometimes as much as 20 miles from the precipitation. These are the so-called “bolts from the blue” that strike from apparently clear sky (in fact, they strike from a nearby thunderstorm, but the sky immediately above the impact point may be free of clouds).

Lightning strikes can be composed of several strokes that give the lightning a flickering appearance, or they can be more damaging “continuing current” strokes in which charge flows continuously over a longer period of time through the lightning channel. Because electricity flows over a longer period of time during continuing current flashes, more heat builds up. For that reason, continuing current flashes are more likely to cause structural fires.

However, any lighting strike can fry electronic equipment, and it doesn’t have to strike a structure directly to cause this type of damage – lightning can be conducted through electrical wires, telephone lines, data cables, and even wet ground.

Damaging Winds

Although Florida, the Gulf Coast, and the southeastern Rockies take the prize for highest frequency of thunderstorms, strong winds (greater than 58 mph) and wind damage are more common farther north, across the southern Appalachian Mountains. Another maximum is centered over eastern Oklahoma and extends into southern Kansas and far western Arkansas.

The bull’s-eye in the southeastern US is not a purely meteorological signal, though, because in order for an event to be recorded as a severe “thunderstorm wind,” a wind gusts over 58 mph must be measured or – and this is the important part – wind-related damage must be reported. Marginally severe winds (under 65 mph or so) tend not to cause significant damage to structures, but they can and do cause significant damage to trees. So we’d expect to find more thunderstorm wind damage in those areas more vulnerable to it – i.e. in areas with plenty of trees that can be broken or toppled by low-end severe (or even sub-severe) winds.

And where do we find a lot of trees? In the southeastern US, but not so much in the Great Plains. When fallen trees or tree branches block roads, take down power lines, fall on cars, or land on roofs, they tend to get reported and end up in the weather data as “thunderstorm wind” events. For that reason, there is a bias in the wind data toward heavily-treed regions, like the southeastern US.

We can see this bias by looking only at very strong wind gusts (greater than 64 kts or 74 mph) that do cause damage to structures or by looking only at measured wind gusts (as opposed to wind damage reports). By either of these measures, the bull’s-eye in the southeast is far less prominent that the relatively low-tree region centered in the southern Great Plains, which is far more prone to the type of weather that produces extreme winds: supercell thunderstorms and severe, organized convection like squall lines.

Hail

Large hail is responsible for billions of dollars of property damage each year – to roofs, siding, HVAC units, vehicles, and crops.

Sub-severe hail (i.e. hail up to 1” diameter) generally does not cause substantial property damage, although it can damage crops and sensitive vegetation. Structural damage begins to occur to shingle and some tile roofing materials at hail sizes of 1” – 1.5” diameter, with the age and condition of the roof contributing to the degree of observed damage. Denting of car metal generally starts with hail around the size of golf balls (1.75” diameter).

As the size of a hailstone increases, so too does its terminal velocity and kinetic energy. It is the rapid transfer of this kinetic energy from the hailstone to the surface it strikes that causes damage, and the kinetic energy of hailstones has been shown to increase exponentially with an increase in their diameters. Strong winds also increase the damage caused by hailstones by imparting an additional horizontal velocity to the hailstones, thereby increasing their kinetic energy. Wind-blown hail is also more likely to strike and damage vertical surface (like the exterior walls of buildings), whereas hail that falls in lighter winds tends to concentrate damage on horizontal surfaces (like roofs).

So where is severe hail (1” diameter or larger) most likely? Unsurprisingly, right in the heart of the Great Plains – from northern Texas, stretching through most of Oklahoma and Kansas, and including western Colorado and southern Nebraska.

In this region of the country, the ingredients for large hail come together like clockwork every spring: a highly unstable atmosphere that encourages explosive thunderstorm updrafts capable of growing and suspending large hailstones, strong vertical wind shear that separates updrafts from downdrafts (rain within downdrafts causes hail to melt more quickly), and reasonably low freezing levels that maximize the depth of the hail growth zone aloft and minimize the amount of time the hail falls through above-freezing temperatures on its way to the ground.

Take note that there’s not much overlap between the hotspots for thunderstorm frequency (Florida and the northern Gulf Coast) and the hotspots for severe hail (the southern and central Great Plains). In other words, the places most prone to thunderstorms are not the places most prone to severe hail.

The reason for this quickly becomes apparent when you consider that thunderstorm season along the Gulf Coast is the summer, and the summer in this part of the country feels very much like living in a sauna for four months. The heat and humidity near the surface typically extends for miles into the atmosphere, with relatively light winds. The freezing level is high and wind shear is minimal: in other words, saunas are not very hospitable environments for balls of ice.

Tornadoes

Although we may think of Tornado Alley as centered on Oklahoma and Kansas (i.e. “those places destroyed in the movie Twister”), tornadoes are a regular yearly occurrence across the central third of the country, including the Great Plains, Midwest, and Southeast. Two prominent hotspots are traditional Tornado Alley centered on Oklahoma, Kansas, and northern Texas and “Dixie Alley” that encompasses Mississippi, Alabama, and Tennessee.

The southern Great Plains are most active during the Spring (late March through May), with tornado activity shifting toward the northern Plains and Midwest as summer progresses. As the rest of the country begins to cool toward fall and winter, Dixie Alley takes over as the tornado hotspot (SPC monthly tornado probability maps).

The seasonality of tornado alley and the meteorology behind its roaming nature is discussed in a previous Blue Skies article.

Summary

Although damaging weather can, and does, occur in just about every state in the nation, there are distinct hotspots for different thunderstorm hazards.

With towering thunderstorms bubbling up many, if not most, afternoons during the late spring through early fall, the northern Gulf Coast and all of Florida are especially prone to lightning-related hazards, as are the southeastern Rocky Mountains out west.

Wind-related damage tends to concentrate in the southern Appalachian Mountains, where dense tree-cover provides ample opportunity for even marginally severe winds to cause widespread damage. The strongest thunderstorm winds, however, are typically found in the southern Great Plains, near the heart of traditional Tornado Alley.

The southern Great Plains are also home to the largest hail and most frequent severe hailstorms due to the potent combination of warm, moist air from the Gulf of Mexico mixing with dry air coming over the Rocky Mountains, ample vertical wind shear, and relatively low freezing levels. However, severe hail (hail 1” diameter or greater), can be found annually across much of the U.S., from Montana to New Hampshire, from Texas to Florida, and just about everywhere in between.

Tornadoes likewise regularly impact the central and eastern half of the country, although strong and violent tornadoes (those rated EF2 or higher on the Enhanced Fujita scale) tend to concentrate in the Great Plains and the Deep South.

As we roll into spring and early summer, remember that while some regions of the country are climatologically more prone to certain types of damaging weather, severe weather truly knows no boundaries of season or place. Stay alert, and reach out to Blue Skies Meteorological Services for a forensic weather analysis if you have questions about the location, magnitude, or timing of a damaging weather event in your area.

The deadly tornado outbreak across Alabama, Mississippi, Florida, Georgia, and South Carolina this past weekend was an abrupt and powerful announcement of the start of the 2019 tornado season. Although tornadoes can and do occur outside of the typical spring-through-summer tornado season (there have in fact been 14 tornado days since the beginning of this year), the majority of tornadoes in the United States do occur between April and July.

An outbreak of violent tornadoes (EF4 or EF5) during the first week of March may seem a bit early, but according to the Storm Prediction Center it’s happened at least 7 times since 1980. Averaged over the course of a year, tornadoes are most likely exactly where you’d expect them: in the Great Plains, with a bulls-eye over central Oklahoma and northern Texas.

However, the majority of early-season tornadoes (January – March) spin up in the Southeast, where spring arrives early. In these areas, warm, moist air from the Gulf of Mexico creeps northward, bringing a taste of spring even as the rest of the country continues to hunker down in winter parkas. But even in the Southeast, that warmth doesn’t last this time of year. Inevitably, a storm system sags southward, ushering through a cold front and a return to winter. When the arctic air behind that cold front meets the warm, moist air ahead of it, severe and sometimes tornadic thunderstorms ignite.

It is this clashing of air masses that fuels the majority of tornadic storms, and for that reason, “tornado alley” roams with the seasons.

By the time April and May roll around, people in the Southeast already have their air conditioners running and have shoved long-sleeved shirts and closed-toe shoes to the back of the closet. The meteorological action shifts north and west, towards Oklahoma, northern Texas, and Kansas – the heart of what has traditionally been labeled Tornado Alley and the birthing ground of many of the United States’ strongest and most devastating tornadoes.

In fact, the city most like to be hit by a tornado is Oklahoma City, which lies dead center in the state and has endured the impact of around 150 tornadoes since record-keeping began in 1890. The Oklahoma City metropolitan area was hit by violent EF4 and EF5 tornadoes in 1999, 2003, and 2013, with the paths of these tornadoes overlapping in several locations. The EF5 tornado that ravaged the suburb of Moore, OK on May 3, 1999 had wind speeds clocked at 318 mph – the strongest ever recorded.

The unique geography of the southern Plains gives rise to its unique and tornado-prone climatology. Wedged between the cool, dry air that spills down the leeward side of the Rocky Mountains and the warm, humid air that’s pumped northward from the Gulf of Mexico, the Great Plains fields some of the most intense and violent air mass clashes in the world.

But by June and July, even the southern Plains have been given over to summer. Storm systems shift northward and focus the tornado threat in the northern Plains and Upper Midwest. By August, as the whole country settles in to the dog days of summer, the tornado threat retreats. Hot and humid air is in place across much of the country, and storm systems are weaker.

An exception is in central Florida, where afternoon thunderstorms are a daily occurrence during the height of summer and occasionally spawn tornadoes, especially when they interact with outflow or sea breeze boundaries. The tornadoes that form under these conditions are generally weak and short-lived, however.

As summer gives way to fall, the tornado risk shifts back to the southeast, where it remains, low but present, throughout the winter.

And in the spring, like a nomad, it starts roaming again.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

“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.

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.

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.

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.)

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.

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.

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.