Forensic Meteorology

Severe Weather Hotspots and Climatology in the U.S.

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.

The most thunderstorm-prone regions of the country are along the Gulf Coast and in the high elevations of the southeastern Rocky Mountains. Source: Koehler, Thomas L: 2019

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.

Intense high-elevation sunlight heats the side of a mountain and initiates an upslope flow that, if moisture and instability are present, can initiate thunderstorms. Credit: David Babb

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.

The sea breeze forms a mini cold-front along the coastline that often triggers thunderstorms along the Gulf Coast and the Florida peninsula. Source: COMET.

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.

A “bolt from the blue”: positively-charged lightning originates near the top of thunderstorms and can travel tens of miles from the base of the parent thunderstorm.

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.

Although thunderstorm wind damage is most frequently reported in the southern Appalachians due to the heavy tree cover in that region, extreme wind gusts over 70 mph most often occur in Kansas and Oklahoma.

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.

The kinetic energy (and therefore damage potential) of a hailstone increases exponentially as a function of its diameter

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).

Severe hail (hail 1″ diameter or larger) is most common across the southern and central Great Plains. This is also the region where the largest hail (hail > 2″ diameter) is most likely.

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.

Hail formation requires a strong thunderstorm updraft that can suspend growing hailstones aloft in the below-freezing hail growth zone. Source: NWS Burlington

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

Statistically, tornadoes are most common in the middle third of the country, with the strongest tornadoes impacting the southern Great Plains and Deep South. However, “tornado alley” migrates throughout the year depending on the season.

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.

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.

Automated Hail Tracks: The Dark Side of Dual Pol

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Last spring we wrote about the capabilities of dual-polarization radar to identify and distinguish between regions of different types of precipitation within thunderstorms and winter weather events (see “Seeing in the Dark”). This powerful technology provides valuable information to operational meteorologists deciding whether to issue a warning for damaging weather and to forensic meteorologists reconstructing that weather and its impact.

It is also used by the dozens of online hail track products marketed to both construction contractors and insurance claims investigators. These automated hail track products can provide a useful first estimate of hail impact, but they are far from perfectly accurate.

Megan Walker Radtke, chief meteorologist and climatologist here at Blue Skies, recently published an article in Claims Magazine discussing the use of hail track products for insurance claims investigation, including when they should be used with caution.

Check out the article in Claims Magazine’s July digital edition.

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.