July 11, 2018

NOAA's Efforts to Fix Critical Weather Radar Gaps Are Slow Going

Doppler radar transformed the way we look at the weather. Tornadoes and severe thunderstorms rarely sneak up on us anymore. However, it only works when it can see the right parts of the storm. Central North Carolina has one of the worst weather radar gaps in the United States. Charlotte is the largest city in the country without adequate low-level radar coverage. There are several similar gaps across the country, but none with so many people at a disadvantage. Congress has tried multiple times over the past few years to rectify the problem. Although a renewed effort to look into gaps in radar coverage became law last year, progress is slow going.

Congress doesn't pay much attention to the weather unless there's discussion about a disaster or climate change. There are a few bills in each meeting of Congress to beef-up funding for forecasting and research, but like most proposed legislation, they never see the light of day. The Weather Research and Forecasting Innovation Act of 2017 represented the most attention Congress has paid to weather forecasting in decades. The bill, which the president signed into law in April 2017, was geared toward improving forecasts and creating better and more timely severe weather warnings.

Among other things, the lengthy piece of legislation specifically directs NOAA to conduct research into critical efforts like increasing tornado warning lead time, increase the accuracy of hurricane forecasts, and work toward better weather modelling.

The bill also directs the agency to identify gaps in the country's network of weather radars and propose a plan to fill those gaps. Lawmakers gave NOAA 180 days after the bill became law to submit to the Senate a report on gaps in radar coverage across the country and 90 days after that to formulate a plan to rectify the problems.

It's been exactly 450 days since the enactment of that legislation. The agency has not yet completed a report on radar gaps or a plan to fix them, a spokesperson for the National Weather Service told me on Tuesday.  When I asked when the agency expected to complete the report, the spokesperson added:

NOAA is working diligently to complete the study and report required. We take this requirement very seriously. Congress was made aware during the formulation of this legislation that such a study and report would take much longer than six months.

Gaps in weather radar coverage is an issue I've talked about for years. The first freelance article I ever wrote was about the radar gap in North Carolina. I've lived near Greensboro for the past eight years.  I'm intimately aware of the occasional sketchiness of radar coverage in this state. The stretch of Interstate 85 between Charlotte and Greensboro has minimal low-level radar coverage, a dangerous gamble with the number of severe storms that regularly traverse over so many people.

Why is this such an issue? It starts with the nature of weather radar and the nature of bureaucracy.

Despite Its Flaws, Radar Today Is Still Better Than Ever

Source: National Weather Service/Wikimedia

Weather radar came into operational use in the United States at the end of the 1950s. The network started with installation of the first WSR-57 radar dish at the National Hurricane Center in Miami on June 26, 1959. Dozens more would be installed over the following years, including the WSR-57 radar atop 30 Rockefeller Plaza in New York City (pictured above) that's still there to this day.

Source: NWS

Early weather radar had limited range and could only see the location and, later on, the intensity of precipitation—a far cry from today's capabilities. The above image shows the terminal that displayed radar data from a WSR-57 radar near Cincinnati, Ohio*. The hook echoes on the monitor are supercells producing destructive tornadoes during the Super Outbreak of April 3, 1974.

There were more than 130 densely-packed radar sites across the central and eastern United States before the network was modernized with the rest of the National Weather Service in the 1990s. The Next Generation Radar (NEXRAD) network was developed in the late 1980s and the National Weather Service began retiring the old radar sites and installing Doppler weather radar (WSR-88D) around the country in 1992.

Weather radar sites before modernization (pre-1989) and after modernization in the 1990s. The radar installed in western Washington in 2011 is not shown. (Source: NWS/National Academy Press | Titles added by author)

The current generation of radar uses the Doppler effect to detect the wind speed and direction within a storm, critical in the detection of storms capable of producing severe winds, hail, and tornadoes. The improved technology also gave the new radar devices a larger radius and higher resolution than the older generations, allowing the use of fewer sites that are spread across a greater distance. Recent upgrades added dual-polarization capability to the existing radar network, giving meteorologists the ability to identify rain, hail, wintry precipitation, and tornado debris.

If Only Earth Was Flat...

Source: NWS Jetstream

Doppler weather radar works by sending out strong radio waves from the radar dish at a slight angle—the standard angle for low-level coverage is 0.5°. The energy reflects off of objects in the atmosphere and the radar measures the strength of the returning beam and the time it takes to return in order to determine the location, intensity, speed, direction of movement, size, and shape of the objects in the beam's path. The radar dish then adjusts its angle upward and repeats this process for several minutes until it has a complete scan of the atmosphere.

This leads to a couple of flaws in radar technology. The first is that large objects like water towers, wind turbines, and mountains can block the beam. A temperature inversion can refract the radar beam back toward the ground and lead to false returns. Radar towers are vulnerable to lightning strikes, wind damage, and aging mechanics, which could render them inoperable at the worst possible time.

Source: NWS Jetstream

Radar is also eventually thwarted by the curvature of the Earth itself. The radar beam grows higher and higher above ground level as it gets farther away from the radar site. This limits a radar site's effectiveness beyond a certain radius when it comes to looking for tornadoes and other severe hazards.

Source: NWS
The beam curving upward with height results in gaps in radar coverage. Most of the largest gaps occur in the Rocky Mountains where the terrain simply blocks the signal from covering some communities, but some of the gaps are due to radar sites being spaced too far apart. The worst gaps east of the Rockies occur over South Dakota, Missouri, parts of the Deep South, and the worst (by number of people affected) is in central North Carolina.

Low-level radar coverage is important because that's where tornadoes form. It's crucial to see rotation within a thunderstorm as close to the ground as possible in order to detect a potential tornado and issue warnings with adequate lead time. 

Central North Carolina Got Overlooked

A radar image of the tornado near Charlotte, N.C. at 2:32 AM on March 3, 2012. The left image shows base reflectivity (precipitation) and the right image shows base velocity (wind). The radar is located to the west near Greenville, S.C.

The urgency surrounding the radar gap in North Carolina surrounds the unease of knowing what can happen without adequate radar coverage in a densely populated area. Charlotte is the largest metropolitan area between Atlanta and Washington D.C. The city and its suburbs are home to more than 2.4 million people, a population greater than that of 15 states.

There's a small Terminal Doppler Weather Radar (TDWR) site at the Charlotte Airport, part of a network of airport-based radars meant to protect arriving and departing flights from dangerous conditions. These radars are different from the WSR-88D in that they have a much shorter range and they're more susceptible to interference.

The lack of low-level coverage in central North Carolina isn't a theoretical game of what-if. An EF-2 tornado touched down a few miles northeast of Charlotte in the middle of the night on March 3, 2012. There was no tornado warning before the storm. The tornado damaged hundreds of homes and injured several people.

The tornado happened quickly and, despite its strength, the rotation was shallow. The beams from nearby Doppler radar sites were far too high to catch the rotation in the storm. The beam from the radar near Greenville, S.C., was 8,200 feet above ground level at the site of the tornado. The radar image above shows the Greenville radar at the time of the tornado. The beam there from the radar in Columbia, S.C., was 9,100 feet high. The tornado signature did show up on Charlotte's TDWR site, but it wasn't particularly strong.

This issue was known long before any new radar sites were built. Years of debate preceded the Weather Service Modernization Act of 1992, which consolidated smaller, more localized offices into the agency we know today.  A subcommittee in the U.S. House of Representatives held a hearing called "Tornado Warnings and Weather Service Modernization" on August 7, 1989, to specifically discuss the proposed closure of the Charlotte weather service office.

The modernization plan divided the Charlotte area between the consolidated NWS offices in Greenville, S.C., Columbia, S.C., and Raleigh, N.C. The new Doppler weather radars sites were co-located with those three offices, leaving Charlotte—a city whose metropolitan area has millions of people—split in thirds with radar coverage to match.

Many of the issues we're dealing with nearly 30 years later came to light in that congressional hearing. The subcommittee heard a variety of opinions on the topic. The head of citizens' emergency preparedness committee at UNC Charlotte pleaded in his testimony not to move city's radar to Columbia for fear of losing low-level radar coverage:

Jack Roper, broadcast meteorologist for WSPA in Greenville, S.C., expressed his concern about the  new radar system's spatial coverage in his part of South Carolina. He characterized the new Doppler network as something that "could be a new Edsel," a reference to Ford's aggressive marketing of a line of cars that failed to meet the ad campaign's lofty expectations and resulted in tremendous financial losses for the company.

The issues were raised again in a letter from Rep. Liz Patterson (D-S.C.) to Sen. Ernest Hollings (D-S.C.) before a June 1991 U.S. Senate hearing on NWS modernization.

Despite all of the concerns voiced in the years leading up to modernization, Charlotte still lost its weather office and its weather radar to smaller cities down the road. Problems with radar gaps were well known when the government started planning the current radar network, especially in central North Carolina, even as the agency said several years after modernization began that there was no lowering of the quality of service in the Charlotte area.

Past Attempts to Fill Radar Gaps

Several congresspeople introduced legislation to improve Doppler weather radar coverage in the years before the 2017 bill became law. The most recent legislative success that directly resulted in a new weather radar came after a push by Sen. Maria Cantwell (D-WA) to secure funding for a new radar site on the Washington coast. The new radar, which went into service in the fall of 2011, was important in monitoring Pacific storms as they approach Washington and Oregon.

Rep. Robert Pittenger (R-NC) and Sen. Richard Burr both introduced the Metropolitan Weather Hazards Protection Act of 2015 in their respective chambers. The bill directed the government to construct new Doppler weather radar sites near big cities in known radar gaps within a year and a half of the bill's passage. The, uh, creative specifications laid out in the bill—"maintain and operate at least one Doppler weather radar site within 55 miles of each city in the United States that has a population of more than 700,000 individuals"—served to limit the scope of the bill to pretty much just Charlotte, North Carolina. The bill passed the Senate by unanimous consent but died after the House took no action.

Rep. Charles Boustany (R-LA) introduced the RADAR Act in 2016, which died without any legislative action. The bill would have required both 1) the operation of at least one Doppler radar site within 55 miles of each state capital and 2) that any future radar sites would be located near at least one county with a population of 130,000+ that doesn't have adequate low-level radar coverage.

The capitals of 13 states, including Boustany's home state of Louisiana, would have been eligible for new radar sites under the proposal. While the bill would have filled in some glaring gaps in coverage, including those in central South Dakota, central Missouri, and southern Louisiana, it essentially told North Carolina that it can wait its turn until the next time around.

All of the radar-related bills introduced in Congress since Cantwell's successful push a decade ago have died in their respective chambers except for the Weather Research and Forecasting Innovation Act of 2017, the effectiveness of which is still pending.

Assuming a similar timeline to the western Washington radar earlier this decade—funding secured in 2009 and the radar made operational in 2011—we likely won't see any new radar sites until the early 2020s.

[Top Image: Pierre cb via Wikimedia Commons]

*I originally said the old WSR-57 radar image was from Wilmington, Ohio. The NWS office is in Wilmington, but the radar is in Covington, Kentucky, near Cincinnati. I've corrected my error.

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July 10, 2018

The Stalled Tropical Storm Off the East Coast Cooled the Gulf Stream Beneath It

Tropical Storm Chris is very close to hurricane strength this afternoon as it finally starts to move northeast a few hundred miles off the North Carolina coast. The storm has barely moved since it formed five days ago, caught between weather systems without any steering currents to shove it along. The storm's stalled motion has induced upwelling in the ocean beneath it, which worked in part to keep the storm from strengthening too quickly.

Chris is one of those storms that gives coastal residents some uneasy relief. The storm is pretty darn close to the United States, and it's never comforting to see a storm approaching hurricane strength right off the eastern seaboard. Chris is pinned between ridges of high pressure to its north, west, and east—the storm got trapped by the same features keeping it from hitting the United States, and there hasn't been anything to steer it away until the ridge holding it in place broke today and a trough lifts the storm out to sea.

The tropical storm sitting over roughly the same spot in the Atlantic Ocean for five days has considerably churned the seawater beneath it. This churning has allowed for upwelling, or cooler water from deep in the ocean to rise to the surface. A buoy near the storm has recorded a precipitous drop in water temperatures over the past couple of days. The buoy measured water temperatures around 82°F on Saturday, July 7, before falling as Chris grew stronger. The buoy's latest measurement recorded waters below 76°F, a six-degree drop in just a couple of days. That's even more impressive when you consider that the buoy is in the Gulf Stream.

The pool of cooler water is readily apparent in daily sea surface temperature analyses. The animation at the top of this post shows the sudden drop in sea surface temperatures beneath the tropical storm between the mornings of July 6 and July 9. Today's analysis, likely showing even cooler waters in spots, will be released tomorrow morning.

The cooler water clearly had an effect on Chris when it started to struggle a bit with its organization on Monday. The combination of dry air wrapping into the storm and cooler water beneath it served to disrupt the tropical storm's structure on Monday. The storm is much better organized today after it mixed out and walled off the dry air and it's starting to lift northeast away from the pool of cooler water it churned up. The storm's look (above) matches its strength now, with a tight core and a clearing eye.

Tropical cyclones strengthen through latent heat release. Warm water on the surface of the ocean evaporates and condenses in the storm, releasing latent heat that provides the instability necessary to sustain thunderstorms around the core of the cyclone. The warmer the water, the greater the latent heat release, and the stronger a storm can get. Cool waters inhibit this process and eventually choke off a storm by weakening the thunderstorms that surround the center of circulation.

The National Hurricane Center expects Chris to reach hurricane strength later today as it accelerates toward the northeast. While the storm will remain far offshore, dangerous rip currents and rough surf are still likely along the East Coast for the next day or two as Chris finally exits the area. The storm's peak strength won't last long—Chris will reach cooler water and less favorable conditions on Thursday, weakening the storm and forcing it to transition into an extratropical cyclone. The latest forecast shows Chris or its remnants clipping southeastern Newfoundland early Friday morning, likely bringing some heavy rain and gusty winds to the provincial capital of St. John's.

[Chart: NOAA | Maps: me | Satellite data courtesy of AllisonHouse]

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July 7, 2018

New Tropical Depression Forms Off East Coast as Tiny Hurricane Beryl Collapses

The circle of life in the Atlantic Ocean is whirring once again as Beryl winds down and Chris winds up. Beryl weakened from a hurricane to a tropical storm on Saturday morning after its surprise performance on Friday. A disturbance between Bermuda and the North Carolina coast finally developed into a tropical depression and it looks like it could make it to hurricane strength as it parallels the coast through next week.

Hurricane Chris?

An Air Force reconnaissance plane investigated Tropical Depression Three this afternoon and found it a bit disorganized as it sits a few hundred miles southeast of North Carolina's Outer Banks. The fledgling tropical depression is bigger now than Hurricane Beryl was at its strongest, truly a sad statement about the latter.

While it's unsettling to watch a storm sit and grow this close to land, the National Hurricane Center expects the cyclone to stay far enough away from the East Coast that the only problems we'll face are rip currents and rough surf. Close is close, though, and it's worth keeping an eye on it just in case things change. It's always a good idea to make sure you have emergency supplies.

Tropical Depression Three is pretty much stuck in place right now, pinned between a stalled cold front to its west and a ridge of high pressure to its east. This will allow the storm to meander for the next couple of days as it gathers strength before a trough picks the storm up and lifts it out to sea early next week.

The depression, which will gain the name Chris when it reaches tropical storm strength, will slowly gather strength thanks to the fact that it's moseying directly over the Gulf Stream. The storm should track directly over or very close to this current of warm water as it lifts off toward Newfoundland next week. Future-Chris could briefly reach hurricane strength before moving over cooler water and into a less favorable environment.

The latest forecast from the NHC shows that Chris will lose its tropical characteristics by the time it reaches Newfoundland, but it will still be a strong cyclone with gusty winds, heavy rain, and rough surf.

Beryl Collapses

Alas, poor Beryl. The loosely amalgamated clump of water vapor now known as Tropical Storm Beryl is clinging to life by a wisp of an updraft. Its low-level circulation is swirling bare, broken free of the convection that once gave it improbable life.

Beryl's triumph was its downfall. The small storm collapsed this morning just as spectacularly as it developed 36 hours ago. Beryl, much like me, fell to pieces after a minor inconvenience, in this case an intrusion of dry air and some moderate wind shear.

Nobody initially expected the itty bitty depression to strengthen into a hurricane based on its size and the hostile environment around it. Tiny hurricanes are fragile and extremely susceptible to adverse conditions. They can strengthen and weaken without much forewarning.

The 5:00 PM EDT update from the National Hurricane Center shows Beryl with maximum winds of 50 MPH in a wind field that only stretches a few dozen miles wide. A tropical storm warning is in effect for the island of Dominica as the system—or at least what's left of it—is forecast to track over the island on Sunday night. Regardless of its organization or official title, the storm or its remnants could bring heavy rain to islands susceptible to flooding and mudslides.

(I updated this post at 7:00 PM EDT with the latest information about each storm.)

[Satellite Images: NOAA | Maps: me]

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Intense Southern California Heat Wave Shatters All-Time Record Highs

The eastern two-thirds of the United States has been mired in an oppressive heat wave for the past couple of weeks, bathing much of the country in a brutal mix of hot temperatures and high humidity. However, while the east has gotten all the headlines (as usual), it's not the only heat in town. The desert southwest and southern California are in the midst of their own historic heat wave, the likes of which have never been recorded in some cities. The heat in the west has more than made up in intensity what it lacks in longevity.

Temperatures easily climbed into the 100s across much southern California and the state's Central Valley, and readings pushed 120°F in the California and Arizona deserts. Downtown Los Angeles reached 108°F on Friday. San Diego saw a high of 96°F. Yuma, Arizona, set a daily record high of 117°F. Las Vegas reached a toasty 112°F—a few degrees short of a record, sure, but still 8°F above normal.

You would expect this type of heat in the desert, but many spots in urbanized areas of California set record highs on Friday. Some of those cities saw their warmest July day on record, and some reporting stations even saw the hottest day ever recorded in decades of weather observations. At least six reporting stations in southern California on Friday broke their all-time record highs.

  • Burbank Airport, which is east of downtown Los Angeles, saw a high temperature of 114°F on Friday afternoon. This broke the all-time record high of 113°F set there in September 1971.
  • UCLA, which is in western Los Angeles, saw its all-time record high of 111°F on Friday, beating the previous record of 108°F set all the way back in September 1939.
  • Van Nuys Airport, northwest of downtown Los Angeles, measured a 117°F high on Friday. The previous all-time record high at the airport was 114°F—however, the airport's records only go back 24 years.
  • Riverside, California, reached 118°F on Friday,  tying the all-time record high first set there back in July 1925.
  • The fire station in Santa Ana, California, south of Anaheim, measured a high temperature of 114°F on Friday afternoon. This was the hottest temperature ever recorded in Santa Ana, where records go back to 1893. The previous all-time record high was 112°F set in June 1917.
  • The airport in Ramona, California, northeast of San Diego, saw an all-time record high temperature of 117°F on Friday. While impressive, records here only go back to April 1998.

The cool water of the Pacific kept communities immediately along the coast much cooler than spots just a few miles inland, but the air was considerably hotter just a few thousand feet above ground level. It was still hot at the surface despite the marine layer's powerful influence. Los Angeles International Airport only ("only") reached 92°F on Friday, but that still beat the record high of 88°F for July 6 set back in 1957.

Temperatures will climb back up into record territory for many of the same areas on Saturday. The heat will start to break on Sunday and should fall back to simply above-normal through early next week.

Why is it so blazing hot? There's a large and powerful upper-level ridge over the western half of the United States. Ridges are associated with sinking, stagnant air; we saw a great example of this last week when several derechos rode around the ridge over the Plains. There are also localized factors at play, such as downsloping winds off of higher terrain (air warms as it descends) and the classic urban heat island effect.

This type of heat is more common of early fall than the middle of summer. Our traditional idea of weather fitting neatly into three-month seasonal blocks kind of falls apart once you travel west of the continental divide. The brutal heat of summer usually plagues southern California once September rolls around, as evidenced by Burbank and UCLA's all-time record highs falling in September.

This is typically the dry season across southern California. Cities like Los Angeles historically see very little rainfall between the end of May and the end of September. Much of southern California has slipped into a moderate to severe drought over the past couple of months. These brutally warm temperatures won't help the cause. Extreme heat helps remove moisture from the ground even faster than normal, which could exacerbate dry conditions heading through the rest of the summer.

[Weather Model: Tropical Tidbits | Temperature Map: me]

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July 6, 2018

Itty Bitty Hurricane Beryl Defies the Odds, Makes Fools of Us All

Looks sometimes aren't deceiving at all. A tiny, good lookin' tropical depression far out in the Atlantic Ocean suddenly and surprisingly developed into a full-fledged hurricane that's so small you could miss it from space without knowing where to look. Hurricane Beryl unexpectedly strengthened into a category one hurricane in less than 24 hours, packing 80 MPH winds around a pinhole eye. The hurricane is about as small as one can get, barely registering larger than your average squall line.

Yesterday I wrote that this storm is "the kind of cyclone that tries to defy the odds" due to its appearance and location, but ultimately couldn't due to its weakness and the hostile environment it's approaching:

The system initially wasn't expected to develop into much of anything, but it started to look more impressive on satellite imagery during the day on Wednesday. This is the kind of cyclone that tries to defy the odds, but fortunately for storm-weary folks near the coasts, the environment is too hostile to allow this storm to buff itself up beyond what we think should be possible. Even a stronger, more solid storm would struggle against the obstacles ahead of T.D. Two.

Oops. Talk about defying the odds.

Hurricane Beryl has maximum winds of 80 MPH this morning as it scoots west toward the Lesser Antilles. The latest forecast from the National Hurricane Center shows the storm possibly reaching the islands as a hurricane before slowly weakening once it enters the Caribbean. Beryl's diminutive size will be its saving grace—even if the worst conditions affect land, it wouldn't be for more than a few hours. The greatest threat with this storm would be flooding and mudslides from heavy rain.

Hurricane Beryl's hurricane-force winds only extend 10 miles away from the center of the eye, and winds greater than 39 MPH only extend out 35 miles. My favorite size comparison for tropical cyclones (and one that always draws ire from some weather folks for its ridiculousness) is to overlay a storm's wind field over the Washington D.C. metro area to show how relatively small it is:

Beryl's hurricane-force wind field is so small that it just barely covers Washington D.C. with just enough room to fit in most of Arlington County, Virginia. The storm's field of 39+ MPH winds would be big enough to stretch from Howard County to Charles County in Maryland.

Meteorologists and weather models have a hard time forecasting the intensity of exceptionally small tropical cyclones; this storm's core isn't much bigger than a healthy supercell. Beryl is tiny, and tiny storms have a history of wildly fluctuating in intensity. On paper, at least, it seems like the storm shouldn't have achieved its current strength. But Beryl is small enough that it found itself a pocket of favorable-enough conditions and took full advantage of what it found.

The storm's structure should insulate it just enough from wind shear and dry air that it could survive into the Caribbean before starting to weaken and fall apart. However, the NHC notes in its latest forecast that predictions are more uncertain than usual because of the hurricane's tiny size. Tiny storms are fragile. Beryl brings to mind Hurricane Danny from 2015, a similarly tiny storm in roughly the same spot that reached major hurricane strength before collapsing as it approached the Lesser Antilles.

Storms like this are humbling for meteorologists and weather enthusiasts alike. I was wrong. They were wrong. We were all wrong. (Crow all around!) Nobody initially expected this storm to strengthen the way it did and they're probably lying to make themselves look good if they tell you otherwise. Predicting the weather is still an inexact science and there's a lot for even the experts to learn about how and why tropical cyclones suddenly intensify, especially itty bitty ones like Beryl.

[Satellite: NOAA | Maps: me]

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July 5, 2018

Hostile Atlantic Ocean Eagerly Waiting to Tear New Tropical Depression to Shreds

Newly-minted Tropical Depression Two is a visually impressive but ultimately doomed cyclone that's far out in the Atlantic Ocean between the Lesser Antilles and Africa. The system in any other year would become a serious concern for interests near the coast, but in this year, this hostile year for the Atlantic Ocean and us all, the storm will last about one-third of a Scaramucci, meeting its untimely fate this Sunday in a swift and meteorologically gruesome fashion.

The National Hurricane Center's first forecast discussion for T.D. Two reads more like the eulogy at a living funeral rather than a dire prediction of tropical woes. The environment is not suitable for significant development. The cyclone may grow strong enough to achieve the name Tropical Storm Beryl before dissipating this weekend at the hands of cooler-than-normal waters, dry air, and strong winds.

The system initially wasn't expected to develop into much of anything, but it started to look more impressive on satellite imagery during the day on Wednesday. This is the kind of cyclone that tries to defy the odds, but fortunately for storm-weary folks near the coasts, the environment is too hostile to allow this storm to buff itself up beyond what we think should be possible. Even a stronger, more solid storm would struggle against the obstacles ahead of T.D. Two.

A quick look at this morning's visible satellite image across the Atlantic shows a small, classic-looking storm that looks like it could have been the nucleus to something more ominous had it formed in a better environment. All that beige fuzz ahead of the storm over the Caribbean is Saharan dust that blew off of Africa ahead of the tropical wave that would become T.D. Two. Dusty desert air is not conducive to a juicy tropical cyclone.

Adding insult to injury is strong wind shear over the eastern Caribbean that will shred the storm to pieces in a hurry. Strong upper-level winds disrupt the updrafts in thunderstorms that try to form and displaces existing thunderstorm activity far away from the center of the cyclone. Both of these work together to kill storms fast. It's something special to watch the thunderstorms in a tropical cyclone floof away (technical term) so fast in dry air and strong wind shear that all that's left is a naked low-level swirl confusedly spinning itself to oblivion.

The remnants of T.D. Two/Beryl will continue into the Antilles through early next week and bring some heavy rain to the area. Aside from a slight chance that a disturbance near Bermuda could briefly develop over the next day or so before also meeting a swift end, the Atlantic will return to its quiet state soon enough.

Related: Here's a Hype-Free Rundown to Help You Keep Track of Storms This Hurricane Season

[Map: Dennis Mersereau | Satellite: NOAA | Model: Tropical Tidbits]

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July 3, 2018

June 29, 2018

How the June 2018 Heat Wave Triggered Three Derechos in One Day

The word "derecho" is usually not allowed on this wholesome website. I try not to use the word unless we're talking about an event that's already happened or the SPC outright uses it in one of their forecasts. That being said, yesterday we saw three derechos in one evening as a result of the growing heat wave. The storms resulted in more than 500 reports of wind damage across a dozen states, claiming one life in Alabama and knocking out power to hundreds of thousands of people. All three storm complexes were strong "mesoscale convective systems," a common type of thunderstorm event during the summer and one you can almost expect to see during a heat wave.

A derecho is a strong, long-lived squall line that produces lots of wind damage along a path stretching hundreds of miles long. The derechos that formed on Thursday were fraternal twins. They were separated by more than a thousand miles, but they all came into existence because of the same ridge producing the heat wave.

The first destructive line of storms moved southwest through Georgia and Alabama on Thursday afternoon. (Another distinct line formed to its southeast in Georgia, but it only produced scattered wind damage reports.) The second derecho began life as a single supercell over Nebraska on Thursday morning that eventually grew into a squall line that raced south into Mississippi before fizzling out after midnight on Friday. A third derecho over the northern Plains produced a wind gust as high as 96 MPH in North Dakota and flipped a small airplane at an airport in Minnesota.

A mesoscale convective system, or MCS for short, is an organized line of thunderstorms that can produce damaging winds across hundreds of miles. The strongest MCSs can last for 12 hours or longer, traversing half the length of the United States before finally petering out.

Most MCSs are easy to spot on radar imagery because they're thick squall lines with a well-defined shield of rain trailing them. The storm comes on suddenly. The hurricane-like winds and rain can last up to 15 minutes before finally subsiding and giving way to a couple of hours of light to moderate rain. The winds can be so violent that people who go through an intense MCS swear they were hit by a hurricane or tornado instead.

What makes an MCS different from other types of thunderstorms?

Thunderstorms "breathe" in the sense that they inhale warm, unstable air in an updraft and exhale cool, stable air with the rain in a downdraft. The vast majority of thunderstorms end up choking on their own downdrafts as the rush of cool air, known as a cold pool, cuts off the updraft and robs the storm of the instability it needs to survive.

When upper-level winds are favorable, though, thunderstorms can merge and share one large cold pool. The storms attach themselves to the leading edge of the cold pool as it races downwind, allowing the storms to thrive as they move into more unstable air. The line of storms doesn't die until they either run out of unstable air or the cold pool "escapes" ahead of the storms, robbing them of the lift they need and causing them to choke on cool, stable air.

The intense winds that can accompany an MCS comes from the "rear inflow jet." Friction and pressure differences within an MCS cause the air to begin circulating within the complex of storms. This circulation leads to the development of a sharp jet of winds that moves from the back of the storms to the front. This rear inflow jet gets shoved to the ground by the downdraft at the leading edge of the thunderstorms, causing the sudden, intense burst of winds that gives a severe MCS its bite.

You can see a mesoscale convective system any time of the year, but they're pretty common around June and July as the intense summer heat starts building over the United States.

I mentioned the possibility when I had a conversation with the little voice in my head on Tuesday:
Q: What kind of severe weather is favored during heat waves?
A: Mesoscale convective systems are a nasty habit of heat waves, especially along the outer edge of the ridge where the dynamics for such thunderstorm events are best. An MCS is commonly known as a squall line or sometimes even a d******.

Q: What's a d******?
A: I'm sorry. That word is censored on this good, moral blog.
Even without ever looking at the Storm Prediction Center's forecast, it's a decent bet to say that severe squall lines are possible when there's a heat wave like this. All of the storm complexes we saw on Thursday formed along the outer periphery of the ridge of high pressure that's allowing it to get so darn hot this week.

Heat waves are a breeding ground for this type of severe weather because of the dynamics involved in their creation. There's usually a sharp temperature and moisture gradient along the edge of a heat-producing ridge. This gradient, a stationary front, serves as the focus along which thunderstorms can develop and organize. Strong instability allows thunderstorms to grow and thrive, and upper-level winds along and north of the ridge of high pressure allows the storms to merge and organize into a single line.

If you overlay Thursday afternoon's radar over surface temperatures at the same time, you can actually see how the thunderstorms are riding the ridge around the heat.

It's not too common to see three separate lines of storms on the same day meet the definition of a derecho, the "d-word" that meteorologists hesitate to use in public for fear of whipping their audience into a panic.

A derecho produces lots of wind damage over a path that's at least 250 miles long. 200 miles? Not a derecho. 250? Ring the alarm. The, uh, specificity of that definition is about as arbitrary as anything else in meteorology; a squall line that only lasts for 50 miles could level a town's power grid and rip the roof off the high school, but it wouldn't technically be a derecho. The term came into the national hype vocabulary after an intense derecho trashed the Washington D.C. and Baltimore metro areas on this date in 2012.

Derechos are serious business. A study conducted in 2005 found that some derechos resulted more costly damage than some of the worst hurricanes to hit the United States. While some meteorologists find endless joy in debating whether or not a squall line really meets the definition, or whether we should use the term at all due to its hypetastic reputation these days, all three of the squall lines that formed yesterday seem to fit the definition for a derecho based on the sheer number and spatial coverage of damage reports each one generated. (If you want to be a stickler and include the requirement of several 75+ MPH wind gusts, as some academic studies do, the one in North Dakota definitely passes muster.)

The Storm Prediction Center is calling for a slight risk of severe weather around the edge of the heat wave over the next couple of days. While the dynamics should be less favorable than Thursday, any threat for severe weather is worthy of attention.

[Maps: Dennis Mersereau]

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