April 19, 2018

The Anatomy of the April 15, 2018 Tornado in Greensboro, N.C.

One of the strongest and longest-track tornadoes to hit North Carolina's Piedmont Triad in years touched down on the evening of Sunday, April 15, 2018. The EF-2 tornado developed just east of downtown Greensboro shortly after 5:00 PM on Sunday and tracked north along a 33-mile path before dissipating south of Danville, Virginia. The tornado damaged more than 1,000 buildings in Guilford County, North Carolina, alone, and resulted in one indirect fatality and at least 14 injuries. The thunderstorm responsible for the tornado left behind a path of wind and tornado damage from central South Carolina through central Virginia.

The Setup

April 15 was the third day of severe weather in an outbreak that began on the Plains on the evening of Friday, April 13. The storms that formed over the two previous days resulted in at least one fatality in Louisiana and injured a dozen more people as strong winds and tornadoes caused a swath of damage focused on Louisiana, Arkansas, and Mississippi.

The system responsible for the previous severe weather continued into the Ohio Valley on the morning of Sunday, April 15. Strong southerly winds near the surface raised temperatures and dew points into the 60s and 70s across the southeast during the day on Sunday, helping to provide enough instability to fuel severe thunderstorms. Winds from the southwest in the middle- and upper-levels of the atmosphere provided the wind shear necessary to organize the thunderstorms into squall lines and allowed some storms to rotate and produce tornadoes.

The Storm Prediction Center issued a risk for severe thunderstorms across the southeast and Mid-Atlantic during the day on Sunday. The threat stretched from the foothills of eastern Ohio south through the Florida Peninsula. An enhanced risk for severe weather existed across portions of the Carolinas and south-central Virginia due to the elevated risk for tornadoes.

Forecasters noted that there was a 10% risk for tornadoes across an area stretching roughly from Savannah, Georgia, north through Martinsville, Virginia, in response to strong wind shear that could allow thunderstorms in or ahead of the squall lines to begin rotating. This was only the seventh time in the past ten years that the immediate Greensboro area found itself under a 10% or greater tornado risk in a SPC forecast.

The Thunderstorm

Most of the storms that formed on April 15 were part of squall lines moving west to east across the southeast. You can trace the individual thunderstorm that produced the Greensboro tornado from its formation around 10:30 AM in southern Georgia all the way until it produced another destructive tornado near Lynchburg, Virginia, almost eight hours later. 

This thunderstorm was the most persistent and destructive out of all the individual cells that formed in the southeast on April 15. The storm began as part of a squall line in southern Georgia around 10:30 AM, quickly strengthening as it moved toward Columbia, South Carolina. The airport in Columbia reported a 74 MPH wind gust as the storm moved through. 

The National Weather Service in central South Carolina confirmed four tornadoes west of Columbia as the storm moved through: one EF-2, two EF-1s, and one EF-0. These tornadoes all had a path length of 3 miles or less, which is common for storms that form as part of a squall line. The tornadoes occurred between 2:00 PM and 3:00 PM.

It appears that the storm gradually acquired characteristics of a supercell as it moved into North Carolina. It developed a sustained mesocyclone (rotating updraft) as it moved east of Charlotte and toward Asheboro. The storm in question eventually separated from its main squall line as it moved along Interstate 74 between Asheboro and Greensboro. 

The storm separating from the squall line appears to have allowed it to fully engage with the favorable environment it encountered—sufficient instability and strong wind shear—to produce the tornado near Greensboro. The rotation weakened as the storm once again merged into the squall line from which it came, and the storm regained its rotation when the storm yet again broke away from its parent squall line as it approached Lynchburg.

The Guilford-Rockingham Tornado

The tornado touched down just north of Interstate 40 on the eastern side of Greensboro, North Carolina, at 5:07 PM. The tornado damaged multiple homes and Peeler Elementary School as it strengthened and moved north.

Hampton Elementary School took a direct hit from the tornado as it reached its peak intensity with 135 MPH winds at 5:10 PM, producing damage that was right on the border between EF-2 and EF-3 intensity on the Enhanced Fujita Scale. Meteorologists from the National Weather Service in Raleigh estimated the tornado's peak intensity by the complete destruction of mobile classrooms at the elementary school.

Television station WFMY had to delay their live coverage of the tornado as their crew and staff sought shelter in a hallway while the tornado tore up neighborhoods not far from the station.

A tree fell on a car near the intersection of E. Cone Blvd. and Cesar St. in Greensboro. The driver passed away from his injuries. This was the only fatality caused by the storm, but it was far enough away from the tornado's track that it appears strong thunderstorm winds took down the tree rather than the tornado itself.

The tornado continued damaging hundreds of homes as it moved north away from the school and out of Greensboro city limits into unincorporated Guilford County. The path of damage shows the tornado slowly changed course toward the northeast until the storm eventually paralleled U.S. Highway 29.

Guilford County Emergency Services estimates that more than 1,000 structures were damaged by the storm in Guilford County alone; 199 homes and businesses suffered major damage (162) or were completely destroyed (37).

Raleigh's storm survey estimates that the tornado was an EF-0 with 80 MPH winds when it crossed into Rockingham County at 5:24 PM.

A significant amount of debris fell on southern Reidsville (where I live) as the tornado weakened and passed a few miles to the southeast of the city around 5:30 PM.

I witnessed several pieces of plywood and insulation fall from the sky before the winds picked up. I took a walk around my apartment complex after the storm and found a significant amount of insulation, large shards of plywood from buildings, many shingles, several large pieces of metal, a computer cable, a window screen, a six-foot strip of vinyl siding, and some other structural debris that fell during the storm. Several large pieces of sheet metal, presumably used as roofing material, also managed to make it up to Reidsville.

It's common to hear of light objects like papers or photographs traveling many dozens of miles after a strong tornado, but the amount of debris that fell on Reidsville surprised me. Most of the debris was able to make it the 15+ miles from Greensboro to Reidsville because it was light enough to easily float in the wind or its surface area was great enough that the debris caught the wind like a sail. Some of the pieces of metal were easily 10 or 20 pounds—so heavy that I couldn't throw them 10 feet let alone the 15 miles they were carried by the tornado—which is a testament to the power of both the tornado and its parent thunderstorm.

It appears that the reason so much debris from Greensboro fell on Reidsville is that the storm weakened as it approached Rockingham County and the tornado itself changed direction even as the winds in the lower- to mid-levels of the atmosphere stayed the same. Doppler radar data shows debris lofted nearly 20,000 feet into the atmosphere as it moved through eastern Greensboro. The radar data shows the debris moving with the thunderstorm over the next 20 minutes as it slowly descends before the bulk of the debris falls out over southern Reidsville.

A couple of minutes later, the tornado restrengthened and reached EF-2 intensity with winds of 125 MPH as it destroyed a mobile home and damaged multiple homes along Grooms Road in eastern Reidsville. The mobile home tumbled several hundred feet across the road, striking a moving vehicle and critically injuring the driver and his son.

The tornado continued through Ruffin, North Carolina, before finally dissipating at 5:46 PM.

Two more tornadoes would occur along the path of this storm for a total of seven during the thunderstorm's lifespan. The storm produced another tornado in the City of Danville shortly after the storm crossed into Virginia, leaving behind EF-1 damage along its 12-mile path. A little more than an hour later, the same storm produced an EF-3 tornado near Lynchburg, Virginia, destroying many homes and injuring at least 12 people along a 20-mile track.

The initial tornado warning for this storm was not issued by NWS Raleigh until 5:09 PM, two minutes after the tornado touched down. The warning was amended at 5:16 PM to indicate that a tornado was confirmed due to the presence of debris on radar. NWS Blacksburg issued a tornado warning for Rockingham County at 5:18 PM, six minutes before the tornado crossed the county line and 15 minutes before it injured people in Reidsville.

You can read the storm surveys conducted in Guilford County and Rockingham County, as well as the text of the tornado warnings for both Guilford and Rockingham Counties.

Triad Tornadoes

Significant tornadoes aren't common in the Piedmont Triad, which encompasses Greensboro, Winston-Salem, Burlington, and surrounding communities. Tornadoes that do form in the Triad are typically weak and short-lived. Before last week, only five tornadoes in the past 20 years to form in this area were rated F2/EF-2 or higher on the Fujita Scale.

Tornadoes are most common in the eastern part of North Carolina where the ingredients for outbreaks are more often present due to the flatter terrain and more favorable environment closer to the ocean. The presence of stable air and influence of the mountains typically reduces (but by no means eliminates) the tornado threat in central and western North Carolina.

Not only was the tornado uncommonly strong for its location, but the Guilford-Rockingham tornado remained on the ground for 33.6 miles—a rare feat for tornadoes in North Carolina. The National Weather Service recorded 1,255 tornadoes in North Carolina between 1950 and 2016. The average path length for tornadoes in North Carolina is just 3.55 miles. Only 18 of those 1,255 tornadoes (1.4%) had a path length of 30 miles or longer, and the vast majority of those long-tracked tornadoes occurred in the eastern part of the state. Sunday's tornado was the 19th such storm.

[Maps and Images by Dennis Mersereau | Radar: Gibson Ridge]

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April 14, 2018

A Race Between the Seasons: Battleground Pennsylvania

There was just a little bit of a temperature gradient across the Northeast and Great Lakes on Saturday. At 5:00 PM EDT, the temperature was 42°F in Boston, 54°F in New York City, and 81°F in Trenton, New Jersey. A strong low-pressure system over the Midwest on Saturday afternoon allowed warm air to surge northward along the East Coast while drawing cold air south from Canada.

The dividing line between "unseasonably cold" and "warm enough to use the A/C" was just a few dozen miles across at the afternoon's peak heating. The three features driving the sharp temperature gradient were an upper-level ridge over the East Coast, a potent low-pressure system located over Illinois, and an area of high pressure over eastern Canada.

The combination of southerly winds and subsidence caused by the ridge aloft brought May-like temperatures to the East Coast on Saturday afternoon. Temperatures in the Washington D.C. area got into the mid-80s today. BWI Airport hit 86°F, Dulles reached 83°F, and National Airport in Arlington made it up to 84°F. The 80s made it as far north as central Pennsylvania and New Jersey.

It's a completely different picture on the other side of those cold and warm fronts. Heavy snow, freezing rain, and sleet fell from from eastern Nebraska through interior sections of the Northeast. Forecasters expected the heaviest snow to fall in the Upper Midwest and northern Great Lakes, where some communities could see up to two feet of snow by the end of the storm. Significant ice accretion is possible in western New York—an ice storm warning includes the cities of Buffalo and Rochester, where up to half an inch of ice could form between this evening and Sunday afternoon, potentially downing trees and power lines.

The bulge of warm air won't extend as far into the Northeast on Sunday afternoon as the low-pressure system weakens, allowing the high in Canada to shove cold air deeper into areas bathed in warmth today. Sunday's forecast high in Trenton, New Jersey, is only 44°F, which is a 37°F drop from what the city saw today. Cold air coming in from the northeast (rather than the northwest) is often called a "backdoor cold front."

The warmth will break for the rest of us on the East Coast for a couple of days when a line of strong-to-severe thunderstorms sweeps through on Sunday afternoon and evening. The Storm Prediction Center has received hundreds of reports of severe weather over the past two days as thunderstorms have made their painfully-slow trek toward the east. A severe squall line over the northern Gulf Coast has barely moved through the day, leading to life-threatening flash flooding in eastern Louisiana and southern parts of Mississippi and Alabama.

A new squall line will develop with daytime heating on Sunday and bring the potential for damaging winds and tornadoes in the Southeast. Damaging winds will pose the greatest threat across areas at risk on Sunday, but tornadoes are possible, especially in any discrete storms that form before the main line comes through.

[Maps: Dennis Mersereau]

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April 12, 2018

'Extremely Critical' Fire Danger Across Southern Plains and Southwest on Thursday

The risk for wildfires across the Plains and the Southwest will increase significantly on Thursday as dry, windy conditions take hold across the region. The Storm Prediction Center—which also issues fire weather outlooks—expects "extremely critical" fire weather conditions to develop over parts of the Plains and Southwest, potentially leading to wildfires that could explosively intensify and rapidly spread. The National Weather Service office in Midland, Texas, characterized the fire threat in their forecast area on Thursday and Friday as "potentially historic."

Fire weather is a function of temperatures, relative humidity, and wind. This is actually one of the few times where the use of relative humidity is preferred over the dew point as a measure of the amount of moisture in the air. High temperatures, low relative humidity, dry vegetation, and gusty winds are all conducive to the rapid growth and spread of wildfires. (You can read more about the SPC's fire weather criteria in this PDF document.)

Wind gusts higher than 30 MPH, widespread high temperatures in the 80s and 90s, and very low relative humidity values will contribute to the potential for even small burns to quickly grow out of control. NWS Midland tweeted on Wednesday night that this was a "potentially historic" fire weather event, invoking the "particularly dangerous situation" wording that's usually reserved for especially severe thunderstorms or other major disasters.

A red flag warning is issued when critical fire weather conditions—those listed above—are occurring or imminent. A fire weather watch is issued when critical fire weather conditions are possible over the next day or so. An enormous chunk of real estate is under one of these watches or warnings for Thursday and even into Friday. The red flag warning includes the entire states of Arizona and New Mexico, significant portions of Colorado, Texas, Oklahoma, and Kansas, as well as small parts of Nebraska and Wyoming. A fire weather watch extends farther east including Oklahoma City, Wichita Falls, and Abilene.

The area under threat for wildfires over the next couple of days has slipped into a deep drought since the end of last year. Nearly half of Oklahoma is in some level of drought; the most recent U.S. Drought Monitor found 15 percent of the state mired in an "exceptional" drought, which is the worst on a five-point incremental scale starting with "abnormally dry." Many of the areas in the worst drought conditions have seen less than 25 percent of their average rainfall over the past 180 days. It's not a coincidence that these drought areas are also under the highest risk for wildfires this week.

Meteorologists will be able to track any fires that develop with incredible clarity and speed thanks to NOAA's new weather satellite GOES-16. The satellite's shortwave infrared product can give meteorologists a look at fires burning with imagery updated every 30 to 60 seconds. This will help emergency officials rapidly get the word out to folks in danger and give first responders a better idea of where fires are located and spreading.

The Storm Prediction Center expects critical fire weather conditions across the desert Southwest and southern Plains to persist on Friday and Saturday. The agency's forecast on Wednesday afternoon noted that extremely critical conditions "appear possible" again on Friday.

[Maps: Dennis Mersereau]

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April 11, 2018

How Do Forecasters Know There's a Tornado Without Seeing It? (Science!)

One of the big problems we run into when we talk about the weather is that people underestimate the risk they face when severe storms approach. The "it'll never happen to me" mindset doesn't only apply to heart attacks and car accidents. The odds that any one of us will find ourselves in the path of a tornado is relatively small. But it still could happen.

It's instinctive to want to run outside and look around when a tornado warning is issued. You want to see the tornado to internalize the threat and seek safety—unfortunately, by the time you can see the tornado, it's usually too late. Meteorologists have some fantastic tools available to them to detect a tornado many minutes before it hits a location, and an important part of severe weather safety is training ourselves to overcome our instincts and trust the warning that an unseen danger is on its way.

Tornadoes were such a sensitive topic in the days before weather radar that meteorologists weren't allowed to publicly mention the word "tornado" for fear of setting off mass panic. After all, tornadoes came by surprise without the technology to detect or predict the storms that produce them. The first successful tornado forecast was issued by meteorologists Ernest Fawbush and Robert Miller at Tinker Air Force Base in Oklahoma City back on March 20, 1948. Fawbush and Miller alerted the commanders at the military installation of the threat three hours before a severe thunderstorm swept across the base and dropped a damaging tornado. Their accurate warning 70 years ago opened the door the advances in tornado forecasting from which we benefit today.

Today, tornado watches and warnings are a ubiquitous part of weather forecasting.

  • A tornado watch is a short-term forecast that means conditions are favorable for thunderstorms that could produce tornadoes over the next couple of hours.
  • A tornado warning is issued when meteorologists detect a thunderstorm capable of producing a tornado, or they have evidence that there's a tornado on the ground. 

It’s easy to confuse a watch and a warning. A watch means you should watch for bad storms, and a warning means you need to act immediately. Tornado warnings are based off of spotter confirmation of a tornado or Doppler weather radar showing strong indications that a tornado is possible or occurring.

Most folks see tornado warnings so often—and personally see a tornado so infrequently—that it can become easy to ignore these warnings as just another false alarm. That's a dangerous gamble to make. Meteorologists don't issue tornado warnings for no reason. There's usually good evidence that a tornado is possible or occurring, and it's all thanks to weather radar.

Weather radar came into use during World War II when soldiers tasked with tracking enemy aircraft found that their radar was picking up rain in the distance instead. This technology revolutionized our ability to track the weather, keeping tabs on everything from snow showers to tornadic supercells without requiring eyes on the ground to scour the skies. Weather radars kept tabs over most of the United States by the 1960s and these devices have undergone upgrades every couple of decades to keep up with scientific advances of the time.

The current generation of radars began service in the early 1990s. Weather radar up until that point could only see the location and intensity of precipitation. The new technology utilized the Doppler effect to detect the speed and direction of precipitation—by measuring the velocity of raindrops, you can accurately calculate the wind speed within a thunderstorm. This became an enormous help in figuring out which thunderstorms were severe and which storms included rotation that could produce a tornado.

The Johns Island, S.C., Tornado

Let's look at a tornado that struck near Charleston, South Carolina, in the middle of the night on September 25, 2015. The EF-2 tornado damaged dozens of homes in Johns Island, but advanced warning helped all residents safely avoid injury despite the tornado touching down after midnight.

Thunderstorms were in the forecast for the Charleston area that night, but there was no tornado watch in effect and the Storm Prediction Center made no mention of a potential for severe weather the night the tornado touched down. The larger environment wasn't conducive for widespread severe weather. Just the right mix of ingredients came together to allow a thunderstorm coming ashore to turn into a supercell and drop a significant tornado. That tornado would have come as a complete surprise if it weren't for Doppler weather radar.

The above image shows the reflectivity image (precipitation) we're so used to seeing on the news or in a weather app. It is possible to see where a tornado might be located in this image if you're experienced in working with radar imagery, but it's not obvious and could easily be overlooked if you're not expecting tornadoes to develop in that kind of an environment. This is where Doppler technology comes into the picture.

If you look at the winds within the thunderstorm west of Charleston, it paints a much different and much uglier picture of what's going on that night.

The velocity products generated by radar images are usually displayed as shades of red and green. Green colors depict winds moving toward the radar while red colors indicate winds moving away from the radar. Brighter colors indicate stronger winds. When you have bright green and bright red colors very close together, forming what's known as a couplet, it indicates strong rotation within a thunderstorm. A look at the velocity image just west of Charleston shows a strong couplet approaching Johns Island. Meteorologists issued a tornado warning when they saw this couplet appear on radar, as the data indicated strong rotation that could produce a tornado.

The 2010s saw a new radar technology called dual polarization (or "dual-pol") added to our existing network of Doppler weather radars. Radar could already detect the location, intensity, and velocity of precipitation, but adding a second radar beam (hence the "dual" in dual-pol) essentially adds a third dimension to radar data. Now you can see both the size and the shape of objects picked up by the radar. This lets you differentiate between raindrops, hailstones, snowflakes, and other objects like birds, bugs, and tornado debris.

That last one—tornado debris—marked another significant leap in tornado forecasting. Tornadoes can loft so much debris into the air that weather radar picks it up as intense returns, creating what's known as a "debris ball" on radar imagery. Dual-pol technology allows meteorologists to see this debris more clearly, giving them the chance to warn people that there really is a tornado on the way. This confirmation helps people know that it's the real deal and hopefully gives them the chance to take the warning seriously before it's too late.

Debris shows up in dual-pol data with a product called "correlation coefficient" ("CC"), which tells you if the objects on radar are similar in size and shape. Uniformly-shaped objects like raindrops and snowflakes have a correlation coefficient between 90 and 100 percent, while dissimilar objects like a wintry mix of precipitation will have a lower percentage.

Tornado debris—including trees, building debris, and vehicles—will have as low of a correlation coefficient as you can register on radar. The tragic mix of debris flying through the air shows up as a conspicuous dark blue splotch on CC imagery, which, when co-located with rotation in a thunderstorm, can be used to confirm the presence of a tornado. All three of the above radar images were taken at the same time. You can see the beginning of debris showing up on CC imagery west of Charleston as the tornado starts lifting debris thousands of feet into the air.

Nine minutes after the initial tornado warning was issued, an EF-2 tornado with 130 MPH winds had torn through several neighborhoods and damaged nearly 80 homes. When you look at reflectivity (top-left), velocity (top-right), and correlation coefficient (bottom-left) together, it paints a dire picture of what could have otherwise been a disaster without advanced warning. You'd never see the tornado since it's after midnight and rain-wrapped. Tornadoes weren't in the forecast. There's no obvious sign of a tornado just by looking at the reflectivity image if you didn't know what to look for.

Doppler and dual-polarization technology gives meteorologists the tools they need to see a tornado long before you do. When a tornado warning is issued, it's issued for good reason. Don't wait to see a tornado if you're ever under a warning. Trust the warnings and use those precious minutes to get to safety.
[Images: Gibson Ridge. | Top Image: A supercell producing an EF-2 tornado in Mobile, AL, on Christmas Day 2012.]

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April 3, 2018

Widespread Severe Thunderstorms Expected in the Central U.S. on Tuesday

A widespread severe weather outbreak is likely on Tuesday in the central United States, covering just about everyone from the Midwest east through the Appalachian Mountains. The Storm Prediction Center expects the worst storms to occur around the Mississippi and Ohio River Valleys, roughly stretching from Memphis, Tennessee, to Dayton, Ohio, where the strongest wind gusts, largest hail, and most significant tornadoes are possible.

All of the ingredients are in place today for severe weather in the middle of the country. A moderate risk for severe weather—a four out of five on the scale used to measure the threat for severe weather—exists from the Mid-South through central Indiana and Ohio. Severe thunderstorms are also possible as far west as the Dallas metro area, as far north as southeastern Michigan, and all the way down to the northern Gulf Coast. Today has all the hallmarks of a classic early spring severe weather event.

A cold front extending from a low-pressure system approaching the Great Lakes will serve as the focal point for thunderstorm development this afternoon and tonight. Warm, soupy air flowing north from the Gulf of Mexico will give the storms the fuel they need to thrive once they get going. Veering winds between the lower-levels and upper-levels of the atmosphere will help thunderstorm updrafts start rotating and form into supercells that can produce the largest hail and most significant tornadoes.

Discrete storms that form today have the greatest chance to produce hail and tornadoes. The greatest threat for damaging winds will occur later in the evening when storms form into one or more squall lines as they move east along the cold front.

The Storm Prediction Center has outlined a large area at risk of significant tornadoes today. A significant tornado is a strong, long-lived tornado that could do significant damage along a long path. It's important to remember that it's impossible to know the strength of a tornado until after the fact, and that even a small or "weak" tornado is a life-threatening emergency. The greatest threat for tornadoes exists across the areas painted under a moderate risk, stretching from northeastern Arkansas through central Ohio.

The threat for severe weather will end from west to east through the nighttime hours. More isolated severe thunderstorms are possible along the eastern seaboard during the day on Wednesday.

[Severe Maps: Dennis Mersereau | Dewpoint Map: Tropical Tidbits]

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March 30, 2018

A Stagnant Weather Pattern Will Give Way to More Cool Weather for the Eastern U.S.

You didn't think winter would go down without a fight, did you? The stagnant weather pattern that's blanketed the United States this week is finally giving way and will be replaced by a pattern that will allow frequent shots of below-normal temperatures to dip across the eastern two-thirds of the United States over the next week.

The pattern we've seen in the upper levels of the atmosphere resembled an omega block, which occurs when a stationary ridge of high pressure over the middle of North America is bookended by low-pressure systems on the western and eastern sides of the continent. An omega block leads to warm, calm weather in the center of the United States and Canada with unsettled weather on either coast.

The blocking pattern we experienced last week kept moving—albeit very slowly. This sluggishness allowed persistent showers and thunderstorms to develop over the southern half of the country, where flash flooding and severe thunderstorms were a serious issue for several days, especially across Texas and Louisiana.

Some areas in Louisiana and Texas saw flash flooding from more than eight inches of rain from heavy rain thunderstorms training over the same spots for several days. Unfortunately, the most plentiful rainfall missed the areas hardest-hit by drought over the past few months. Areas that saw more than an inch of rain almost completely avoided parts of the country that are in some level of drought.

What can we expect over the next few days? Temperatures for most of us east of the Rockies will follow one of those classic, springtime up-down-up-down trends as storms and disturbances come and go. The Climate Prediction Center shows below-average temperatures for most of us east of the Rockies, while above-average temperatures and dry conditions continue for the western section of the country.

Snow should mostly be relegated to the Rockies and the northern states, where a fast-hitting winter storm could drop up to eight inches of snow in Minnesota and Wisconsin on Friday and Saturday. There's a quick chance of snow or a wintry mix from Sunday into Monday from the central Plains through the Mid-Atlantic, but it shouldn't amount to much if it snows at all. The sun angle is high enough this time of year that wintry precipitation won't stick around very long if it accumulates at all.

Our next chances at significant weather should emerge toward the middle and end of next week, when heavy rain and thunderstorms are possible along and east of the Mississippi River, and a potential heavy rain event could welcome the first weekend of April for parts of the West Coast.

[Model: Tropical Tidbits | Rain Map: Dennis Mersereau | Temp. Outlook: NOAA / CPC]

March 22, 2018

A Note to the New Owners of The Weather Channel

The Weather Channel is under new ownership. The Hollywood Reporter published news on Thursday that Entertainment Studios recently closed a nine-figure deal to buy the Atlanta-based weather behemoth away from Comcast, NBC, and the venture capital firms that have owned the network for the past ten years.

Knowing how media companies changing hands tends to lead to changes, I decided to greet the news by writing an open letter to the folks at Entertainment Studios with the hope that they fully understand the power of their new acquisition.


Congratulations on your recent purchase. You now own one of the most powerful platforms for safety information and science education in the United States. The Weather Channel has been a cornerstone of the weather community since 1982 and its reach has saved and changed countless lives across its decades on the air.

The spark that energizes The Weather Channel is unlike anything that goes into any other project on television. It's one of a rare group of networks whose sole mission at its inception was to serve its viewers. They're not there just to tell you about the weather. These dedicated meteorologists—some of whom have been in front of the camera longer than I've been alive—are there to explain what's going on and help their audience stay safe. True weather coverage is service journalism in a world that increasingly needs it.

I grew up on The Weather Channel. I was the geeky 8-year-old who set the VCR to record segments of Weather Center during the day so I could watch it after school. I looked up to the meteorologists who explained the weather to me every day. The late Dave Schwartz's quirky sense of humor and friendly presentation style had a lasting effect on my own sense of humor and how I approach writing and speaking to audiences. One of the highlights of my childhood was the day I got to stand in The Weather Channel's parking lot while visiting family in Atlanta. The picture I took that afternoon is still in its frame 18 years later.

My story is far from unique. It would be hard to come across a meteorologist, an aspiring student, or a weather enthusiast of any age in the United States who wasn't inspired by The Weather Channel. That says nothing of the countless lives saved by the network's devoted coverage of severe weather events. I know people who religiously hinge on every word spoken by Dr. Greg Forbes and Carl Parker and Jim Cantore because their voices were the voices folks heard while they took refuge from storms raging around them. The rapport The Weather Channel's experts have built with their viewers is immeasurable in terms of its value. Trust is a crucial component of how people respond to severe weather.

I've written quite a bit about The Weather Channel over the years. Not all of it was flattering. I strongly disagree with the network's winter storm naming practices. It's a betrayal of the network's mission that there's no actual weather on the air between the hours of 8:00 PM and 5:00 AM during the week and only a dozen hours of weather coverage during the weekends. I've lost count of how many times I've wanted to get information about bad weather only to find fat guys peeing in the woods or angry truckers cussing at each other instead of, y'know, the weather.

My criticism of the network comes from my love for what it's done and what it can still do. And there's no other network on television that would engage with critics as frequently and openly as The Weather Channel. The network's winter weather expert had an open exchange with me several years ago on a blog post I wrote criticizing winter storm names. They even allowed—heck, encouraged!—me to speak my mind about my disagreements with the channel when I appeared as Dr. Marshall Shepherd's guest on an October 2015 episode of WxGeeks.

The fear of change with new ownership is well-founded. The Weather Channel's programming took a hard turn toward entertainment when Comcast/NBC/Bain bought the network back in 2008. Ratings are money, after all, and only destructive weather is good for ratings. But things got so bad at one point that Jim Cantore, the undisputed face of the network, felt the need to speak out during a severe weather outbreak in April 2010:

Things have gotten better since then, but Comcast/NBC's ownership left a lasting mark on the network through hours and hours of reality programming each day. The allure of reality programs to keep the lights on and turn a profit is understandable—that's the whole point of running a business, after all—but The Weather Channel spent decades positioning itself as the leader in weather. The network has a unique responsibility as the leading source of weather information on television to continue providing timely information and analysis to its viewers. Any further expansion of entertainment programming will betray this trust.

The smartphone revolution changed the way most of us get our weather information. Most of us now have a personal emergency alert system in our pocket wherever we go. We also have more apps than we know what to do with, and many of those apps pull weather information from questionable sources. But the smartphone revolution isn't all for the best.

Apps alone cannot tell the whole story. Knowing the high and the low and a cute cloud emoji is okay at a glance, but most weather events take more than a passing look to understand what's going on. You can't get the full story on a major snowstorm on the East Coast or tornadoes in Alabama or potential flooding or fires in the West just by looking at two numbers and an emoji.

The weather requires nuance and an expert explanation of what's going on. We need details. Most television and web editors don't care much about the nuances of weather anymore. People are missing key points about weather forecasts and then they get upset when they think the forecast was wrong or they found themselves unprepared for what was to come.

That void is why I started this blog. That's why it's so disheartening to see so much reality programming on The Weather Channel. That's why it's scary to think of what can happen to this important network in the future. There are already too many information vacuums when it comes to the weather. We can't afford one more on television.

I'm looking forward to seeing what Entertainment Studios will do with this crucial platform going forward. I hope they stay true to the network's core mission of keeping its viewers informed and safe.

[Top Image: The Weather Channel's first moment on the air on May 2, 1982, via YouTube]

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March 21, 2018

Here's How Thunderstorms Are Able to Form With Temperatures Near Freezing

Loud cracks of thunder and vivid lightning are pretty far down on the list of things you expect to hear and see when the temperature is near freezing, but that's exactly what folks in North Carolina were greeted to on Tuesday night as a line of thunderstorms swept through the area. It was cold enough that some of the storms produced sleet. Thundersleet isn't a common phenomenon—especially in this part of the country—so this rare meteorological treat adds to the already-phenomenal winter North Carolina has seen this year.

Thundersnow and Thundersleet

We're used to hearing about thundersnow during the cold months. The video of Jim Cantore hollering and frolicking in a blizzard as lightning zaps around him was an instant classic. But you have to be in just the right place at the right time to see it.

Thundersnow can occur in nor'easters and lake effect snow. There's often so much lift created by the dynamics of a powerful nor'easter that this lift can have the same effect as air rising on a warm day. The strong lifting motion leads to convection which allows lightning and thunder to accompany heavy snow. Lightning is also possible during lake effect snow since the snow bands coming off the lakes form through similar processes that generate pop-up thunderstorms during the warm months.

The thundersleet we saw in parts of North Carolina and extreme southern Virginia formed through a different process.

Elevated Convection

A typical thunderstorm forms as a result of warm air rising from the surface. Air rises more quickly when there's a large temperature difference between the lower levels and the upper levels of the atmosphere. The speed at which air rises determines the strength of a thunderstorm.

A temperature inversion can stop air from rising from the surface. An inversion occurs when a warm layer of air forms on top of a cooler layer of air. The surface temperature north of Greensboro, North Carolina, on Tuesday night was 35°F when the line of thunderstorms swept through, but the air just a few thousand feet above ground level was nearly 50°F. It was too cold near the ground for surface-based convection to occur.

The atmosphere is much more complicated than the simple diagrams they teach us in school. Air doesn't always have to rise from the surface for a thunderstorm to develop. Air can actually start rising from thousands of feet above the surface if the inversion layer is warm enough.

The poorly-annotated jumble of lines above is called a SKEW-T chart, which allows us to trace the air temperature and dew point through a column of the atmosphere. Data collected by the instruments attached to weather balloons are plotted out on these SKEW-T charts, and the resulting graph is a tremendous help in all types of weather forecasting situations

The above SKEW-T chart was generated by the HRRR weather model for Greensboro, North Carolina, at 9:00 PM on Tuesday, right around the time the line of storms rolled through the area. The data shows that the air temperature at the surface was around 35°F at the time of the storms. The temperature dropped steadily for a few thousand feet above ground level before sharply rising in the inversion layer between 3,000 and 4,000 feet. The air finally started steadily cooling off again above 4,000 feet.

The inversion layer was so warm that it actually allowed air to start rising from the top of the layer all the way to the upper levels of the atmosphere, feeding the storms the instability they needed to produce heavy rain and lightning.

The SKEW-T chart above also shows a tiny little slice of the atmosphere below freezing a few hundred feet above the surface, which explains how some of the precipitation fell as sleet. Sleet forms when a snowflake falls through a layer of warm air but doesn't completely melt before reentering subfreezing air. The ice crystals left behind in the newly-formed raindrop serve as the nucleus allowing the droplet to freeze into the ice pellets we know as sleet.

Loud Thunder

If anyone wasn't aware that it was sleeting outside, they sure knew that it was thundering. The thunder was loud and it seemed like some of the cracks would never stop rumbling.

The same inversion that allowed the thunderstorms to develop also helped make the thunder so loud. The sound of the thunder was amplified through "atmospheric ducting." The sudden change in temperature acted as a sort of barrier that reflected some of the sound waves back down toward the surface and caused it to echo across long distances. The echoing effect caused by the inversion allowed one crack of thunder to reverberate for much longer than normal, leading to that ultra-satisfying rolling thunder sound.

[Radar: WSV3 | Sounding: PivotalWeather | Video: Dennis Mersereau]

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