May 26, 2020

The Worst Is Yet To Come For California's Central Valley Heatwave


An intense heatwave gripping California's Central Valley will continue through the end of the week. Forecasters expect high temperatures to crack 100°F across the region through Thursday before things tick closer to normal this weekend. 

Monday began the sweltering four-day slog as temperatures climbed into the 100s through California's Central Valley. Sacramento, Merced, and Bakersfield all reported a reading of 100°F at 4:00 PM on Tuesday, with temperatures in the 90s creeping as far west as San Francisco Bay.

The heat will keep on going through the end of the week. Here are the week's forecast high temperatures from the National Weather Service as of Tuesday afternoon.

Wednesday, May 27th:


Thursday, May 28th:


Friday, May 29th:


Temperatures will return to a more reasonable state by this weekend as the upper-level ridge responsible for the heat begins to weaken and move off to the east.

This week's heat wave is the result of a strong upper-level ridge that's parked over the western United States. The ridge intensified when it got pinched between two upper-level lows, one over Texas and the other over the eastern Pacific Ocean. The intensity of the upper-level ridge is the primary driver of the unusually intense heat. High pressure aloft fosters sinking air, which heats up as it descends toward the surface. The intense subsidence leads to crystal clear skies and an intense sunshine that roasts communities stuck under the pattern.

Some daily and monthly record highs are in danger of falling during this heatwave. There's a chance that Fresno will break its monthly high temperature record of 107°F on Wednesday. Sacramento will likely break a few daily records before the heat abates, and the capital could come close to tying its monthly record high of 105°F.

The geography of the Central Valley and the relatively dry air that dominates the region allows this part of California to get pretty toasty during the warm months. The average first triple-digit reading occurs on June 11 in Sacramento, June 2 in Fresno, and May 30 in Bakersfield.


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May 16, 2020

Tropical Storm Watches Issued For North Carolina Coast As Depression Forms In Atlantic


Tropical Depression One formed off the Florida coast on Saturday afternoon, starting this year's hurricane season early for an unprecedented sixth year in a row. Forecasters expect the system to strengthen into Tropical Storm Arthur by early next week as it brushes the North Carolina coast. Heavy rain, rough seas, and gusty winds are likely as the system approaches land.

We've been watching this tropical disturbance for the past couple of days as it slowly moved out of the Caribbean into the southwestern Atlantic. A cluster of heavy showers and thunderstorms persisted long enough for the disturbance to root itself at the surface with a closed low-level circulation. It's easy to see the tight swirl of the circulation on radar out of Melbourne, Florida, this afternoon. 


Environmental conditions are favorable—and its access to the Gulf Stream sufficient enough—that the National Hurricane Center expects the depression to strengthen into Tropical Storm Arthur overnight Saturday into Sunday. It should follow a general north-northeasterly track over the next couple of days, with the center of the storm passing quite close to North Carolina's Outer Banks on Monday.

There are several large-scale features pushing and pulling on the system that will guide its ultimate path. A ridge of high pressure over the western Atlantic will keep it from simply jogging out to sea, while an approaching trough should eventually weaken the ridge and allow the system to curve away from land. The strength of each feature and the timing of the trough's approach are both critical in the exact path of the storm.

A tropical storm watch is in effect for many of North Carolina's coastal counties given the projected path of the storm. It wouldn't take much of a westward deviation from the forecast track to bring the core of the storm closer to the coast. We're probably not looking at too many impacts either way—minor flooding from heavy rain and scattered power outages from gusty winds would be the biggest threat to most folks on land. Rough surf and rip currents will pose a threat to anyone at southeastern or Mid-Atlantic beaches through early next week.

Remember that the cone of uncertainty is the average margin of error in the National Hurricane Center's forecast track during previous hurricane seasons. The center of the storm stays within that cone about 66% of the time. 

The National Hurricane Center issues advisories on storms every six hours—at 5:00 AM/PM and 11:00 AM/PM EDT—with updates every three hours in between when watches and warnings are in effect.

[Satellite Pic: NOAA]


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May 14, 2020

A Preseason Tropical Storm Could Form Off The Florida Coast This Weekend


This year's Atlantic hurricane season could start early for the sixth year in a row. A tropical disturbance moving through the Florida Straits has a high likelihood of developing into a tropical or subtropical storm off Florida's east coast on Friday or Saturday. While it's likely that the system will stay offshore, it's still too early to say what—if any—impacts we could see in the United States from the potential storm.

Bubbling Disturbance


The disturbance is slowly moving northeast across the Florida Straits tonight. Radar imagery out of Key West shows broad rotation within a disorganized batch of showers and thunderstorms. The system should move over The Bahamas by Friday morning, where the disturbance could produce several inches of rain through the weekend. 

Forecasters at the National Hurricane Center give the disturbance an 80 percent chance of development through the weekend, and it's likely we'll have a full-fledged tropical or subtropical storm come Saturday night.

The system—which would earn the name Arthur—will encounter favorable conditions for organization and strengthening once it clears the Florida Peninsula to the east. The disturbance will move through an environment with relatively light wind shear, as well as the moisture and warm waters (it'll be over the Gulf Stream!) necessary to sustain thunderstorm development.

Impacts

Source: WPC
It's likely the system will remain out to sea for the duration of its life cycle, but it's close enough to the coast that anyone from Florida to New England should watch it carefully and prepare for heavy rain or power outages if it changes direction.

Here are the impacts we're likely to see as things stand right now:

—Heavy rain will affect southeastern Florida and The Bahamas. Several inches of rain could lead to flooding in vulnerable areas if heavy rain occurs in a short period of time.

—Rough seas and rip currents are likely up and down the southeast coast as this storm strengthens. The stronger the storm, the greater the likelihood for dangerous waves and rip currents. 

We'll have a clearer understanding of its impacts once the system develops and forecasters and models get a better handle on its structure and surroundings.

Tropical vs. Subtropical

This system could exist as a subtropical depression or subtropical storm for at least a portion of its life. A subtropical cyclone is a low-pressure system that exhibits characteristics of both tropical and non-tropical lows. A subtropical cyclone isn't fully tropical, but it's just tropical enough—and brings the same impacts—that it deserves a name and the full treatment from the National Hurricane Center.

A tropical cyclone consists of a tight-knit cluster of thunderstorms wrapped around a center of low pressure. These thunderstorms act as the engine that allows the storm to maintain itself and strengthen. A tropical cyclone has an intense core of winds that gradually decreases in intensity with distance from the center of the storm. Temperatures throughout a tropical cyclone are uniformly warm throughout the system. 

A subtropical cyclone can see its main cluster of thunderstorms develop many dozens of miles from the center of the storm, giving it a severely lopsided appearance on satellite and radar imagery. Most subtropical storms derive at least some of their energy from upper-level winds rather than solely from those thunderstorms. Subtropical systems also have some cold air wrapped up in the core of the system. These systems can (and often do) transition to fully tropical cyclones if they're in a favorable environment.

The differences between tropical and subtropical are mostly academic. The messaging and impacts are largely the same no matter what you call the system, so subtropical systems should be treated the same as any other named system.

Six Preseason Seasons In A Row


We're poised to see the the sixth Atlantic hurricane season in a row that saw its first named storm develop before the "official" start of hurricane season. The chart above shows all the preseason storms we've seen since 2003. 


Atlantic hurricane season runs from June 1 to November 30. As we've seen this year, last year, the year before that, the year before that, the year before that, and one more year before that, these dates are somewhat arbitrary. Hurricane seasons are based on a mixture of climatology and the simple fact that the first day of June and last day of November are cute cutoff points for messaging purposes. 

The concept of a preseason storm is almost entirely an Atlantic Ocean phenomenon. Preseason storms happen here from time to time but they certainly seem more common than they used to be. It's still unclear whether this is a function of more favorable environments for storms in May, a better ability to detect wayward storms, or looser qualifications for naming/advising on storms.


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May 8, 2020

Trump's Hurricane Sharpie Map Was A Warning Flare For The Federal Coronavirus Response


We hit the point of no return the moment Donald Trump took his Sharpie to a days-old hurricane forecast in the Oval Office.

It seems like we've passed a thousand of these inflection points over the last three years.

He issued a controversial pardon while Hurricane Harvey made landfall in Texas because people were already watching the news and his ratings would be higher.

His administration slow-walked aid to Puerto Rico in the wake of Hurricane Maria. The president himself lied about how much money the territory got from the federal government, then responded to criticism by saying Puerto Rico didn't sufficiently show its appreciation to him.

He stood amid the charred ruins of Paradise, California, and said the fire might have been prevented if only the state had raked the forests more often.

So it seemed like just another eyeroll when the president held up a National Hurricane Center forecast map so freshly altered by the ink of his pen that the black semicircle protruding from the expert forecast shimmered a bit under the lights of the cameras.


Three days earlier, the president had erroneously warned that Alabama (among other states) would be hit "much harder than anticipated" by scale-topping Hurricane Dorian. No forecast on September 1, 2019, placed the hurricane's projected path near Alabama. It seemed like a normal mistake he'd make and meteorologists quickly corrected the president's misinformation. The National Weather Service office in Birmingham even tweeted that the storm would steer clear of Alabama.

As it turns out, NWS Birmingham was unaware of the president's tweet at the time of their correction. They only reacted to the onslaught of phone calls and messages initiated by the president's false claim. The corrections incensed the president, who spent the next couple of days digging-in and doubling-down on his assertion that Alabama was at risk no matter how much the experts said otherwise.

The president's mistake consumed the White House, which suddenly found itself more concerned with Trump's image than the category five hurricane churning perilously close to Florida's coast. The issue seemed to come to a head on September 4 when Trump manually extended the cone of uncertainty on a days-old hurricane forecast to make it look like he was right all along, proudly holding up the altered map for cameras to see.

He presented outdated and falsified information to the public as a scale-topping hurricane loomed near the country's coast. That wasn't even the worst of it.

We'd soon find out that there was an intense behind-the-scenes campaign to force NOAA and the NWS to back up the president's faulty claims, including threats to fire NOAA leadership if they didn't issue a statement denouncing their own experts for contradicting the president's tweet. NOAA complied and issued an unsigned statement.

That fiasco—that wholesale demolition of expertise for the benefit of the president's personal image—foreshadowed the federal response to the coronavirus pandemic.
Image Source: New York Times
Today, the death toll from the coronavirus in the United States stands at 75,000 people and climbing, and it's likely that number is an undercount. That's a staggering number. It's more than double the average number of people who die of the flu every year. It's higher than the deadliest year on record for car crashes in the United States (56,278 in 1972). The coronavirus will likely register as one of the top causes of death in the United States in 2020, and it's only May.

And at every step of this pandemic—from the first confirmed cases on American soil to governors prematurely reopening their states—things were made worse by valuing political whims over expert assessments.

The first heads-up about the impending pandemic reached the upper echelons of the Trump administration in January. Warnings grew more dire through February as the virus spiraled out of control in other countries and it was clear the United States would follow a similar trajectory. The United States encountered persistent testing delays that compounded the crisis and let the virus spread undetected for weeks. 

No warning struck as hard as one that came during a briefing on February 25, 2020, when Dr. Nancy Messonnier, a vaccine expert with the Centers for Disease Control, warned that it was a matter of when—not if—a bad outbreak of coronavirus would start spreading in the United States. Dr. Messonnier punctuated her remarks by describing how she warned her own family that the virus' spread would lead to "significant disruption of our lives." 

An already-plummeting stock market fell even harder on news reports of Dr. Messonnier's warnings, hurting Trump's prized metric for gauging his success as president. The Trump administration said the doctor spoke out of turn, and the president threatened to fire her if she didn't walk back her warnings.

The following day, Trump insisted during a press conference that the 15 confirmed cases at that point would "be down close to zero" pretty soon.

One day after that, Trump said the virus would go away on its own. "It’s going to disappear. One day—it’s like a miracle—it will disappear."

Trump called criticism of his administration's coronavirus response "the new hoax" during a rally on February 28, 2020.

The country saw its first confirmed coronavirus death on February 29, 2020, though it's likely the first actual death occurred weeks earlier.

Case counts quickly multiplied as testing ability came online throughout the country, finally giving us a glimpse of the extent of the community spread that had occurred before most communities took mitigating actions.

On March 9, as states began to roll out their stay-at-home orders, Trump (now, falsely) tweeted that the flu was deadlier than the coronavirus:


As the number of cases exploded through the middle of March, most states instituted stay-at-home orders of varying strictness, pausing the national economy to the tune of millions of jobs and trillions of dollars. This sudden economic crash cut at the heart of Trump's reelection pitch.

Trump's daily coronavirus press conferences, which began as an effort to stabilize the economy, slowly turned into venting sessions and virtual campaign rallies as the days wore on.

He openly bragged about the ratings for his briefings. He tweeted on five different occasions that his coronavirus briefings got bigger ratings than The Bachelor (March 29, March 29 (again), April 8, April 9, and April 21). He only started reining in the televised events after he openly wondered if doctors should inject patients with disinfectant to cleanse their systems of the virus.

Flouting his own government's suggestions that states stay the course until there's a significant decrease in serious cases, the president started pressuring states to reopen for the benefit of the economy. "Reopen" protests broke out in some states. Pictures and footage from the rallies showed an unmistakable trend: the vast majority of protesters wore Trump hats, Trump shirts, waved Trump flags, or drove vehicles adorned with stickers and signs advertising the president's reelection campaign.


The president appeared to take notice that his own supporters dominated these protests. Trump began April 17 by tweeting "LIBERATE MINNESOTA," "LIBERATE MICHIGAN," and "LIBERATE VIRGINIA," firing the starter gun for supporters to congregate in each state's capital and demand that their governor allow life to return to normal even as the virus continues to spread. Men carrying rifles wandered the halls of Michigan's capitol building as elected officials attended a legislative session wearing bulletproof vests. A day later, Trump tweeted that Michigan's governor should relent to their demands.

Now, as the death toll surpasses each bracket Trump predicted we'd stay under for him to have done a good job, reports indicate that Trump and his surrogates are preparing to publicly question the veracity of the death toll and claim that it's overinflated so states and hospital systems can grab federal dollars and make the president look bad at the same time.

He downplayed the virus because he thought it would hurt him. He threatened to fire an expert because he thought her warnings imperiled him. He egged on the protests because the attendees supported him. He's on his way to questioning the death toll because it looks bad on him.

He altered a hurricane forecast and upended NOAA during a disaster to save face over a simple mistake he made in a tweet one morning.

A category five hurricane became him.

It seemed easy to brush it off at first because hey, it's just the weather, right?

The bad weather served as a warning of what came next.

[Top ImageTwitter/@WhiteHouse]

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May 7, 2020

A Festive Mid-May Cold Snap Will Make Things Interesting For A Change


It's almost the middle of May. Crops are germinating. Flowers are in bloom. Leaves are fully leafed. So of course we're talking about the potential for accumulating snow! Welcome to a topsy-turvy weather pattern that's sure to bring a festive verve to the monotony of staying at home. A burst of heat will build across the western United States while a late-season cold snap in the east brings frosty temperatures to many and snow to some.
Source: Tropical Tidbits

A wavy jet stream will allow a sharply divided weather pattern to develop across the United States by the end of the week. A steep ridge will build west of the Rocky Mountains while a hearty trough digs south through the Great Lakes. The above-average temperatures out west and below-average temperatures back east will threaten to break daily temperature records on both ends of the spectrum.


The National Weather Service's high temperature forecast for Saturday is a stark example of what a pronounced ridge-trough pattern looks like. The west will register a solid "balmy" with 80s reaching Seattle and upper 90s as far north as California's Central Valley. Some cities out west will see record high temperatures over the next few days. Outside the heart of the deserts, it usually doesn't get this warm for a while yet.


Back east, Saturday would be considered a beautiful day if it wasn't May 9th. Temperatures will dip well below normal for this time of year; folks on the northern Gulf Coast could see their breath on Saturday morning. Such a late freeze will threaten crops and gardens if they're not protected.

We'll see lots of comfort-related complaining about the sudden resurgence of cooler weather this weekend, but it's not all bad. This kind of forecast is a dream come true for cold weather folks like me. It won't be long until we're in a months-long slog of hot and muggy weather with occasional severe storms. Everyone will complain about that soon enough. Enjoy the chill while we've got it.

Where there's cold air and precipitation, there's a chance for...


A low-pressure system will swoop across the Mid-Atlantic and Northeast on Friday evening into Saturday morning, bringing the potential for snow on the northern edge of the system. Models are a bit wishy-washy on who might see what—it's May, give them a break—but the National Weather Service does paint some accumulating snow across interior parts of the Northeast on Friday.

The map above doesn't cover the entire period when snow is possible. The map stops at Friday evening, while the bulk of the potential snow could occur overnight into Saturday. The data is also a little off because NWS Buffalo hadn't issued their snowfall forecast when I created the map, hence the conspicuous emptiness in western New York. But it gives you a good idea at where there's a chance for wintry weather to end the week.

There's an outside chance that snowfall totals will wind up edging a bit higher and nudge closer to big cities like Boston. Any snow that does fall won't stick around very long given borderline temperatures and a sun angle that's equivalent to what we see in August.


The Climate Prediction Center's latest outlook shows that it's likely we'll see below-normal temperatures continue across much of the eastern United States through next week, although not to such an extent as we'll see this weekend. Things should return to a more May-like state by next weekend.

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April 23, 2020

A Powerhouse Supercell Managed To Travel 300 Miles Across Three States


A real powerhouse of a supercell thunderstorm rolled across hundreds of miles of southern woodlands on Wednesday evening, ramping up in east-central Texas on Wednesday afternoon before finally petering out nearly eight hours later over the Mississippi River near Natchez, Mississippi. The supercell produced multiple tornadoes along its path, even prompting a tornado emergency as it approached Fort Polk, Louisiana.



Wednesday saw a classic springtime severe weather event across the southern Plains and lower Mississippi Valley. A line of tornadic supercells developed along a dry line in central Oklahoma, including a tornado that killed two people and grew to frightening strength in a matter of seconds.


The environment was just as ripe for severe weather farther to the south in Texas. A single supercell managed to survive for eight hours as it tracked from College Station, Texas, to Natchez, Mississippi. The video above shows a radar loop (video by me, radar by College of DuPage) of the storm along its track from College Station to Natchez. It's the southernmost storm that begins on the bottom-left and ends just right of center in the frame.
The thunderstorm near College Station that'd grow into the long-track supercell. (Gibson Ridge)

A thunderstorm began to develop broad rotation near College Station, Texas, around 3:00 PM CDT, slowly growing more organized over the next couple of hours as it moved east of the town. Things got going in a hurry as the supercell moved south of Lufkin, Texas, around 5:30 PM CDT, producing the first in a series of tornadoes that would touch down between there and central Louisiana through the evening.

A tornado east of Jasper, Texas. (Gibson Ridge)


The supercell neared peak strength once it got into far eastern Texas, cycling through stronger and weaker phases as it moved into Louisiana. The radar image above shows the storm around 7:30 PM CDT, as it approached the Texas/Louisiana border. It's broken down into four panels:

  • The top-left panel shows reflectivity, or precipitation.
  • The top-right panel shows velocity, or wind. Green shows wind blowing toward the radar (to the east), while red shows wind blowing away from the radar. When you have a strong, tight cluster of red and green right next to each other, it shows strong rotation within a storm.
  • The bottom-left panel shows correlation coefficient. The radar beam can tell us how similar in shape and size the objects are that it's intercepting. Uniform objects like raindrops have a high correlation coefficient (red/purple), while mismatched objects like tornado debris have a very low correlation coefficient (blue). When you see a dark blue dot in the same spot as tight rotation, you're looking at debris swirling around a tornado.
  • The bottom-right panel is normalized rotation, or NROT. This is an algorithm within Gibson Ridge's radar program that helps you quickly spot areas of rotation. Higher NROT values can indicate stronger rotation.
A tornado approaching Fort Polk, Louisiana, on April 22, 2020. (Gibson Ridge)

This storm appears to have produced several strong tornadoes across eastern Texas and central Louisiana. The National Weather Service in Lake Charles, Louisiana, had to issue a tornado emergency for Fort Polk, a military training site that houses thousands of troops and their families, as a confirmed tornado moved very close to the heart of the base.

The storm continued producing possible tornadoes as it passed southeast of Alexandria, Louisiana, before the updraft weakened and the storm finally started falling apart as it crossed the Mississippi River south of Natchez, around 11:00 PM CDT. The storm powered along almost uninterrupted for eight hours (!!!) before it finally fell apart.

A supercell is powered by a rotating updraft. Strong wind shear can cause horizontal rolling motions in the atmosphere. If a strong thunderstorm can develop in that environment, the updraft will push that rotation into the vertical and the updraft itself will begin to rotate. This rotating updraft then tilts downwind, allowing the storm to ingest unstable air and vent cooler air without choking itself off like a "normal" thunderstorm would.

This was a rather unusual testament to the endurance of a supercell when it's in a near-pristine environment. We usually don't see this kind of a marathon run outside of major outbreaks.

We're used to hearing about supercells after a tragic tornado or major hailstorm, but the vast majority of supercells are relatively—relatively—weak and don't last for very long. Supercells can be extremely fragile, falling apart if their outflow moves too quickly or if nearby storms contaminate the environment ahead of them with stable air. It's quite something to watch a supercell on radar practically disappear in just a few frames.  But there was nothing in this storm's way and it took full advantage of the situation.

Update: I changed the headline from "Tornadic Supercell Travels Farther In One Day Than You've Gone In The Last Two Months" to "A Powerhouse Supercell Supercell Managed To Travel 300 Miles Across Three States" because...well, do I really need to explain? 


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April 19, 2020

Another Dangerous Severe Weather Outbreak Is Possible In The Southeast On Sunday


Another severe weather outbreak is possible in the southeast on Sunday, affecting many of the same areas hit by severe weather just last weekend. A moderate risk for severe weather is in effect from Louisiana to Georgia, but just about everyone in the region is at risk for dangerous storms on Sunday and Sunday night. All modes of severe weather are possible, including the risk for strong, long-track tornadoes.

Source: Tropical Tidbits


A low-pressure system will move across the southeast during the day on Sunday. Thunderstorms will fire across the risk areas as the low pushes east through the day. Strong instability and wind shear will allow the storms to grow severe. The above model image for Sunday afternoon shows winds at the 850 mb level (about 5,000 feet or so), showing the low-level jet that will allow some storms to grow into supercells and pose a significant tornado threat.



This map looks similar to last weekend's severe weather threat The Storm Prediction Center paints a threat for severe thunderstorms across most of the southeast, with the greatest threat centered on Louisiana, Mississippi, Alabama, and Georgia. A moderate risk for severe weather covers many of the same areas that were under the greatest risk last week; this Sunday's threat covers a greater area, from northern Louisiana to central Georgia.

Storms will ramp up in a hurry during the afternoon and continue east through the nighttime hours. This is going to be another sleepless night for folks in Georgia and parts of the Carolinas.

Forecasters will refine these forecasts with updated information through the day on Sunday. Beware the sharp cutoff on the northern edge of the risk areas; it wouldn't take much of a shift to bump the threat for dangerous storms a few dozen miles to the north.

The most serious threat is tornadoes. The greatest threat for tornadoes exists in and around the moderate risk area, but any severe thunderstorms in the region have the potential to produce tornadoes. Remember that these percentages seem rather low, but we're talking the probability of tornadoes—the red shading means there's a 15% probability of a tornado within 25 miles of any point in the shaded area. The black hatching indicates the risk for strong, long-track tornadoes.



While tornadoes grab all the attention, there's a threat of large hail and damaging winds with any of the storms that form across the risk areas on Sunday. The strongest storms could produce hail the size of golf balls or larger.

There's also a significant wind threat on the eastern portion of the risk area in Alabama and Georgia. Storms will eventually coalesce into one or more squall lines capable of producing intense straight-line winds of 70+ MPH. This is a heavily wooded area, so communities are susceptible to widespread tree damage if a well-organized line can get going. It's a good idea to make a mental note of large trees or limbs hanging over your home and avoid those rooms when storms are on the way.

One caveat, like always, is that storm mode will determine who sees which hazards. Discrete thunderstorms will have the best opportunity to take advantage of the strong wind shear and live up to their full potential. That's what happened last week in Mississippi, where a big, messy cluster of storms to the north produced extensive wind damage while storms farther south had free rein to produce intense tornadoes.

As I pointed out last week, never count on the forecast flopping. Be prepared for the worst and hope for the best. Keep an eye on the radar and make sure you have multiple ways to receive severe weather warnings. Check that emergency alerts are activated in your phone's settings. Don't rely on tornado sirens for tornado warnings—sirens are outdoor warning systems that aren't designed to be heard indoors, and these aging systems prone to failure due to glitches, power outages, and high winds simply drowning them out.

Oh...one more thing. If you follow plenty of weather folks on social media, you might see chatter throughout the day on Sunday that the Storm Prediction Center might (or should, depending on their point of view) upgrade some areas to a high risk, or a full 5/5 on the scale used to measure the threat for severe weather. Don't worry about the difference between a moderate risk and a high risk. Some of the worst severe weather days have occurred during moderate risks. No matter how you phrase it, Sunday could be another significant severe weather day in the southeast and anyone in the region should be glued to the weather until the threat clears out.


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April 11, 2020

A Significant Severe Weather Outbreak Is Possible Across The Southeast On Easter Sunday



A multi-day severe weather event will peak on Sunday with the possibility of a significant severe weather outbreak across the heart of the southeastern states. A moderate risk for severe weather is in effect on Sunday for parts of Louisiana, Mississippi, and Alabama, with the threat for dangerous storms radiating out from there to include communities from eastern Texas to eastern Georgia. Conditions appear favorable for storms to support strong, long-track tornadoes, as well as widespread damaging winds and some instances of very large hail.

The Setup

Source: Tropical Tidbits


This weekend's severe weather setup has its roots in the same upper-level low that brought so much rain and snow to California over the last week. Parts of California have seen more than half a foot of rain at lower elevations and many feet of snow at higher elevations as a result of the stalled system.

California's storm got pinned over the West Coast by a large ridge of high pressure over the central United States. A trough dipping south out of Canada pushed the ridge out of the way, unblocking the upper-level low and allowing it to move east across the United States. The trough generated a low-pressure system at the surface over the southern Plains, setting the stage for the severe weather.

Sunday's Outlook



All indications point to a large-scale and potentially significant severe weather outbreak on Sunday. A moderate risk for severe thunderstorms—a 4 out of 5 on the SPC's ascending scale—exists from central Louisiana to central Alabama, with an enhanced risk (3/5) covering a huge area from the northern Gulf Coast to the Ohio River.

Storms will start across the western risk areas on Sunday morning, progressing eastward through the afternoon and evening hours. Severe thunderstorms will reach Georgia and the Carolinas overnight on Sunday into Monday.

Tornadoes



The banner headline on Sunday is the tornado threat.

The SPC's outlook on Saturday afternoon painted a 15% risk for significant tornadoes across the moderate risk areas, with a 10% risk radiating out from there to include New Orleans, Mobile, Atlanta, and Memphis. 10-15% doesn't seem like much, but it's a relatively large risk considering we're talking about tornadoes.

This is a common setup for the middle of April. We see this type of threat at least once a year...often more than that. Tornado safety is embedded in the culture around these parts.

Not every storm will produce a tornado. Not every tornado will be enormous, nor will every tornado incur a high rating on the Enhanced Fujita Scale. But the setup on Sunday looks similar to the kind of days that produce significant tornadoes. That should give you some sense of how serious the situation could be if the event lives up to its full potential.

Source: Tropical Tidbits


If storms are able to develop during the period when wind shear is just right, they'll have plenty of "spin" to work with throughout the lower levels of the atmosphere. The graphic above shows the winds at 850 mb (about 5,000-ish feet). The area of intense southerly winds over the southeastern states is known as a low-level jet; winds in the lower levels of the atmosphere are ripping along at 50+ knots, gradually veering toward the southwest and west as you get higher up toward the jet stream. This twisting of the winds will allow strong thunderstorms to develop into supercells capable of producing strong tornadoes, large hail, and damaging winds.

Hail



As you'd expect with a significant severe weather setup, hail is going to be a big deal. The strongest storms will be capable of producing hail the size of golf balls or larger, which is big enough to total a car and cause serious damage to homes.

This kind of hail will easily shatter a window, especially when it's blowing around in wind gusts of 60+ MPH. Even "just" a severe thunderstorm warning is reason enough to get away from windows and take cover. Shattering glass from windows and skylights is dangerous enough without all the other threats thrown in the mix.

Wind



The threat for damaging winds is always the most widespread risk on a severe weather day. Wind gusts of 60+ MPH are enough to bring down trees and power lines, leading to widespread power outages. Trees are vulnerable to snapping in high winds right now because they're in full leaf and this is the first real stress they've experienced since last summer. Stay mindful of limbs and large trees that loom over your home and avoid those rooms when a storm is on its way.

Flooding



Don't sleep on the risk for flash flooding from heavy rain. Several inches of rain are likely whether or not you experience any severe weather. The Weather Prediction Center's excessive rainfall outlook paints a moderate risk for flash flooding across a large portion of the southeast on Sunday. Natural waterways will quickly overflow their banks during a heavy thunderstorm, and street flooding is a high likelihood as well.

Monday



The low-pressure system will reach the Great Lakes by Monday and trigger one last round of severe thunderstorms over the eastern states before the cold front moves out over the ocean. The SPC's latest forecast shows a risk for severe storms from northern Florida to upstate New York, with the greatest risk existing along the coastal Carolinas.

Many of Monday's storms will occur early in the morning when folks are asleep, so make sure you have a way to receive severe weather warnings that'll wake you up.

Words of Caution

1) The caveats.

The best case scenario is that only a handful of severe thunderstorms form and meteorologists get hollered at for a week because people got upset and scared "for nothing." We see a few notable setups under-deliver every year. Last year was the infamous high risk over central Oklahoma; conditions looked primed for a horrendous tornado outbreak, but storms had trouble forming in the area where all the parameters for tornadic development were maxed-out.

Historic tornado outbreaks are rare and memorable for a reason. Big severe weather outbreaks have so many failure points that it's hard for a big one to live up to its full potential. Supercells can quickly merge into a squall line and rob each other of the dynamics they need to produce tornadoes. Showers and storms earlier in the day can muddle the instability and stunt storm development later on when the wind shear is most favorable for supercells to develop.

While it's hard for a day to achieve its potential, never bet on a bust. If there's a risk, assume it's going to happen and plan accordingly. It's great if it doesn't. There's still a lot to learn about meteorology. But it's a testament to how far the science has come that forecasters can raise the alarm days ahead of time when some of our immediate family members remember the days when you had to see the tornado to know one was possible.

2) Know the jargon.

You'll see a lot of insidery terms and model images flying around on social media tonight and tomorrow, often without explanation. Not all meteorologists gear their posts and language toward the general public; lots of meteorologists use Twitter to talk to other meteorologists. However, some pages, especially on Facebook, like to post this stuff because it gets your Uncle Filbert to share it with all his friends.

Source: Pivotal Weather


One of the most commonly shared images these days is "updraft helicity." This graphic appears to show tornado-like tracks moving across the at-risk areas. Updraft helicity measures the strength of the rotating updrafts in thunderstorms simulated by the model.

People who aren't familiar with these graphics tend to fixate on the exact track of the paths—not unlike spaghetti model charts during hurricane season—and go "whew, I'm in the clear" or "oh no, we're doomed." Those updraft helicity charts are best used to see where thunderstorms have the best opportunity to turn into supercells that could produce tornadoes. The stronger the signal, the more favorable the environment. It's never a good idea to focus on the specific tracks.

Source: Pivotal Weather


Another routine visitor during tornado season is the Significant Tornado Parameter (STP). Meteorologists have developed a ton of indices to boil down lots of different dynamics into one bite-sized number to quickly identify areas of hazardous conditions. The STP is one of the parameters that got traction because it's relatively easy to talk about with folks who aren't weather buffs.

The STP is an algorithm that combines different measures of instability and wind shear to highlight how favorable an environment is to produce significant (EF-2+) tornadoes. It's not perfect, but it's good for a quick evaluation of where things could get ugly if storms can fully engage with the environment around them.

You might also see meteorologists and enthusiasts talking about "CAMs," which became one of those must-use buzzwords a few years ago for people who need to prove their worth (or make themselves sound smart, but that's another issue) on social media. A CAM is a convection-allowing model, such as the NAM or the HRRR. These models allow the simulation of thunderstorms, which is helpful during severe weather events. Global models like the GFS and European can miss the small-scale features that help meteorologists fine-tune their forecasts. Meteorologists can use models like the NAM or the HRRR to help forecast the placement, motion, and type of severe thunderstorms up to a day in advance.

Safety Steps

1) Stay. Home.

The potential for a tornado outbreak during a pandemic is an unusual situation, and it's one that meteorologists and emergency managers have been dreading since the COVID-19 crisis started.

Most states are under some version of stay-at-home orders or guidance right now. Theoretically, people shouldn't be gathering at church on Easter Sunday or piling into homes for a big dinner on Sunday evening. If everyone is mindful of the safety guidelines, folks staying home instead of gathering in large crowds should limit the potential for mass casualties that would come from a tornado outbreak on a normal Easter Sunday.

That's how things are supposed to be.

In practice, though, we have widespread reports of churches intentionally rebelling against stay-at-home orders and people ignoring the orders to visit family and friends under the misguided illusion that they can't get sick or that they're not carrying the virus. That's a horrible idea given that we're approaching the apparent peak of COVID-19 infections across the country, but it's an exceptionally bad idea on a day when dangerous severe weather is possible.


Many folks live in homes that are susceptible to high winds and would be incur serious damage or complete destruction in even a low-rated tornado. I wrote about this conundrum a few weeks ago. What do folks do when they rely on community shelters that might not even be open? There aren't many good answers, but it's best to think about it now rather than when the tornado warning is issued.

2) Don't look for the tornadoes.

Seriously. The terrain in the southeast absolutely sucks for spotting tornadoes. In most areas, all you'll see are dark clouds scraping the tops of the trees until the tornado is right on top of you. I know lots of people still like to see the tornado coming before they'll dive into their basements/closet/bathroom/whatever. Not only does the terrain here obscure tornadoes until it's too late, but the atmosphere is so moist in the southeast that most tornadoes are rain-wrapped, making them impossible to spot until it's too late.

If there's a tornado warning, trust the threat is there and seek shelter like you saw it yourself.

3) Wear shoes, jeans, and a helmet.

If your home is damaged by a tornado, strong winds, or even a serious pelting of large hail, you'll have to walk through a myriad of sharp debris to get out and assess the damage. You'll need to wear shoes and jeans to walk across debris like broken glasses, jagged pieces of wood, electrical wires, pipes, and nails.

Also, it may look dorky, but wearing a bicycle or motorcycle helmet while you're sheltering from a tornado could save your life. It's unpleasant to talk about, but a leading cause of death during a tornado is a fatal blow to the head by flying debris. A helmet can't stop you from getting hurt if the worst happens, but it can keep you alive.

4) Watch the radar and listen for warnings.

This is the kind of setup that will likely produce long-lived storms. It's likely that folks in the path of the most serious storms will be able to watch them on the news for an hour or more before they approach their communities. Be proactive. Stay aware. And please make sure the emergency alerts in your smartphone's settings are still activated for tornado warnings.


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