April 29, 2021

Strong Winds Will Blow Across The Mid-Atlantic And Northeast On Friday

Who doesn't love a good windstorm? (Most people, it turns out.) High winds are likely on Friday as a cold front pushes across the East Coast, prompting wind alerts from the southern Appalachians to northern New England. Some communities could see wind gusts of 50-60 MPH on Friday afternoon and evening, which could be enough to bring down trees and cause power outages.

A low-pressure system over New England will move east over the Atlantic Ocean overnight Thursday into Friday. A center of high pressure will build in over the Great Lakes behind the front, and the strong pressure gradient between the high over the lakes and the low over the ocean will lead to blustery conditions across the region.

Modeled winds at the 850 mb level of the atmosphere on Friday evening. SOURCE: Tropical Tidbits

Winds in the lower levels of the atmosphere (around 5,000 feet or so) will crank up as they blow over the Northeast and Mid-Atlantic on Friday afternoon. Daytime heating causes the atmosphere to overturn as warm air rises and cool air descends to take its place. This churning motion—called "mixing"—can nudge some of those fast winds down to the surface, leading to the potential for strong wind gusts that could down trees and power lines.

A high wind warning is in place for parts of the Mid-Atlantic covering Delaware, much of New Jersey, central Maryland, and southeastern Pennsylvania, including Baltimore, Wilmington, and Philadelphia. These areas could see wind gusts as high as 60 MPH when the winds peak late on Friday. Wind advisories are in effect for the potential for 40-50 MPH wind gusts from New England on down the spine of the Appalachians.

Flights to, from, or over the Mid-Atlantic and Northeast could experience severe turbulence at times thanks to the strong winds, a point worth keeping in the back of your mind if you're flying somewhere on Friday.

Dry air behind the front will lead to an increased fire risk for some areas. The Storm Prediction Center's fire weather outlook issued on Thursday afternoon paints an elevated risk for fire weather conditions in parts of eastern North Carolina and Virginia, but local NWS offices issued fire weather watches for a much wider area that extends west to include upstate South Carolina and the Piedmont in North Carolina and Virginia. (The SPC fire weather outlook will update again in the very early morning hours on Friday.)

Gusty winds (and the associated wildfire risk) will wane overnight Friday into Saturday as the low pulls away from the Canadian Maritimes and the pressure gradient weakens. 

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April 21, 2021

Dry, Windy Conditions Increase Wildfire Risk In Southeast And Southwest On Wed. & Thur.

Two corners of the country will face an increased risk of wildfires on Wednesday and Thursday as a result of dry, windy conditions spreading behind a cold front that brought unusual late-season snow to folks farther north. The Storm Prediction Center expects "critical" fire weather conditions to exist over the next couple of days in parts of Arizona, New Mexico, and the Carolinas. The fire risk will subside in the southeast with sunset on Wednesday, while the Southwest will see another critical fire weather day on Thursday.

A powerful cold front is steadily marching its way across the U.S. toward the Atlantic Ocean today. It'll finally reach the coast by tonight, leaving behind gusty winds and freezing temperatures in its wake. The cold front brought snow down to the southern Plains on Tuesday, bringing many communities their latest or second-latest snowfalls on record. 

Even areas that don't fall below freezing will see humidity plummet and winds intensify as the front moves through. Just about the entire country save for a few corners of Texas and Florida will experience comfortable dew points over the next couple of days. The combination of dry air, gusty winds, and dry vegetation will increase the chances of wildfires in parts of the Southwest and southeast during the day on Wednesday and Thursday.

Fire weather conditions exist when dry air, gusty winds, and warm temperatures increase the chances that even a small spark could ignite dry vegetation and lead to an out-of-control wildfire. Most wildfires are relatively small—a far cry from the catastrophes we've seen out west recently—but even a fire in an area as small as a field or patch of woods is a hazard to nearby buildings, motorists, folks with respiratory issues, and the safety of crews that have to fight the flames.

Most of South Carolina and a portion of North Carolina will experience critical fire weather conditions during the day on Wednesday, which means that any burns or sparks could ignite a fire that spreads out of control in the windy conditions. While the region has seen tons of rain in recent...well, years...it's been relatively dry for the past couple of weeks. It's been dry enough that dense vegetation could burn quickly and spread to nearby areas.

The greatest concern for wildfires on Wednesday and Thursday is in the Southwest, where the region is mired in an increasingly serious drought. The Storm Prediction Center's fire weather outlook for Wednesday shows critical fire weather conditions likely across much of Arizona and western New Mexico, with an elevated fire risk radiating out from there to include parts of Utah and Colorado on Thursday. The fire risk will continue here through Thursday (shown above), moving a bit east. 

The three categories in the SPC's fire weather outlook work somewhat similar to the way their severe weather outlooks work, but the conditions necessary to prompt issuance of one of the three risk levels—elevated, critical, and extremely critical—vary from one region to the next depending on their dryness and expected conditions. The SPC has a decent explainer on their website (in PDF format) explaining the criteria necessary for each category.

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April 15, 2021

How An Alabama Beach Town Saw Softball-Size Hail

Residents of Orange Beach, Alabama, woke up to the horrendous sound of hail a few hours after midnight on Saturday, April 10. A supercell thunderstorm moved over the city and pelted the beachside community with hailstones as large as 4.00" in diameter, which is roughly the size of a softball. Hail that big usually stays on the Plains rather than the Gulf Coast. It takes a strong storm to create that kind of hail where that kind of hail doesn't belong. Here's how it happened. 

The Setup

A severe weather event unfolded across the southern United States beginning on April 9 and continued through April 10. The Storm Prediction Center issued a moderate risk for severe weather in anticipation of a strong squall line developing in Arkansas and Louisiana that would eventually move east toward Mississippi and Alabama. 

The storms played out more or less as forecasters expected, with plenty of reports of strong wind, large hail, and occasional tornadoes scattered across the areas outlined in the agency's outlook. During a severe weather event like this, you'll often hear during severe weather coverage that the storms could come in multiple rounds. Severe weather outbreaks in the southern United States often happen in two rounds, with one batch of discrete thunderstorms developing in the unstable air ahead of the squall line, followed by the squall line itself.

If there's enough instability and wind shear present, those discrete thunderstorms can develop rotating updrafts, becoming supercell thunderstorms. This one-two punch is the situation that led to enormous hail falling on Orange Beach, Alabama, during the April 9-10 severe weather event.

The Hailstorm

A supercell that developed over the northern Gulf of Mexico came into good view of the radar in Mobile, Alabama, around 2:30 AM CDT. The supercell was rather mean looking when it was over the open water, with strong rotation showing up on radar's velocity (wind) data and a pronounced hook echo on reflectivity (precipitation) imagery. 

Hailstones can grow inside of a thunderstorm as long as they remain within reach of the updraft and the updraft can sustain their weight. A stronger updraft, like one you'd find in a supercell, can support larger hailstones. Radar revealed that this supercell developed a huge hailcore as it reached peak strength over the northern Gulf of Mexico.

You can see the hailcore in 3D radar imagery just before the storm crossed the coastline. This radar image is from around 2:56 AM:

In this image, the word "WEST" is directly over Mobile Bay, and the thunderstorm is just about to cross the coastline into Baldwin County. I've dimmed all the colors on the radar's color scale except for the highest returns, highlighting the thunderstorm's huge hailcore. 

The supercell rapidly weakened and lost its structure as it moved from the water over land. This sudden weakening caused all of that enormous hail in to fall to the ground in unison, leading to a tremendous hailstorm in communities near the coast. An animated look at that 3D radar imagery shows the hailcore rapidly falling out as the storm moves ashore:

The National Weather Service in Mobile issued a severe thunderstorm warning as the supercell neared the coast in southern Baldwin County. The initial warning at 2:50 AM called for quarter size (1.00") hail. An updated warning issued at 3:06 AM called for hail up to 2.00" in diameter, which is roughly the size of an egg you'd buy at the store. Forecasters upped the potential to "tennis ball size" hail, or 2.50" in diameter, by 3:34 AM as the storm pushed inland.

Ultimately, folks in and around Orange Beach wound up witnessing hailstones the size of baseballs and softballs, which caused a tremendous amount of damage to vehicles, windows, roofs, and just about anything breakable that was left exposed to the elements that night. 

Given the intensity of the supercell required to support such giant hailstones, it's really rare to get this kind of hail so close to the water. In fact, these were the largest hailstones ever recorded in southwestern Alabama—not an easy feat given the region's track record with nasty severe weather outbreaks.

Hail History

Hail that big is rare, period. 

The National Weather Service recorded more than 363,000 reports of hail of all sizes between 1955 and 2019, ranging from tiny beads of ice the size of a green pea to grapefruit-size hail that left craters in the ground when it impacted the surface. More than 85,000 of those hailstones measured the size of a golf ball (1.75" in diameter) or larger, accounting for about one-quarter of all the hail reported in the United States.

It's tough for hail to grow larger than that. Only 0.5% of the hail reported in the country during that 64-year period—or a little under 1,800 reports—came in for hail the size of a softball or larger. The map above shows all recorded instances of ≥ 4.00" hail across the United States. The concentration of major hail on Plains sticks out like a sore thumb. The number of reports falls off with distance from the central states.

There are a few instances of huge hail in places you normally wouldn't expect it, such as an exceptionally damaging hailstorm in Miami, Florida, on March 29, 1963, and a powerful supercell in north-central Oregon on July 9, 1995. But while there are some big-time hail reports near the Gulf Coast in Texas and Louisiana, reports of very large hail were non-existent along the Gulf Coast between New Orleans and Key West until a few days ago.

[Radar Images via GR2Analyst]

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