....'which includes two Level 5 out of 5 “high risk” areas for extremely dangerous storms.'
Posted by
prayformojo (aka mayhem)
Mar 31 '23, 12:43
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An outbreak of severe weather, including the potential for “strong to potentially violent tornadoes” from “long-track” storms, is expected across the central United States and the Mississippi Valley on Friday. An expansive area of concern has been plastered across outlook maps issued by the National Weather Service’s Storm Prediction Center, which includes two Level 5 out of 5 “high risk” areas for extremely dangerous storms.
These two most threatened zones cover southeast Iowa, including Iowa City, northeast Missouri and west central Illinois, as well as east central Arkansas, northern Mississippi and southwest Tennessee, together with Memphis. Such high-risk zones are rare and reserved for the most severe weather situations. It’s been more than two years since the last high-risk issuance.
Thunderstorms will develop explosively during the afternoon and evening hours in multiple zones, with the Corn Belt first. The Storm Prediction Center issued a rare “particularly dangerous situation” tornado watch for central and eastern Iowa, western Illinois, northern and central Missouri and southwest Wisconsin until 8 p.m. Central time. “Parameters are favorable for the potential for strong/violent tornadoes and very large hail,” the center wrote.
Another vigorous batch of rotating supercell thunderstorms, each of which could produce one or more significant tornadoes, will materialize between central Missouri and southern Arkansas. The Storm Prediction Center also issued a particularly dangerous situation tornado watch for this zone until 8 p.m. Central, noting “a few intense tornadoes are likely.”
A third tornado watch was issued for northeast Texas and northwest Louisiana until 9 p.m. Central.
The watch zones include population centers such as Des Moines; Springfield and Peoria, Ill.; Shreveport, La.; St. Louis; Little Rock; and Memphis. Evening commutes could be dangerous, air travel will be rocky, and widespread disruptions to social plans, transportation and commerce are inevitable.
Dangerous storms could blast across some of the same parts of Mississippi devastated by deadly tornadoes one week ago.
Thunderstorms will merge into an 800-mile-long line along the cold front Friday evening, and they should march east toward Chicago, Indianapolis, Louisville and Nashville. It’s unclear how much of a punch the thunderstorms will still be packing, but at least scattered instances of severe winds, as well as a few tornadoes, can be anticipated.
Tornado season is here. Here is what you need to know.
Very rarely is an area this large exposed to a severe weather hazard this widespread or great in magnitude. Residents should have multiple ways to receive severe weather warnings — whether through mobile devices, weather radios, or trusted television or internet sources — and review sheltering plans with loved ones. Particularly throughout this evening, nobody in the severe weather zone should be more than five minutes away from a safe sheltering location.
Areas affected
Severe storm risk map for Friday. (NWS)
Surrounding the Level 5 high risk zone, a Level 4 out of 5 risk of severe weather stretches from central Iowa all the way south to just north of Greenville, Miss. — where widespread dangerous storms are probable. Surrounding that zone is a Level 3 out of 5 “enhanced risk,” which creeps as Far East as Middle Tennessee and as far north as northeast Iowa and southern Wisconsin. Huntsville, Nashville, Louisville and Chicago are in the enhanced zone where severe storms will be numerous.
A Level 2 out of 5 risk — with more scattered severe storms — spans from Kansas City, Kan., and northeast Texas all the way to Milwaukee, Columbus and Chattanooga. The western periphery of this zone represents uncertainty with respect to where storms will initially fire; in case they crop up an hour or two ahead of schedule, they would be farther west. The eastern edge of this zone marks low confidence in how far the storms will march before meeting their eventual overnight demise.
Overall, more than 85 million Americans face an elevated threat of dangerous storms.
Expected severe thunderstorm impacts
Northern areas (Illinois, Iowa, northeast Missouri)
Initial thunderstorms across central Iowa will blossom between 12:30 and 2:30 p.m. Central time. They’ll erupt rapidly, exploding from puff-like cumulus clouds to full-fledged monstrous thunderstorms within 60 to 90 minutes. The first thunderstorms will be rotating thunderstorms or supercells, capable of producing a few tornadoes, very large hail and damaging winds.
The tornado threat in central Iowa may be lessened by a lack of shear, or changing wind directions with altitude, which helps storm spin. However, as storms move into eastern Iowa and especially central and northern Illinois, they’ll encounter winds more favorable for sustained rotation. That could make for some tornadoes, perhaps significant. At the same time, cells will be merging into a line, which would favor damaging straight-line winds over tornadoes. It’s unclear which threat will predominate during the mid-evening, but hazards will be present.
The straight-line wind risk is increasing, too, with a few gusts of 70 to 80 mph possible by Friday evening. Storms will efficiently mix jet stream winds from aloft down to the surface in the form of damaging gusts. By nightfall, storms in this region will be primarily straight-line wind producers and the tornado threat should wane.
A high-resolution model simulation of a line of thunderstorms progressing east ahead of a cold front spiraled east by an advancing low-pressure system. Note the snow on the system's back side. (Tomer Berg/PolarWX)
Southern areas (eastern Arkansas, northwest Mississippi, western Tennessee)
Signals point to a very concerning setup for severe evening weather. Thunderstorm will blossom during the midafternoon into an environment ripe for rapid supercell develop. Low-level spin will be abundant and will overlap with plentiful CAPE, or fuel, for thunderstorms. That stems from Gulf of Mexico warmth and moisture pumped northward ahead of an eastward-advancing cold front.
A high-resolution model simulation of a warm, humid air mass wafting north as frigid air crashes south in the storm's wake. It's along that interface of battling air masses that severe weather will form. (WeatherBell)
Initially, it was unclear whether supercells (rotating thunderstorms) would form or whether storms would organize into a line. However, confidence has increased in the likelihood of several supercells, which could persist for several hours or more as they advance east-northeast in the steamy spring air mass. The greater Memphis area could face several rounds of supercell thunderstorms, each of which could produce tornadoes. At least one or more of the tornadoes that will threaten the Mid-South and Lower Mississippi Valley could be in the strong to violent category. That would connote twisters of EF3+ strength on the 0-to-5 scale for intensity.
The Gulf of Mexico is very warm. That could mean a bad tornado season.
The tornado risk will remain moderate to high through an hour or two after sunset. Then a mix of damaging straight-line winds and quick-hitting erratic tornadoes will accompany the thunderstorm line as it pushes east.
A high-resolution model simulation of a number of rotating supercells expected near Memphis this evening. (WeatherBell)
Ohio and Tennessee valleys and Appalachians
Thunderstorms will be progressing eastward rapidly during the first half of the overnight. By then, they will have merged into a line spanning the cold front. Strong to damaging straight-line winds of 55 to 65 mph and a few tornadoes are expected.
It’s unclear how far east thunderstorms will survive. Ordinarily, the loss of daytime heating would spell storms’ demise. Once thunderstorms outrun their heat and moisture, they tend to weaken quickly. In circumstances like these, with incredibly strong winds aloft, however, storms may be able to make it all the way to the Appalachians with a continued brief high wind or very low-end tornado risk.
A weather model simulation of fierce jet stream winds aloft, which is spurring the severe weather setup. (WeatherBell)
Some additional severe thunderstorms are possible in the eastern United States on Saturday — particularly in Pennsylvania and New York. “Strong thunderstorms may develop and organize east of the lower Great Lakes into the Hudson Valley and northern Mid Atlantic region Saturday, accompanied by a risk for severe wind gusts,” the Storm Prediction Center writes.
Other storm hazards
The powerful storm systems spawning the severe thunderstorms will also generate a sprawling area potentially damaging non-thunderstorm winds. Wind advisories — for gusts up to 50 mph — are in effect for nearly 55 million people from the central states to the Ohio Valley into Friday. These advisories are expected to be extended to the East Coast for Saturday. High-winds watches are in effect for the central Appalachian Mountains for gusts up to 65 mph.
With high winds affecting such a sprawling area, along with the severe thunderstorm threat, many tens of thousands of power outages are possible in the eastern half of the nation.
On the north side of this storm system, heavy snow is expected from South Dakota to the Upper Peninsula of Michigan. Blizzard warnings cover much of central and eastern South Dakota and southwest Minnesota on Friday into early Saturday because of a dangerous combination of heavy snowfall and high winds, which could reduce visibility to near zero.
Minneapolis, which has already seen one of its Top 10 snowiest seasons on record, is expecting 5 to 8 inches of snow and wind gusts up to 45 mph. It’s under a winter storm warning until 7 a.m. Central time Saturday.
As a testament to the volatile nature of this storm, parts of southern Wisconsin could see severe thunderstorms Friday evening before cold air sweeps in, ushering in wind-whipped snow by Saturday morning.
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