Saturday, May 27

Severe storms to sweep through South amid snow and flooding to the north

The active weather comes amid a clash of the seasons responsible for a temperature roller coaster. Much of the Mid-Atlantic and Southeast will see temperatures some 15 to 25 degrees above average before an abrupt return to winter by Friday afternoon.

The National Weather Service Storm Prediction Center, meanwhile, has hoisted a level 3 out of 5 “enhanced risk” for severe weather over Mississippi, Alabama and western Tennessee, noting that “damaging winds and a few tornadoes are possible.” Nashville and Memphis are included in this elevated risk zone.

On the system’s backside, heavy snow could accumulate to nearly a foot deep in some locales. Kansas City, Detroit and Ann Arbor, Mich., are all under winter storm warnings, with a plowable snowfall likely.

Severe thunderstorms likely over Deep South

A cold front, marking the leading edge of cool, dry continental air from the west-northwest, was draped from southeast Oklahoma to near Laredo, Tex. Ahead of it, a mild, moisture-rich air mass was overspreading parts of the Eastern Seaboard, Tennessee Valley, Mid-South and Gulf Coast.

In advance of the front, temperatures will climb into the lower 70s. That will provide some instability, or juice, for thunderstorms, but instability will not be robust. That will probably be compensated for by a fierce low-level jet stream, with a highway of strong winds just a few thousand feet above the ground that will draw northward vast amounts of moisture from the Gulf of Mexico.

Events like this tend to be “boom or bust,” with little middle ground in between an overachieving severe weather outbreak and a lesser episode with more isolated storms. Assuming temperatures warm sufficiently, tornadoes may be widely scattered with more widespread strong to damaging winds.

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Storms were already ongoing over Arkansas, eastern Oklahoma and eastern Texas on Thursday morning, and will shift east with time. A broken squall line of storms, with some rotating storms or supercells embedded, will intensify during the afternoon hours as daytime heating peaks.

The most likely areas to be affected include western and Middle Tennessee, northern and central Mississippi and western Alabama. Cities such as Jackson, Vicksburg, Starkville and Tupelo in Mississippi, Huntsville, Tuscaloosa and Florence in Alabama, and Nashville, Memphis and Jackson in Tennessee are in the highest-risk category.

A level 2 out of 5 “slight risk” for severe weather surrounds the bull’s eye, and blankets a broader area of the South and Mississippi Valley that includes almost all of Arkansas, northern Louisiana, Bowling Green and Lexington, Ky., and Birmingham.

A marginal risk of severe thunderstorms exists to the east over the Georgia, the southern Appalachians and the Carolina Piedmont. A few storms may rumble through Thursday night or very early Friday, but any severe weather should be isolated at most.

Strong winds from the Gulf of Mexico to Maine

Wind advisories stretch from the Gulf Coast all the way to Washington County in Downeast Maine, spanning a continuous 1,750 miles. Nearly a third of the U.S. population is expected to see gusts of 40 mph or more during the next 24 to 36 hours. The wind stems from two mechanisms at play.

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First is a strong pressure gradient, or a change of air pressure with horizontal distance. Air rushing from offshore high pressure to low pressure will be bent rightward by the Earth’s rotation, curving up the Eastern Seaboard.

At the same time, that breeze will be reinforced by the strong jet stream. Any showers or downpours that form, moreover, will effectively mix that momentum down to the surface in the form of strong wind gusts.

Even behind the frontal passage, which will occur during the early morning hours Friday along the Interstate 95 corridor, gusty winds will continue as chilly air blows in from the northwest. That “cold air advection” will maintain a blustery day before the breeze finally abates Friday evening.

On the back side of the system, moisture wrapping northwest around the low pressure center will fall into subfreezing air. That will deposit a strip of moderate to heavy snowfall, primarily during Thursday. Snow will plaster a zone from northwest Oklahoma and central Kansas through the Midwest and into the Great Lakes.

Kansas City, where thundersnow was reported Thursday morning, is under a winter storm warning. A general 4 to 8 inches of snow is likely by the time precipitation wraps up late Thursday.

The same is true in Springfield, Ill., South Bend, Ind., and Detroit, where warnings are also in effect. The band of heaviest snow may only be 100 miles wide, but there could be a few double-digit snow totals within that swath.

Meanwhile, heavy rain will occur where temperatures are above freezing near and just east of a stalled warm front draped northwest of the Ohio River. A widespread inch to two inches of rainfall will be common in southern Missouri and Illinois and most of Ohio, as well as in western New York State on the shores of Lakes Erie and Ontario.

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Recent cold weather over the Great Lakes and interior Northeast has allowed ice to become thick over many rivers. The sudden surge of warmth and influx of rainwater will spur a breakup of said ice, possibly triggering ice jams and river overflow.

Flood watches and warnings are up for a lengthy swath of the eastern U.S. from eastern Missouri to Vermont.

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