Heavy Snow, Flooding And Severe Weather To Affect Eastern Half Of U.S. – The Washington Post

A developing storm system is set to unleash multiple modes of hazardous weather as it treks across the eastern half of the Lower 48. Heavy snow, perhaps totaling more than a foot, is possible across parts of the Upper Midwest and Great Lakes, while flooding and severe thunderstorms threaten areas in warmer air to the south.

Temperatures will spike ahead of the storm system, leaping 20 degrees or more above seasonal norms and flirting with 70 degrees as far north as D.C. Readings will plummet behind the system as it drags a strong cold front all the way to the Eastern Seaboard.

It comes after the tranquil weather that dominated last week, spelling a sudden end to the quiet stretch as a multifaceted arsenal of threats looms.

On Tuesday morning, the impetus for the system, a lobe of high-altitude cold air, low pressure and spin, was diving southeast in the skies over California. That will shift toward the Four Corners and transfer its energy by Wednesday to a zone of low pressure in the Texas Panhandle — low pressure that will then slide through the Ohio Valley.

Millions of Americans from Kansas City to Chicago to central Michigan will find themselves on the storm’s cold side, while places such as Dallas; Jackson, Miss.; Memphis; Birmingham, Ala.; and Atlanta will bask in the system’s mild “warm sector.” That divide will govern who will see what — heavy snow or severe thunderstorms — before a 30-degree drop in temperatures as Old Man Winter regains control late in the week.

A dryline, or sharp boundary between moist air to the east and encroaching arid air from the Desert Southwest, will take shape over Central Texas on Wednesday. A slightly muggy air mass ahead of it will provide ample fuel for thunderstorms as the approaching boundary kicks pockets of air upward.

The National Weather Service Storm Prediction Center has issued a Level 2 out of 5 “slight risk” for severe thunderstorms on Wednesday for much of central and eastern Oklahoma and adjacent north-central and northeastern Texas. That includes the Red River, the Interstate 35 corridor from Dallas to Oklahoma City, the 287 stretch from near Fort Worth to Vernon, and additional population centers such as Wichita Falls and Paris, Tex., and Tulsa and Lawton-Fort Sill, Okla.

A few initial supercells, or rotating thunderstorms, may form very late in the day Wednesday, bringing the threat of widely scattered damaging winds and a low-end tornado. But severe weather risk has trended down somewhat. That’s because the front appears a bit delayed in its passage, coming through later Wednesday when daytime heating is limited.

On Thursday, the potential for severe weather will shift east into Louisiana, Arkansas, Mississippi, western Tennessee and Alabama, where a broad “slight risk” has been drawn by the Storm Prediction Center. By then, the storms will have probably merged into a broken, messy squall line, with gusty to damaging winds and a risk for some embedded tornadoes. However, cloud cover streaming east in advance of the storms will inhibit sunshine and cut back on severe weather chances somewhat.

A few severe thunderstorms can’t be ruled out late Thursday in the Southeast, but the risk is low.

Flood watches are already up in eastern Illinois, most of Indiana, a few counties in southern Michigan and northwestern Ohio and east of Lake Erie in western New York State. Additional watches may be issued. Moisture will train, or continuously ride along, a stalled warm front as the system makes its cross-country journey beginning Wednesday night. A broad swath of 1 to 1.75 inches is likely, and a few spots may close in on 2.5 to 3 inches.

Indianapolis, Toledo and Buffalo could all see instances of flooding, with nearly 2 inches expected to fall.

“Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations,” wrote the Weather Service. “Frozen ground, ice jams, and additional snow pack melt will help lead to increased runoff leading to minor to moderate river flooding.”

Elsewhere, the East Coast will probably see a general 0.4 to 0.75 inches of rain, such as in D.C., Baltimore, Philadelphia and New York. Amounts should be similar in Boston.

Moisture wrapping around the low-pressure center will deposit a strip of moderate to heavy snowfall, with millions in line for plowable snows. The band of heaviest snow may be only 100 or 150 miles wide, meaning there will be a steep gradient, or change with distance, in totals on either side of the band.

A few flurries or pockets of freezing rain are possible in southeastern Iowa, southern Wisconsin, the Chicago area and parts of Michigan in the predawn hours Thursday, but the bulk of the snow will be materializing in the Oklahoma Panhandle and central Kansas. From there, the snow will increase in coverage and intensity, expanding northeast through the Kansas City area, northwestern Missouri well north of Interstate 44, southeastern Iowa, northern Illinois, southern Wisconsin and the northern two-thirds of Michigan. It will last through Thursday evening in northeast zones.

There could be icing between the heavy snow zone and where ordinary rainfall brings flooding.

At this point, the best we can do forecast-wise is identify a large zone set to receive a 4-to-8-inch snowfall. It includes northern Oklahoma, much of eastern and southeastern Kansas, the Kansas City area, southeastern Iowa, northern Illinois around Davenport, Peoria and Chicago, and the majority of Michigan, especially north and west of Detroit. Lansing, Flint and Grand Rapids, Mich., appear primed for heavy accumulation.

Within that zone, there will likely be totals in the 8-to-16-inch range. Weather model “ensembles,” or large groups of simulations, are highlighting a decent shot that someplace gets more than a foot, but confidence is low in precisely where.

Temperatures will crash as the snow departs Thursday night, with Chicago waking up near zero degrees and the Twin Cities in the negative double digits.