Friday, September 29

Sprawling storm to unleash heavy snow, high winds, flooding and severe weather

Double-digit snow totals are possible in the Rockies and across the Upper Midwest and Great Lakes, with moderate to heavy rain likely to trigger river flooding just to the south. More than 30 million people are under flood watches from Kansas to Vermont, while ice jams are probable in the interior Northeast because of heavy rainfall on frozen rivers amid mild temperatures.

In the southern and southeastern United States, unseasonable warmth 25 degrees or more above average will set the stage for strong to severe thunderstorms that present the risk for damaging winds and an isolated tornado or two.

The temperature contrasts will fuel strong winds; advisories and high-wind watches affect more than 75 million people in parts of the Southern Plains, Midwest, Ohio Valley and Northeast.

It all comes as an intense high-altitude disturbance that dropped into Southern California on Tuesday ejects from the Southwest. That disturbance will then intensify a nascent zone of low pressure over the Texas and Oklahoma panhandles that will ride northeast before exiting interior New England by Friday.

That disturbance brought thunderstorms in Southern California on Tuesday, unleashing enough hail to whiten the ground surrounding the Rose Bowl Stadium. It ended a record streak without precipitation in the Sierras and produced thundersnow just inland from San Diego.

As the low-pressure zone sweeps from Oklahoma to Vermont, southerly winds in advance of it will draw warm, moist air northward. Frigid air will sweep in from Canada in the storm’s wake. For those on the storm’s cold side, it’s time to gas up the snowblowers.

Two days of strong to severe thunderstorms are likely with the storm system, although the passage of the instigating cold front has trended slower, compared with previous forecasts. That means a lower potential for severe thunderstorms on Wednesday, since the front will move through eastern Texas and Oklahoma around dark, missing out on the daytime heating that would provide more energy for storms.

On Wednesday, the main triggering mechanism will be a cold front preceded by a dry line, or the leading edge of arid air pushing east from the Desert Southwest that will clash with pockets of mild, moisture-rich air streaming north from the Gulf of Mexico.

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Storms may wait until around or after Wednesday night or early Thursday to get going along the Red River in southwestern Oklahoma and north-central Texas. They will sweep east along Highway 287 to the Interstate 35 corridor, with cities such as Dallas, Fort Worth, Wichita Falls, Tex., and Tulsa and Lawton-Fort Sill, Okla., included in a Level 2 out of 5 “slight risk” for severe weather advertised by the National Weather Service Storm Prediction Center.

A strong low-level jet stream means winds will be howling just a few thousand feet above the ground; some of the momentum will probably be mixed down to the surface in thunderstorms in the form of strong wind gusts. An isolated tornado can’t be ruled out amid the change in wind speed and/or direction with height, but the odds are low. Any twisters would probably be quick-hitting and brief.

Similar uncertainty will exist Thursday as the storms enter a slightly more favorable atmospheric setup. Instability necessary for violent storms will be limited because of cloud cover obstructing incoming sunlight, but a strengthening low-level jet stream, drawing moisture from the Gulf of Mexico, will compensate for it.

A broad swath of the Deep South and Mississippi River Valley is included in an elevated risk zone for severe weather. Memphis, Nashville, Jackson, Miss., and Huntsville Ala. are included in a level 3 of 5 “enhanced” risk zone where there is potential for not only damaging winds but also “several — and possibly locally strong/damaging — tornadoes,” according to the Storm Prediction Center.

Storms will fade Thursday night, but a few rumbles of thunder are possible from the nation’s capital through the Southeast before the front clears the coast Friday.

Moisture will pool along a stalled warm front draped from near St. Louis to Toledo and Buffalo, and into southern Ontario on Thursday. Areas along and north of Interstate 70 could see hefty rain totals as precipitation trains, or moves repeatedly over, the same areas.

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A general 1 to 1.5 inches is likely from eastern Missouri through Indiana and Ohio and Lakes Erie and Ontario. Flood watches are up in the northern Ozarks, most of Missouri, western Illinois, lakeside counties of Ohio and Pennsylvania, and a broad stretch of western New York State.

The rain could bring river flooding to some ice-clogged waterways of the interior Northeast.

Winter to date has been unusually cold across much of the Northeast, with temperatures over the past 45 days averaging as much as 5 degrees below normal. The cold has been most severe in the vicinity of the Great Lakes, where an active jet stream has allowed continued pockets of Arctic air to overspread the region.

As ice coverage has increased atop area waterways, so, too, has concern for potential flooding when a thaw comes. When river ice breaks up and flows downstream, which can happen when temperatures warm, large chunks of ice sometimes become stuck on bridges and river bends in dam-like formations. The results can be problematic.

“River ice has been unusually thick across much of [the Interior Northeast], with the prolonged cold air … which is pretty unusual,” wrote Adam Gill, a meteorologist at the National Weather Service in Binghamton, N.Y., in an email.

Flood concerns that arise from ice jams are exacerbated if new water is also entering rivers and streams. In midwinter, this often happens through rain or snowmelt, both of which could be concerns as temperatures surge into the 60s with this storm system. However, in this case, Gill notes that a period of warmth last week has melted a large portion of the snowpack, and the expectation that much of the Northeast will receive less than an inch of rain limits the likelihood of significant runoff.

The storm will also deposit a strip of heavy snowfall, with double-digit snow totals possible in areas that see jackpot accumulations. Moisture wrapping northwest around the counterclockwise-spinning pinwheel of low pressure will be drawn into a cold air mass, transforming it into snow. Nearly 30 million people are under winter storm watches or warnings and winter weather advisories

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Much of the snow is focusing in the Rockies on Wednesday, and Denver is under a winter storm warning for 4 to 8 inches of snow through Thursday morning.

By Thursday, most of the snow will shift to the area from northern Oklahoma and central Kansas to Missouri and Illinois and across northern Indiana and Michigan. The band of greatest snowfall may be only 100 or 150 miles wide, which makes predicting the axis of most significant accumulations challenging. Subtle shifts of 10 or 20 miles will have major bearings on final amounts.

The heaviest may remain just southeast of Chicago. The Windy City is under a winter weather advisory, with 2 to 4 inches of snow expected.

Kansas City is under a winter storm watch, with 4 to 8 inches probable.

Similar totals will exist into central Illinois and northern Indiana, as well as in southern Michigan, where advisories, watches and warnings have yet to be hoisted. Somebody could end up with 10 to 14 inches of snow when all is said and done.

Wind advisories occupy a massive zone of real estate in the eastern United States, with gusty to locally damaging winds expected.

Gusts of 40 to 50 mph are possible in the Great Lakes vicinity, with 35-to-40-mph gusts in parts of the South on Thursday.

Across south coastal New England, gusts topping 55 mph could be in the offing Thursday evening into Friday morning as momentum from aloft is mixed down. That will especially be true along the immediate coastline and in Plymouth and Bristol counties in Massachusetts, as well as on Cape Cod and the islands, which are under a high wind watch. Here, the winds could cause scattered power outages.

Winds will slowly abate on Friday.

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