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Nicole’s remnants to bring heavy rain, tornado threat to eastern U.S.

Heavy rain will fall from the Gulf Coast to Canada

Rainfall forecast from the National Weather Service through Saturday night. (WeatherBell)

Nicole struck eastern Florida early Thursday as the nation’s first November hurricane in 37 years, and though it’s now far removed from warm ocean waters, it’s not done yet. The remnants of the tropical cyclone will deliver a strip of heavy rainfall from the southeast United States to Canada, all the while contributing to a rare late-season tornado threat for parts of the Mid-Atlantic.

History-repeating hurricane paths in Florida amaze meteorologists

All tropical storm, hurricane and storm surge alerts have been dropped, the system disintegrating into a tropical depression — a leftover swirl of low pressure. Now the concern shifts to a risk of tornadoes in the Mid-Atlantic. A tornado watch is in effect for much of eastern Virginia and southern Maryland, until 6 p.m. Eastern time. The watch includes counties just south of Washington, D.C. A tornado watch that had been in effect for eastern North Carolina through 3 p.m. was allowed to expired.

Through 3 p.m., although numerous tornado warnings had been issued between North Carolina and central Virginia, including around Richmond, no twister had yet been confirmed.

Nicole officially made landfall near Vero Beach, Fla., around 3 a.m. Thursday as a Category 1 hurricane with sustained winds of 75 mph. As the storm blasted ashore, it unleashed peak gusts of 84 and 80 mph near Daytona Beach and in Melbourne. An elevated weather station at Cape Canaveral, 120 feet off the ground, clocked a gust to 100 mph.

On Thursday morning, up to 350,000 customers in the Sunshine State had lost power, but reports that service had been restored to all but 40,000 customers as of Friday morning.

A storm surge, or rise in ocean water above normally dry land, of 3 to 4 feet brought minor to moderate flooding along Florida’s Atlantic-facing shoreline, but erosion from large battering waves proved a bigger problem. At least a dozen structures in Daytona Beach were rendered uninhabitable as angry seas undercut the cliffs on which they were perched.

The storm unloaded about 3 to 6 inches of rain in eastern and northern Florida.

Nicole now

As of 10 a.m. Eastern time Friday morning, Nicole was a tropical depression with maximum sustained winds of 30 mph. Centered 35 miles north of Atlanta, it was zipping to the north-northeast at 23 mph.

Nicole’s air pressure was rising as the low pressure center “fills in” with air. It’s akin to how a stirred-up eddy in your morning cup of coffee eventually slows down and the dip in the fluid flattens.

Because of that, there isn’t as much of a gradient, or change of air pressure with distance, to support strong winds. That’s why all the winds associated with Nicole are below tropical storm force. It’s like sledding; you’ll accelerate faster if the gradient, or slope, is greater and the hill is steeper. Since Nicole’s gradient is weakening, its winds are diminishing.

That said, it’s still a blob of moisture that is working northeastward, and an unseasonably warm, humid air mass is moving north ahead of it. Dew points in the mid- to upper 60s will surge as far north as the Mason-Dixon Line, setting the stage for a few dangerous thunderstorms Friday afternoon.

Growing tornado threat

Dry air is entering Nicole’s circulation from the west, the same direction from which a cold front was approaching. That influx of dry air is a blessing and a curse: On one hand, it erodes Nicole’s circulation from the inside out and hastens the demise of its core. On the other hand, that dry air helps to kick up the warm, humid air in advance of Nicole, generating strong to severe thunderstorms.

Those thunderstorms will build into a highly “sheared” atmosphere. In other words, Nicole is inducing a change of wind speed and/or direction height. That will encourage downpours and thunderstorms to rotate and perhaps even produce a few tornadoes.

The Storm Prediction Center has highlighted a Level 2 out of 5 risk for severe weather to account for this potential. Charlotte, Raleigh, Richmond, Virginia Beach and Wilmington, N.C., are included. A Level 1 out of 5 marginal risk encompasses Charleston and Columbia, S.C.

D.C., Baltimore and Philadelphia present a bit more uncertainty. They’re in the Level 1 risk zone, too. That’s because they’re facing a classic HSLC, or High Shear Low Cape, setup — infamously tricky for meteorologists to forecast. On the one hand, wind dynamics strongly support rotating thunderstorms and a tornado threat. Conversely, instability, or fuel for thunderstorms, will be rather limited. How exactly these ingredients combine, and in what ratio, remains to be seen.

Nicole to race by D.C. region Friday, with rain and possibly tornadoes

Off and on, storms will continue throughout Friday afternoon and evening. There’s a likelihood that additional tornado watches will be needed to accommodate this potential, especially in Virginia, during the evening. More targeted warnings will be issued on a local level if it’s suspected by meteorologists that a tornado is imminent or occurring.

Heavy rain

That tornado risk occurs in the “warm sector” of the storm. To the west, temperatures won’t be as warm, but the impending cold front will help to focus Nicole’s moisture and squeeze it out of the air — analogous to wringing out a washcloth.

It appears the bulk of the heaviest rainfall will be west of the Acela Corridor and Interstate 95, leaving places like D.C., Philadelphia, New York and Boston walking a fine tightrope. Considerably higher amounts of rain will fall to the west, with a broad 2 to 3 inches over the Appalachians. To the east, only a quarter to a half inch will fall near the coast.

The greatest rain totals will accompany “upslope flow” in western North Carolina, or where air is forced up the mountains. That will drop up to 6 inches on the eastern slopes of the southern Blue Ridge.

“Isolated flash, urban, and small stream flooding will be possible today across the southern and central Appalachians, particularly in the Blue Ridge Mountains,” the National Hurricane Center wrote. “Heavy rain and isolated flooding impacts will extend north through eastern Ohio, west central Pennsylvania, into western New York and northern New England by tonight into Saturday.”