The remains of Nicole bring heavy rain and tornado danger to the eastern United States

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Nicole hit east Florida early Thursday as the country’s first November hurricane in 37 years, and while it’s now a long way from warm ocean waters, it’s not over yet. Tropical cyclone remnants will deliver a streak of heavy rains from the southeastern United States to Canada while also contributing to a rare late-season tornado threat for parts of the mid-Atlantic.

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All tropical storm, hurricane and storm surge warnings were dropped, the system resolved into a tropical depression – a leftover whirlpool of low pressure. Now concern is shifting to the threat of tornadoes in the mid-Atlantic. A tornado watch is in effect for much of eastern Virginia and southern Maryland until 6:00 p.m. Eastern time. The clock includes counties south of Washington, DC A tornado watch that was in effect for eastern North Carolina until 3 p.m. may have expired.

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

Nicole made official landfall near Vero Beach, Florida around 3 a.m. Thursday as a Category 1 hurricane with sustained winds of 75 miles per hour. As the storm swept ashore, it triggered peak gusts of 84 and 80 mph near Daytona Beach and in Melbourne. An elevated weather station at Cape Canaveral, 120 feet above the ground, clocked a gust at 100 miles per hour.

As of Thursday morning, up to 350,000 customers in the Sunshine State had lost power, but PowerOutage.us reports that service had been restored for all but 40,000 customers by Friday morning.

A storm surge, or a 3 to 4 foot rise in seawater over normally dry land, caused minor to moderate flooding along Florida’s Atlantic-facing coast, but erosion from large waves proved a major problem. At least a dozen structures in Daytona Beach were made uninhabitable as angry seas eroded the cliffs on which they perched.

The storm discharged about 3 to 6 inches of rain across eastern and northern Florida.

As of 10 a.m. Eastern Friday morning, Nicole was a tropical depression with maximum sustained winds of 30 miles per hour. Centered 35 miles north of Atlanta, it was flying north-northeast at 23 miles per hour.

Nicole’s air pressure rose as the low pressure center “filled” with air. It’s akin to how a swirling vortex in your morning cup of coffee eventually slows down and flattens out in the liquid.

Because of this, there isn’t as much gradient or barometric pressure change with distance to support strong winds. Because of this, all winds associated with Nicole are below tropical storm strength. It’s like sledding; You accelerate faster when the incline or incline is greater and the hill steeper. As Nicole’s gradient weakens, the winds ease.

That means it’s still a blob of moisture, acting northeast, and an unseasonably warm, moist airmass is moving north ahead of it. Dew points in the mid to high 60’s will rise north to the Mason-Dixon line, setting the stage for some dangerous thunderstorms Friday afternoon.

Dry air enters Nicole’s circulation from the west, the same direction a cold front approached. This influx of dry air is both a curse and a blessing: on the one hand, it undermines Nicole’s circulation from within and accelerates the demise of his core. On the other hand, this dry air helps stir up the warm, humid air in front of Nicole, creating strong to severe thunderstorms.

These thunderstorms will build up into a highly “sheared” atmosphere. In other words, Nicole causes a change in wind speed and/or directional altitude. This will encourage downpours and thunderstorms to turn and maybe even spawn 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, NC are included. A Level 1 of 5 marginal risk includes Charleston and Columbia, SC

DC, Baltimore and Philadelphia show a little more uncertainty. They are also in the Level 1 risk zone. That’s because they face a classic HSLC or High Shear Low Cape setup that’s notoriously difficult for meteorologists to forecast. On the one hand, the wind dynamics support strong rotating thunderstorms and a risk of tornadoes. Conversely, instability or fuel for thunderstorms will be more limited. Exactly how these ingredients combine and in what ratio remains to be seen.

Nicole drives through the DC area on Friday, with rain and possible tornadoes

Occasionally the storms will continue throughout Friday afternoon and evening. There is a likelihood that additional tornado watches will be needed to account for this potential, particularly in Virginia during the evening. More targeted alerts are issued at a local level when forecasters suspect a tornado is imminent or occurring.

This tornado risk occurs in the “warm sector” of the storm. Temperatures won’t be as warm in the West, but the upcoming cold front will help concentrate Nicole’s moisture and force it out of the air – analogous to wringing a washcloth.

It appears that the bulk of the heaviest rainfall will occur west of the Acela Corridor and Interstate 95, leaving places like DC, Philadelphia, New York and Boston walking a fine line. Significantly higher amounts of rain will fall in the west, with a wide 2 to 3 inches over the Appalachian Mountains. To the east, only a quarter to a half inch will fall near shore.

The greatest amounts of rain will accompany upslope flow in western North Carolina, or where air is being pushed up mountains. That will drop as much as 6 inches on the eastern slopes of southern Blue Ridge.

“Isolated lightning, urban and small river flooding will be possible today in the southern and central Appalachian Mountains, particularly in the Blue Ridge Mountains,” wrote the National Hurricane Center. “Heavy rain and isolated flooding will stretch north through eastern Ohio, west-central Pennsylvania, western New York and northern New England from tonight through Saturday.”

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