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

Around an inch of rain is probable, and a few twisters are not out of the question, especially south and east of Washington

Rainfall forecast from the National Weather Service. (weatherbell.com)

Unseasonably warm and humid air is on its way back to the Mid-Atlantic ahead of the remnants of Tropical Storm Nicole, which will barrel through the region Friday. While Thursday is calm, Friday will feature rain chances picking up by morning, with intermittent waves of showers throughout the day.

The main impacts in the D.C. area will include on-and-off periods of heavy rain that could lead to isolated flash flooding, and gusty winds from the south and southeast.

Given a lot of atmospheric spin associated with the remnants of Nicole, a tornado threat also may develop. The odds of twisters are somewhat higher south and southeast of Washington, toward Southern Maryland, Richmond and the Virginia Tidewater.

Tropical Storm Nicole lashing Florida, set to drench eastern U.S.

The National Weather Service Storm Prediction Center has placed the D.C. area in a Level 1 out of 5 risk for tornadoes, while areas to the south are in a Level 2 out of 5 risk zone.

Additionally, the Weather Service has placed the region in a Level 1 of 4 risk for excessive rainfall. The odds of heavy rain will increase to the west and northwest of the D.C. area.

Storm details in brief

Timing: Shower chances increase during the predawn hours Friday, especially southwest of the area, and become likely by sunrise. Additional waves of rain pass during the day. Rain should end late Friday night.

Coverage: Expect on-and-off showers, coming through in waves, and perhaps some thunder. The showers will be fast-moving but may be quite heavy at times.

Hazards: The primary concerns are heavy rain, gusty winds and a risk for an isolated tornado or damaging wind gusts. The chance of flooding is fairly low because the area has been rather dry lately.

Rainfall projections: A widespread 1 to 1.5 inches is most probable. Toward the mountains, 2 to 3 inches could fall. Southern Maryland and the Delmarva Peninsula probably see closer to half an inch to an inch.

How Nicole will influence the region

On Friday morning, the center of a weakening Nicole will be over central Georgia, as shown below, with showers working northward through the Mid-Atlantic.

The storm features a very large wind circulation. High pressure retreating to the north will assist in tightening the pressure gradient across the Mid-Atlantic, thus keeping wind speeds elevated. Expect winds to gust frequently from 20 to 30-plus mph Friday and potentially higher in any thunderstorms.

As Nicole transitions from a tropical to more of a mid-latitude storm, a warm front will form (red scalloped lines above) that may be a focus for any tornadic activity. Meanwhile, a strong cold front and deep dip to the jet stream will be approaching the East Coast from the Ohio Valley.

The center of Nicole’s remnants will join up with that front, perhaps near the spine of the Appalachians, as a plume of deep tropical moisture moves northward to the storm’s east. The strong uplift on the west side of Nicole’s remnants will interact with tropical moisture to generate a swath of potentially very heavy rain over the Appalachians, with more showery weather to the east.

By Friday evening, Nicole’s remnants will rapidly exit to the northeast, and skies may actually start to clear by midnight.

Why strong winds and tornadoes are a risk

While the fuel for the types of storms that could generate tornadoes will be limited in the D.C. area, the wind shear (change in wind direction and or speed with altitude) will be significant. That combination of ingredients may set the stage for low-topped rotating thunderstorms. Those cells, in turn, may bring gusts of damaging (50-60 mph) wind to the surface in a couple of spots, as well as generate brief tornadoes.

Inland tornadoes spawned by tropical remnants tend to be short-lived and weak, but those characteristics also make them difficult to detect with radar, thus hindering the issuance of timely warnings.

At this point, the Storm Prediction Center feels the highest tornado threat will be just to the south of D.C. However, we caution that this zone could extend farther north if the air mass remains unstable into the late afternoon and early evening hours.

How much rain?

Rain totals will be highly track-dependent. As of now, around an inch total in Washington seems reasonable. A shift in the forecast track farther east would bring higher totals closer to the area. The region has been quite dry recently, so the threshold rain amounts to trigger flash flooding locally are high.

Overall, the predicted track of Nicole’s remnants has shifted west, somewhat lowering potential rainfall in the immediate area.

Here are the amounts projected by different models:

  • European (ECMWF): 0.50-1 inch+
  • American (GFS): 0.75-1.5 inches
  • American (NAM): 0.50-1 inch
  • Canadian (GEM): 0.75-1.5 inches
  • ICON: 0.75-1.5 inches

Additional track shifts are possible, which would affect the rainfall forecast. But we don’t expect big changes, now that we’re within a day of the event.

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