Nicole will run in the DC area on Friday, with rain and possible tornadoes




Abnormally warm and humid air is on its way back to the mid-Atlantic ahead of the remnants of Tropical Storm Nicole, which will pass through the region on Friday. While Thursday is calm, Friday the chance of rain will resume in the morning, with intermittent waves of showers throughout the day.

Major impacts in the DC zone will include intermittent periods of heavy rain that could lead to isolated flash flooding and gusty southerly and southeasterly winds.

Given the strong atmospheric rotation associated with the remnants of Nicole, a tornado threat may also develop. The chances of tornadoes are somewhat higher south and southeast of Washington, south of Maryland, Richmond and the Virginia Tidewater.

Tropical Storm Nicole hits Florida and is expected to flood the eastern United States

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

Additionally, the Weather Service has placed the region in a Level 1 out of 4 risk level for excessive precipitation. Chances of heavy rain will increase west and northwest of the DC area.

Hourly: The chance of showers increases during the pre-dawn hours on Friday, particularly in the southwest of the region, and becomes likely around sunrise. Additional waves of rain pass during the day. The rain is expected to end late Friday evening.

Cover: Expect intermittent showers, in waves, and possibly thunder. Showers will move quickly but can be quite heavy at times.

Hazards: The main concerns are heavy rain, gusty winds, and the risk of an isolated tornado or damaging wind gusts. The risk of flooding is quite low as the area has been rather dry lately.

Precipitation forecast: A general 1 to 1.5 inches is most likely. Towards 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 north across the mid-Atlantic.

The storm has a very large wind circulation. The retreat of high pressures to the north will help to tighten the pressure gradient across the mid-Atlantic, thereby keeping wind speeds high. Expect frequent gusty winds of 20-30 mph on Friday and potentially higher in any thunderstorms.

As Nicole transitions from a tropical storm to a mid-latitude storm, a warm front will form (red scalloped lines above) which could be the focus of any tornadic activity. Meanwhile, a strong cold front and a deep dip to the jet stream will approach the east coast from the Ohio Valley.

The center of Nicole’s remnants will join this front, possibly near the Appalachian spine, as a plume of deep tropical moisture moves north to the east of the storm. Strong uplift on the west side of the Nicole remnants will interact with tropical humidity to generate a potentially very heavy rain band over Appalachia, with more rain to the east.

On Friday evening, the remnants of Nicole will quickly move out to the northeast and skies may actually start to clear around midnight.

Why strong winds and tornadoes are a risk

While fuel for the types of storms that could generate tornadoes will be limited in the DC zone, wind shear (change in wind direction and/or speed with altitude) will be significant. This combination of ingredients can pave the way for low-level rotating thunderstorms. These cells, in turn, can bring damaging gusts of wind (50-60 mph) to the surface in a few places, as well as generate brief tornadoes.

Inland tornadoes spawned by tropical remnants tend to be short-lived and weak, but these characteristics also make them difficult to detect with radar, hampering timely warnings.

At this point, the Storm Prediction Center estimates the highest tornado threat will be just south of DC. However, we warn that this area could extend further north if the air mass remains unstable in the late afternoon and early evening.

Rainfall totals will be highly track dependent. Right now, a total of about an inch in Washington seems reasonable. A change in the forecast track further east would bring higher totals closer to the area. The area has been quite dry recently, so the threshold rainfall amounts to locally trigger flash floods are high.

Overall, the predicted trajectory of Nicole’s remains has shifted westward, somewhat reducing potential precipitation 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 changes are possible, which would affect the precipitation forecast. But we don’t expect big changes now that we are less than a day away from the event.

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