Tropical Storm Andrea promises stream of rain up East Coast; eyes D.C. area Friday


Tropical Storm Andrea forecast cone from the National Hurricane Center as of naming the system.

5:45 p.m. update: The system in the Gulf of Mexico has become well enough organized for the National Hurricane Center to upgrade it to Tropical Storm Andrea. Andrea is the first named storm of the season, which officially started on June 1. Tropical Storm Warnings are now up for parts of the Florida Gulf Coast from Boca Grande to Ochlockonee River. Watches are up on the Atlantic seaboard from Flagler Beach, Florida to Surf City, North Carolina.

Previously…

Satellite image of Gulf of Mexico tropical disturbance, which has a 60 percent chance to become a tropical depression or storm. (National Hurricane Center)
Satellite image of Gulf of Mexico tropical disturbance, which has a 60 percent chance to become a tropical depression or storm. (National Hurricane Center)

An area of disturbed weather in the Gulf of Mexico is becoming more organized and the National Hurricane Center gives it better than even odds (60 percent chance) of becoming at least a tropical depression. Then, the moisture-rich weather system promises to cruise up the East Coast, dumping heavy rain in many locations between today and Saturday.

The amount of rain that falls in the D.C. area will depend on its exact track.

The system is currently positioned over the central Gulf of Mexico and has generated substantial rains over the Florida peninsula. Most of the system’s thunderstorms are east of the center. This asymmetric configuration isn’t favorable for rapid intensification but just a modest uptick in its strength is required for it to reach tropical depression status. It could even become the 2012 Atlantic hurricane season’s first named storm, which would be Andrea.

As the system comes ashore in northern Florida late Thursday, some of its moisture will be drawn northward along a front draped along the East Coast, increasing rain chances in the Southeast and Mid-Atlantic. The opportunity for heavy, persistent rains will wait until the system’s center approaches – which is most likely Friday into Friday night for the D.C. area.


Forecast model tracks for tropical disturbance (UCAR)

The Short Range Ensemble Forecast model indicates at least a 50 percent chance of 1 inch or more rain between Friday morning and Saturday morning in the D.C. area.

(National Weather Service)
(National Weather Service)

Other models indicate the heaviest rain may well fall west, east, and/or north of the District.

For example, the NAM model simulates the heaviest rain falling west of the District.


NAM model indicates 2-4 inches of rain will mainly west of D.C. between Thursday through Saturday, with 1-2 inches in the immediate area (StormVistaWxModels.com)

But the GFS and European models suggest the heaviest rain to the east and north.

The European and GFS models suggest the heaviest will tend to fall east and north of the D.C. area from this tropical system. (StormVistaWxModels.com)
The European and GFS models suggest the heaviest will tend to fall east and north of the D.C. area from this tropical system. (StormVistaWxModels.com)

The National Weather Service Weather Prediction Center rain map looks like a compromise between the various models, forecasting moderate rains over the District and heavy rains to our west, east and north.


The National Weather Service’s rainfall forecast through Saturday

The Mid-Atlantic River Forecast Center has issued an outlook highlighting the possibility of river flooding in northern Maryland into Pennsylvania this weekend.

The bottom line is that a rich plume of moisture will stream north, but exactly where it focuses is to be determined. Heavy rain amounts of 1-3 inches or more, capable of some flooding, cannot be ruled out. But it’s also possible we get largely bypassed, with more intermittent showery weather and amounts around one inch or less.

As a first call, subject to change, I’ll project 1-1.5 inches of rain across the metro area.

In any event, widespread severe flooding isn’t likely, because the system will head up the coast rather quickly.

Stay tuned for updates.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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Jason Samenow · June 5, 2013