What tropical storm Andrea means for Washington, D.C. area; flash flood watch

* Flash flood watch for much of D.C. area *

Tropical storm Andrea over the Gulf of Mexico (NOAA)
Tropical storm Andrea over the Gulf of Mexico (NOAA)

Tropical storm Andrea, packing peak winds of 60 mph, is in the process of coming ashore in north Florida. But rain, not wind, is the hazard the Mid-Atlantic needs to focus on as the storm surges northeast into the region.

How much rain are we talking about?

A good general estimate is 1-3 inches for the metro region between late this afternoon and Saturday morning, but with locally higher amounts possible.

The latest GFS models dumps closer to 2-4 inches on the region (see below), whereas other models are not quite as aggressive.


GFS model simulation of total rainfall between this morning and Saturday morning. (WeatherBell.com)


Here is my assessment of rainfall possibilities:

Less than 1 inch: 25 percent (if the storm tracks farther east than forecast)
1-2 inches: 25 percent
2-3 inches: 25 percent
3-4 inches: 15 percent
4 inches or more: 10 percent

Another way of looking at this? There is a 75 percent chance of at least 1 inch of rain, a 50 percent chance of at least  2 inches, and 25 percent chance of more than 3 inches.

Area in green under a flash flood watch Friday morning through evening.
Area in green under a flash flood watch Friday morning through evening.

The National Weather Service has posted a flash flood watch from Friday morning to Friday evening for the potential of 2 to 4 inches of rain, and isolated amounts to 6 inches.

Particularly from late Friday morning into Friday evening, a period of very heavy rain is possible, that could lead to localized flash flooding.  If you live in an area that normally floods during heavy rain events, be prepared.

Some flooding of area rivers is also possible in the wake of the storm, especially north of the District (useful link: National Weather Service river flooding briefing)

What’s the timing of the storm and when is the heaviest rain expect?


Radar & lightning:Latest regional radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall.

A plume of moisture, loosely associated with the storm, is already streaming into western parts of the D.C. region. It’s raining in  Loudoun and Frederick counties.  Through the afternoon, this area of rain will expand east and northeast. By 7 p.m., most of the region should have light to moderate rain.  And rain will continue overnight with roughly 0.5-0.75 inches by morning.

Sometime late tonight into early Friday morning, there may be a break in the action.  The brunt of the storm comes through between late Friday morning and early-to-mid evening.  Rain tapers off overnight Friday, but lingering moisture means there could be a few pop-up showers and storms Saturday, especially during the afternoon and evening hours.

Is there a wind threat?

The strongest winds from the remnants of Andrea will pass east of the center.  As the center is forecast to pass through southeast Va. and the Delmarva peninsula, that places us west of the center, reducing the risk of high winds.

However, as the center makes it closest approach and we experience the strongest rain bands (probably Friday afternoon), we may record a few strong wind gusts, perhaps in the 20-35 mph range.  With that, I don’t see power outages as a major concern with this storm.

What about tornadoes?

Like the strongest winds, the tornado threat with tropical systems usually focuses east of the center.  Thus, tornadoes are not expected in the D.C. area.

Do we need to worry about storm surge and coastal flooding?

At the moment, just minor coastal flooding is forecast, at high tide tonight, and perhaps Friday as well, with water levels one foot or so above normal.  Coastal flood advisories may be necessary says the National Weather Service.

Chesapeake Bay and Atlantic shore (beaches) impacts

For those of you along the Chesapeake Bay and especially the Virginia, Maryland and Delaware beaches, the effects of Andrea will be more intense – with generally heavier rains and stronger winds than in the D.C. area. But this is not a repeat of Sandy.

Gale-force winds are a good possibility, but just isolated gusts may be damaging.  Peak sustained winds should be in the 20-40 mph range, with some gusts to 50 mph possible.

The closest location that may have a small risk of tornado activity is probably around Virginia Beach.

Here’s an excellent infographic summarizing the impacts expected for southeast Virginia and the Maryland and Virginia beaches from the National Weather Service:

If you’re interested in specific impacts on the Delaware and New Jersey beaches, please see this excellent National Weather Service briefing: Tropical Storm Andrea Briefing, June 7th-9th, 2013

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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