3:40 p.m. update: The latest update from the European model tracks the storm a bit more to the east, and forecasts less rain than previous runs. We may lower our forecast rain totals some in our PM Update coming out around 5 p.m. Also, the onset time for the rain will probably focus towards the latter end of the windows given below from the District and points north and northwest – most likely between 6 and 8 p.m.

Original post, from 12:22 p.m.



Satellite image of developing Nor’easter (NASA)

A developing coastal storm – that will slowly track up the Mid-Atlantic coast through Thursday – brings the possibility of heavy rain to the region. The rain is likely to begin late this afternoon and this evening from southeast to northwest and should continue at least through the first half of Thursday.

The storm has an impressive feed of moisture which is surging up the East Coast, originating just north of Cuba.

Given the potential for heavy rain, the National Weather Service has issued a flood watch from 6 p.m. tonight to 6 a.m. Thursday given the potential for 1-2 inches of rain, with locally heavier amounts.

Frequent questions



Radar & lightning: Latest regional radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click 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

Where is the rain right now?

Track the storm’s progress on regional radar and the loop to the right.

When does the rain start?

The rain could begin as early as around 3 p.m. around Fredericksburg, Va. and as late as 8 p.m. or so around Baltimore.

Here are estimated start times for different locations:

Fredericksburg, Va. and Southern Md.: 3-5 p.m.

Fairfax, Va. and Washington, D.C.: 4-6 p.m.

Rockville, Md.: 5-7 p.m.

Columbia and Baltimore, Md.: 6-8 p.m.

Below is a simulated radar showing the predicted onset of the rain from the HRRR model. It’s a general guide and won’t exactly match reality:



HRRR model simulated radar simulation for onset of rain later today (WeatherBell.com)

When will the heaviest rain fall?

Most models suggest the most likely time for heavy rain is between roughly 8 p.m. tonight and 4 a.m. Thursday morning.

When will the rain end?

This is a trickier question.

Right now, my thinking is that steady rain tapers off between 4 a.m. and 10 a.m. from southwest to northeast. However, some light rain may linger into Thursday afternoon – with the chance of rain after 10 a.m. around 30 percent in the southwest part of the area and 50 percent to the northeast.

In southwest areas like Centreville and Manassas – especially – skies may brighten as Thursday afternoon wears on, whereas clouds could linger into Thursday night in areas to the northeast towards Annapolis and Baltimore.

How much rain are we talking about?

This is another tricky question. Model forecast rainfall varies from half an inch to about 2 inches in the District.

I generally favor around 0.5-1 inches west of I-95, and 1-2 inches along and east of I-95 – including D.C. (60 percent likelihood).

The National Weather Service’s rainfall forecast – below – appears reasonable to me – which paints 1-1.5 inches up the I-95 corridor.



Click to enlarge. (National Weather Service)

If the storm tracks farther west than forecast, more widespread totals of 1-2 inches would be possible, with locally higher amounts (boom scenario, 20 percent likelihood). The latest NAM model, below, supports this wet scenario:



NAM model storm rainfall simulation (NWS)

But if the storm tracks east, areas west of I-95 could receive less than half an inch and even areas along and east of I-95 might have trouble topping one inch. That drier solution (bust scenario, 20 percent chance) is favored by the GFS, below:



GFS model storm rainfall simulation (WeatherBell.com)

Will the Redskins game be affected at FedEx field Thursday night?

If the storm takes more of an inland track and is slow to exit, overcast skies and even some drizzle could linger for the game. But, more likely, it’s mostly cloudy and dry – with temperatures around 60-65 degrees.

What will coastal impacts be?

The storm’s most significant impacts will occur over the Delmarva Peninsula, where 1-3 inches of rain are likely along with some strong winds. At the Atlantic beaches in Maryland and Delaware, gusts could reach 30-40 mph and a gale warning is in effect.



(National Weather Service)

Late tonight, minor coastal flooding is possible along the western shores of the Chesapeake Bay around high tide from the easterly wind fetch.