Since August, Washington, D.C.’s weather has been about as good as you can expect.  We’ve had no shortage of sunny comfortable days.  And when it’s been hot, it hasn’t been that hot. Rain, with the occasional exception, has been hard to come by.

Enter a coastal storm, developing off the Southeast coast today, forecast to meander off the Mid-Atlantic coast beginning tomorrow and *possibly* remaining there until early next week. It may not only deliver several consecutive days of cloudy weather, but also significant rainfall. The storm is made up of some of the remnants from tropical storm Karen.

There’s model disagreement about how long this storm will stick around and how much rain it will send ashore. That will depend on the exact track of the storm and the persistence of high pressure to the northeast (that will help lock the storm in place), which are both uncertain.

The European model simulates 2 to 3 inches of rain between Wednesday night and Monday morning, whereas the GFS model simulates just over an inch in the same period.

Total rainfall forecast through Monday morning on European model (

Total rainfall forecast through Monday morning on GFS model (

While that entire Wednesday night through Monday morning period may well be cloudy, models produce most of the rain on Thursday and Friday (with very light rainfall amounts over the weekend).

But even Thursday and Friday, different simulations of the SREF model show a wide range of possible rainfall totals, from essentially no rain to over 3 inches in Washington, D.C.

Rainfall forecast through Friday night from different simulations of the SREF model. Each green line represents a different simulation and is a time series of the cumulative rain over time (Penn State)

What’s becoming increasingly clear is that from Wednesday night through Friday it’s likely to be cloudy and at least damp. Exactly how much rain falls, and whether rain continues at times over the weekend is more of an open question.

Maximum wind speed forecast for offshore winds through Friday (Coastal Emergency Risks Assessment)

Along the coast, strong onshore winds are likely to bring high surf, beach erosion and coastal flooding – perhaps over an extended period. Coastal flood advisories have already been posted for the Atlantic beaches.