Storm clouds at the U.S. Capitol on April 6. (Navin Sarma via Flickr)

The upcoming weekend is one jam-packed with outdoor events, most notably the March for Science on the Mall on Saturday. Meanwhile, a slow-moving storm system, loaded with moisture, will crawl into the Mid-Atlantic region. Substantial rainfall is likely, but the exact timing and intensity — still coming into focus — will play a pivotal role in how disruptive this wet weather becomes.

Timing

Right now, our best guess is that while some spotty showers or sprinkles are possible any time Saturday, the onset of steady rain will hold off until late in the afternoon or evening, after most of the March for Science activities.


Global Forecast System model simulation of rainfall Saturday and Sunday.

As the actual March is set to start at 2 p.m. Saturday, it’s not out of the question some light rain showers intercept marchers — depending on how late it goes.

Forecast models agree that the bulk of the weekend rain should occur Saturday evening and overnight. Some rain may continue into Sunday, especially from the District south. But there’s likely to be a sharp cutoff in the rain in the region, and areas north of that cutoff zone may have a mostly dry, though overcast and cool, Sunday.

This same storm system will meander off the Mid-Atlantic coast Monday and Tuesday, meaning the continuation of cool, cloudy weather and the chance for showers, especially in our southeastern areas.

Rainfall amounts

Whatever rain falls will be beneficial for the region, still mired in moderate drought. Rainfall is running below normal in April for the 11th straight month.


(U.S. Drought Monitor)

Models are in good agreement that areas well south of Washington, toward Fredericksburg, are likely to receive substantial rainfall of at least an inch. However, forecast amounts decrease some as you head north.

The National Weather Service is predicting about an inch of rain through Sunday evening in Washington, 0.7 inches around Baltimore and more than 1.5 inches around Fredericksburg.


Forecast rainfall through Sunday night from the National Weather Service.

Its forecast amounts are similar to projections from the European model:


European model rainfall forecast through 5 p.m. Sunday. (StormVistaWxModels.com)

But the latest GFS model forecast suggests lower numbers — just 0.3 inches in Washington, 0.1 inches in Baltimore, but 2.0 inches around Fredericksburg.


GFS model rainfall forecast through Sunday night.

The Canadian model forecast looks like a blend of the Weather Service and GFS:


Canadian model rainfall forecast through Sunday night.

The bottom line is that there is the potential for substantial rainfall in the region this weekend, of at least one inch. But the best chance for such soaking rainfall is south of the District.

As we get closer to the weekend and more confidence in the exact track of the storm system, we’ll be able to refine the forecast for rainfall timing and amounts. We’ll post another update Friday.