At the moment, showers are in the forecast every day next week starting Monday while cooler-than-average conditions return by Tuesday. Two to four inches of rain could fall.
Computer model forecasts indicate the dreary pattern may not give way until next weekend (May 22 and 23).
The weather disturbance is known as an upper-level low and contains a deep pool of chilly air. Upper-level lows can generate a lot of precipitation, especially when they get stuck in the same general area and can draw moisture off the Atlantic Ocean. Next week, the possible presence of subtropical storm Arthur offshore the Mid-Atlantic may even increase the flow of moisture into the region.
However, upper lows are notoriously erratic and difficult to predict when they diverge from the atmosphere’s steering currents. They can meander aimlessly, making it difficult to forecast rainfall placement, timing and amounts.
In recent days, computer model forecasts for rainfall in the Washington area next week have shifted around substantially as they’ve struggled to get a handle on the upper low’s path.
Most models now suggest the low will dive from the Upper Midwest on Sunday into the Southeast by Tuesday, where it stalls for several days before lifting north. While not predicted to position itself directly over Washington, the low’s position to the southwest will allow moisture transport into the region.
“Guidance [from models] differs on exactly where this upper low sets up,” the National Weather Service wrote in its morning discussion. “No matter what, the entire long term period looks wet, with below average temperatures.”
Although it’s likely to be unsettled, cooler-than-normal and rainy for a significant portion of next week, it’s unlikely to rain continuously. There should be pauses and perhaps even intervals of sunshine.
On the other hand, we can’t rule the possibility of an extended period of heavy rainfall at some point and even flooding if the upper low positions itself in a way to maximize moisture transport over the region.
“This system has the potential to bring a significant amount of rainfall over several days, given the right placement,” the Weather Service wrote.
While models are wavering in their rainfall forecasts, here’s how much they are predicting between Monday and Friday next week:
- European (ECMWF): 3.6 inches
- American (GFS): 2.7 inches
- Canadian (GEM): 3.9 inches
- United Kingdom (UKMET): 1.7 inches (through Wednesday only)
Specifics regarding the temperature forecast are also in flux and will depend on where the upper low positions itself on given days and the amount of cloud cover and rain.
While temperatures should warm into the 70s on Monday ahead of a passing cold front, most days after that may struggle to escape the 60s or even the 50s due to the cooler air behind the front and the proximity of the upper low (and its clouds and rain).
If the upper low behaves as predicted and weakens and exits by next weekend, more normal late May warmth should then return. Average highs climb into the upper 70s by the end of next week.