First of all, the weekend is still looking nice for the DC metro region. However, before we reach that time, there is an unusual weather situation for mid-June that is currently unfolding for parts of the Mid-Atlantic region including this area. As a result of this unfolding weather pattern, occasional rain and thunderstorms are likely from later today through tonight in a northwest-to-southeast corridor extending from southwestern Pennsylvania to the DC metro region. Some of the rainfall in this corridor will be heavy at times and some of the thunderstorms that form will likely reach strong-to-severe levels.
An upper level low with very strong dynamics for this time of year is currently centered over the Great Lakes and it is going to drop southeastward over the next 12-24 hours crossing over West Virginia and western Virginia along with an associated surface low and cold front (above, upper-level forecast maps from 00Z NAM model run). In addition to this approaching upper-level feature, a boundary zone already exists across the DC metro region and it is adding “fuel to the atmospheric fire”. Very warm and humid air currently sits across central and southern Virginia and much cooler air sits across northeast Maryland and this temperature contrast will play a role in the unfolding weather over the next 12-24 hours. The immediate DC metro region will likely see temperatures jump this afternoon as the sun tries to break out and this low-level warming will act to destabilize the atmosphere ahead of the main event tonight.
Occasional rain is likely to break out later today or early tonight in the DC metro region (> 4pm) and thunderstorms are likely to mix into the picture in this same time period and some may reach strong-to-severe levels. The main threat from any severe thunderstorm will be damaging wind gusts given the unusually strong winds in the upper atmosphere, but isolated tornadoes cannot be ruled out anywhere in the corridor from southwestern PA to the DC metro region. In addition to the threat for damaging wind gusts, flash flooding is also a real concern. The entrenched air mass is full of moisture that is ready to be “squeezed out” by strong dynamics in the atmosphere. “Training” may take place in the overnight hours in this zone between southwestern PA and DC with multiple thunderstorms possibly rolling over the same areas in an extended period of time. In fact, some computer forecast models have predicted more than two inches of rain in a rather short period of time (above, NOAA total precipitation forecast map) and I believe some spots in this zone could actually see as much as five inches of rain by tomorrow morning.
The good news is that this system will slide southeast of the DC metro region on Friday and the weekend is looking quite nice throughout the entire Mid-Atlantic region including the DC, Philly and NYC metro regions.
Meteorologist Paul Dorian