Washington DC (LWX) — the National Weather Service forecast in plain English · Plaincast

What has changed

A Severe Thunderstorm Watch was issued for the majority of the forecast area through 8 PM this evening. A Freeze Watch was issued for location to the west of the Blue Ridge for Saturday night into early Sunday morning.

Messages

1) Severe Thunderstorms are possible this evening as a strong cold front moves through.

2) Frost and/or freezing temperatures are possible this weekend.

3) Nor'easter to develop off the Carolina coast late in the weekend lingers much of next week.

Discussion

KEY MESSAGE 1. Severe Thunderstorms are possible this evening as a strong cold front moves through.

Skies remain mostly sunny across the majority of the forecast area, which has allowed temperatures to climb into the lower to mid 80s. Dewpoints have also risen into the 60s to near 70 within southerly flow. This unseasonably warm and humid airmass coupled with cooling temperatures aloft are allowing instability to build over the area. Thunderstorms are in the process of forming along a strong cold front which is just starting to move into western MD and the WV Panhandle. The coverage of storms is expected to gradually increase as the front moves into an environment further east characterized by slightly higher instability, with the greatest coverage of storms expected to the east of the Blue Ridge this evening where the highest dewpoints (upper 60s to near 70) are located.

Confidence in the timing of the storms is very high since they're forming directly along the front. The storms are located over Western MD and the WV Panhandle currently, should reach the Blue Ridge by around 5-6 PM, impact the I-95 corridor between 6 and 8 PM, and then Southern MD shortly thereafter.

In terms of hazards, damaging winds appears to be the primary threat, with ample flow in place just off the surface. With around 40-50 knots of effective bulk shear, some supercellular or bowing structures could be possible, which could locally enhance the threat for damaging winds. However, relatively weak low-level lapse rates could be a limiting factor for damaging winds. As thunderstorm coverage increases to the east of the Blue Ridge, we'll also be losing daytime heating, which will only make the low-level lapse rates even poorer. While the environment doesn't appear to be the most favorable for tornadoes, a brief tornado or two can't be completely ruled out. The threat for severe thunderstorms will immediately come to an end once the front moves through, with winds picking up out of the northwest behind the front. A Severe Thunderstorm Watch remains in effect for the majority of the forecast are through 8 PM.

KEY MESSAGE 2. Frost and/or freezing temperatures are possible this weekend.

Strong cold advection will ensue within northwesterly flow behind the cold front tonight through the day tomorrow. Upslope showers in the Alleghenies may even end as a few flakes of snow late tonight, but no accumulation is expected. Temperatures are forecast to drop into the 40s by daybreak tomorrow (mid- upper 30s mountains). A few showers could linger from Central Virginia to Southern Maryland tomorrow morning, but gradual clearing is expected tomorrow afternoon as the upper trough axis progresses to our east and large scale subsidence starts to increase. Temperatures will be much cooler tomorrow, with highs in the 50s (40s mountains).