11:10 AM | **Threat of downpours and strong-to-severe thunderstorms later today/early tonight...weekend coastal storm threat**
Low pressure will slide across southeastern Canada today and whip a strong cool front across our area later tonight. As a result, there are likely to be numerous showers later today into early tonight and perhaps multiple lines of convection (i.e., thunderstorms) passing through the region. Any shower or storm that forms later today could bring downpours to the region with the chance for localized flash flooding and there can be frequent lightning, small hail and damaging wind gusts as well in some spots. The front clears the region by early tomorrow morning setting us up for a much nicer air mass in the Wednesday-to-Friday time period, but an unusually strong summertime upper-level trough promises to bring us unstable weekend weather.
Today's cool frontal passage
A cool front is advancing towards the I-95 corridor and dew points have climbed into the lower 70’s ahead of it in a broad southwesterly flow of air. With clearing skies now in the I-95 corridor, daytime surface heating will act to destabilize the atmosphere at the same time the cool front closes in on the region. In fact, the surface-based "Convective Available Potential Energy" (CAPE) - an instability parameter for the troposphere - is already quite high at mid-day all along the I-95 corridor from DC-to-Boston
This overall setup should lead to numerous showers and scattered strong-to-severe thunderstorms in the I-95 corridor - most likely in the 2-8 pm time period. Any shower or storm that forms later today can produce heavy rainfall for a short period of time and there can be damaging wind gusts as well with potential strong-to-severe thunderstorms. The thunderstorms may come in separate lines of convection during this late afternoon/evening time period. In DC, the MLB All-Star game is ultimately likely to have a chance to get played with late evening improvement; but perhaps not until after a delay on the front end. The cool front swings through the region late tonight and there can be clearing skies after midnight. Much less humid air will pour into the region on Wednesday and it’ll stay relatively comfortable through the day Friday with plenty of sunshine expected each of the next three days.
Weekend coastal storm threat
An unseasonably strong upper-level trough will form over the Midwest early this weekend and the I-95 corridor will be situated on the unstable front end of its trough axis for much of Saturday and Sunday. This type of pattern in the middle of the summer can lead to some significant rainfall and strong thunderstorms - even the potential formation of a coastal storm (tropical or sub-tropical). As a result, showers and thunderstorms are likely this weekend – perhaps impacting both days – and there can be some heavy rainfall in at least portions of the Mid-Atlantic region. Stay tuned.
Meteorologist Paul Dorian