As I discussed yesterday, for thunderstorms to reach their full potential, sunshine is necessary to destabilize the atmosphere. We should have some intervals of sunshine, but it’s unclear how many and for how long - they may be limited.
Link: Satellite loop
The National Weather Service Storm Prediction Center (SPC) has maintained the severe thunderstorm risk level at slight, but still could upgrade the risk this afternoon. Despite that conservative classification, I’d argue this is a high-end “slight risk” day. It’s not often, for example, SPC predicts a 10 percent chance (that’s a non-trivial number) of a tornado within 25 miles of a given point in the D.C. area - see graphic above. And the 30% chance of damaging wind is significant.
Here are my latest thoughts on how the day will evolve with the qualifier that predicting the intensity and timing of thunderstorms is inexact:
* There will probably be two primary rounds of activity
* Round one may begin as early as midday to early afternoon west of town (especially towards the I-81 corridor) - and towards the mid-to-late afternoon for the rest of the area. This round will consist of scattered activity - hit or miss (40-50% chance any given area gets hit). A few of these storms may become severe and rotate - spawning a tornado or two - especially later this afternoon in any areas that get a lot of sunshine.
* The most sunshine and, thus, highest severe weather potential may end up southeast of D.C. towards southern Maryland and the northern neck of Virginia.
* Rounds 2 is likely this evening with the front and should be widespread (80% chance of rain). It may come in the form of a nasty squall line with heavy rains capable of localized flooding, some strong to potentially damaging wind gusts, and perhaps an isolated tornado. This looks to come through starting between 7 and 9 p.m. from west to east, and may interfere with the Nats game. It should wind down between midnight and 2 a.m.
Here are some relevant excerpts from the discussion from SPC about today’s storm threat:
* THE PRIMARY SEVERE THREAT WILL BE MID-LATE AFTERNOON AS PRE-FRONTAL STORMS FORM OVER CENTRAL VA AND SPREAD NORTHEASTWARD INTO MD.
* SEVERAL MITIGATING FACTORS CONTINUE TO WARRANT MAINTENANCE OF SLIGHT RISK OUTLOOK. EARLY MORNING VISIBLE IMAGERY SHOWS WIDESPREAD CLOUDS AND SCATTERED SHOWERS ACROSS THE REGION. THIS COMBINED WITH FORECAST MARGINAL MID LEVEL LAPSE RATES WILL LIMIT UPDRAFT ACCELERATIONS. ALSO...WHILE LOW LEVEL WINDS ARE SUFFICIENT FOR ISOLATED SUPERCELL STORMS AND A FEW TORNADOES...THEY DO NOT APPEAR AT THIS TIME TO INCREASE CONCERN FOR SIGNIFICANT DAMAGING WINDS/GREATER THAN 65KT GUSTS/ OR STRONG TORNADOES.