Severe Thunderstorm Watch likely!
Mesoscale Discussion 1323 NWS Storm Prediction Center Norman OK 0932 AM CDT Wed Jul 21 2021 Areas affected...Parts of northern Virginia...the District of Columbia...central/eastern Maryland...Delaware...southeastern Pennsylvania...New Jersey...southeastern New York including Long Island...Connecticut...Rhode Island and southeastern Massachusetts Concerning...Severe potential...Watch likely Valid 211432Z - 211530Z Probability of Watch Issuance...80 percent SUMMARY...Scattered intensifying thunderstorm development is possible through Noon-2 PM EDT, posing a risk for at least marginally severe hail and locally strong surface gusts. With storms likely to become more widespread thereafter, a severe weather watch probably will be issued. DISCUSSION...A slowly southward advancing surface cold front has reached southeastern New England and the northern Mid Atlantic coast, with large-scale ascent likely to increase in a corridor along and ahead of it through early afternoon. This appears likely to be aided by forcing ahead of substantive mid-level short wave trough gradually turning east of the lower Great Lakes region. The strengthening lift will coincide with a narrow pre-frontal corridor of rapidly steepening near-surface lapse rates aided by daytime heating. Lower/mid tropospheric lapse rates are also relatively steep and, despite seasonably modest low-level moisture, it appears that this will contribute to mixed-layer CAPE in excess of 1000 J/kg (up to around 2000 J/kg). Although low-level wind fields are generally weak, 30-50+ kt cyclonic flow in the 500-300 mb layer is contributing to sufficient shear to support organizing clusters of storms. Isolated supercells are possible initially, and these storms may pose a risk for (mostly) marginally severe hail, before the risk for potentially damaging wind gusts become more prominent later this afternoon. Areas of deepening convective development are already evident along this corridor, and a severe weather watch will probably be issued within the next hour or two.
The 2nd and 3rd images are for 5 PM and 6 PM from the HRRR model.