This year has been off to a slow start with regards to severe thunderstorms, with the only notable outbreak thus far having occurred in February. The first notable severe weather potential of the summer is expected today as a squall line approaches the area late this evening, producing heavy rainfall and frequent lightning with a risk of strong wind gusts and small hail, although the highest severe weather risk will remain south of the area. Drier and cooler conditions will return for the upcoming week, however, as an unseasonably cool air mass returns to the region.
Weather Prediction Center (WPC) surface analysis valid at 1200 UTC (8 AM EDT).
The latest WPC surface analysis, posted above, depicts a 997 hPa surface cyclone positioned near northern Michigan, with a warm front extending down through Pennsylvania and Maryland, positioning the tri-state area in its cool sector with a southeasterly flow. A band of light to moderate rain associated to some extent with mid-level frontogenesis moved though the region last night and is currently near northern New York state. As of 11 AM, an area of scattered heavy showers within a broad region of onshore southeasterly flow and low-tropospheric isentropic lift developed over northern New Jersey ahead of the approaching warm front, with radar estimates suggesting rain totals near 1/4 to 1/2 inch.
0z WRF-NMM hour 18, valid at 1800 UTC (2pm EDT), depicting precipitable water in mm.
The warm sector of the cyclone is associated with a moist and relatively warm air mass; widespread cloud cover remains per the latest visible satellite, although warm surface temperatures and high humidity are contributing to decent instability. In addition, the East Coast is embedded within a corridor of high moisture content, with precipitable water (the amount of liquid if the entire tropospheric water vapor column was condensed into water) values exceeding 48 mm, or 2 inches. The combination of a warm, unstable and moist air mass along with sufficient synoptic-scale forcing for ascent, is expected to support the potential for severe thunderstorms later this afternoon and evening.
Northeast US Analysis
0z WRF-NMM hour 20, valid at 2000 UTC (4 PM EDT), depicting surface-based CAPE (J/kg).
As previously noted, the atmospheric environment is favorable for the development of strong to severe thunderstorms later this afternoon. For a severe weather event to materialize, a combination of instability, moisture, vertical wind shear and forcing for ascent is very often required, and today features at least some of each aspect.
The map above depicts an axis of 2000+ J/kg surface based CAPE from Virginia up through the warm front in central New Jersey and eastern Pennsylvania, which is largely overlapped with vertical wind shear upwards of 30-35 knots. There is some low-level directional shear near the warm front, veering from southeasterly at the surface to southwesterly aloft, although otherwise the wind profile aloft is largely unidirectional, implying a generally straight line hodograph and a higher likelihood for linear convection with strong wind gusts as the primary risk. Any discrete convection, which is likely to occur earlier in the afternoon within the warm sector, may be more likely to feature a small hail and isolated tornado risk, with the tornado risk higher towards the warm front, especially near southeast Pennsylvania, southern NJ, Maryland and NE Virginia, where low-level directional shear will be higher.
12z HRRR hour 10, valid at 2200 UTC (6 PM EDT), depicting surface-based CAPE
The actual cold front continues to lag behind in Ohio, where a separate round of scattered thunderstorms is currently ongoing. Over the Northeast US, however, thunderstorms are primarily expected to develop along a pre-frontal surface trough just downwind of the Appalachians near central Pennsylvania, continuing east while organizing into a squall line given the generally unidirectional wind profile. The squall line is expected to move through northern NJ towards 6-8pm, and NYC and locations east towards 7-10pm. Given the moist environment, the primary risks in the tri-state area are likely to be heavy rainfall rates exceeding 1 inch per hour, and strong wind gusts over 40-50 mph especially west and south of NYC. An isolated tornado cannot be ruled out south of I-80 where the combination of directional shear and instability will be likely present, although the highest risk of severe storms will remain south of the tri-state area.