It’s been a very active week across the nation in terms of severe weather, and this Thursday, the next target for strong storms will be parts of the Northeast. All modes of severe weather will be present, including a few tornadoes, damaging winds, and moderate hail. This risk will span from Upstate New York down through northern Virginia, with the greatest risk across Pennsylvania.
The main culprit for these severe storms will be low pressure tracking east across the Great Lakes, with showers and storms forming ahead of its warm and cold fronts. Tonight into Thursday morning, there will already be a few showers, and possibly isolated thunderstorms, that push east ahead of the cold front across portions of the Northeast. These showers will not be severe, but may make for a slower morning commute, especially in the New York City area.
Now as we move into tomorrow afternoon, daytime heating will help to destabilize the atmosphere while dew points, or humidity levels, continue to rise associated with a southerly wind. This will allow for thunderstorms to form ahead of the cold front. An area of strong thunderstorms is expected to form midday near Lake Erie, and it’s these storms that should expand into a convective line, trekking across much of New York and northern Pennsylvania through the afternoon. This is what the HRRR model suggests, and the NAM model is also in similar agreement. This line will remain severe over the Mid-Atlantic states, but once it tracks into New England, it will weaken.
Just to the south is the area that we are most concerned about in terms of severe weather. Most of Pennsylvania, down toward the Mason-Dixon Line looks to be quite active, especially between 2-8 pm EDT, as scattered thunderstorms push to the south and east. Some of these storms will be super-cells, and could drop tornadoes.
Below is a forecast sounding from the NAM model for southeastern Pennsylvania Thursday afternoon. It’s confusing to understand what this graphic is showing, but we would like to point out how the lapse rates suggest a conditionally unstable atmosphere, 64 knots of wind shear from the ground up to 6 km, nearly 3000 J/kg of CAPE (Convective Available Potential Energy) at the surface, and a near-saturated atmosphere — all of which are ingredients for severe weather and more specifically tornadoes.
By midnight Thursday night, the storms will move offshore, making for a dry rest of the night across the Mid-Atlantic while a moderate rain continues to affect northern New England.