Hazards:

• Heavy Rain/Flooding: The heaviest rain, with a swath of 2-3″ amounts expected, will be focused to the west of Isaias’ center of circulation, generally across portions of the mid-Atlantic and the Hudson Valley.
• Wind: The strongest wind gusts will be found to the east of Isaias’ track, arcing from the immediate mid-Atlantic coast northward into Southern New England. The most intense gusts, perhaps as high as 55-75mph in spots, will extend from the Jersey Shore northward into portions of Connecticut and central Massachusetts. Across southeastern and central New England, gusts should be somewhat less intense, but may still reach 40-55mph.
• Tornadoes: Tornadoes are possible from the Jersey Shore northward into southern and perhaps central New England tomorrow. Have multiple ways to get a warning and be ready to act quickly if one is issued for your location.

Discussion: As of 03.21Z, Tropical Storm Isaias was approaching the South Carolina coast, and is expected to make landfall this evening as a strong tropical storm or weak hurricane. Guidance is in good agreement regarding the track of Tropical Storm Isaias, brining it generally northward across eastern North Carolina, the DelMarVa, and the Hudson Valley, with tracks diverging just a bit as the storm advances into Vermont or New Hampshire. When referencing the track in this forecast, we will be sticking with the official guidance from the National Hurricane Center (NHC), which is in alignment with the aforementioned numerical guidance.

GOES-16 “Sandwich” imagery reveals moisture streaming northward ahead of the cyclone amid southwesterly deep-layer flow. In the 200-300mb layer, a jet streak is beginning to take shape above the eastern Great Lakes, and is expected to intensify and lift northward. As Isaias advances northward, it will couple with the right entrance region of this strengthening jet streak, and should benefit from strong upper level diffluence. Given the degree of upper level support, the overall intensity of the cyclone should maintain itself as it moves northward, even once the transition to a more extra-tropical structure is underway.

Along and to the west of Isaias’ track, a swath of heavy rainfall totals is expected. Precipitable Water values near and in advance of the storm are exceeding 2″, and strong 850mb moisture transport is pulling rich tropical air northward and wrapping it around the cyclone, where on the northwest side, heavy rainfall will result. The Weather Prediction Center (WPC) has issued a Moderate Risk for excessive rainfall across a broad region of the Carolinas and mid-Atlantic States — a Slight Risk for excessive rainfall extends into western and northern New England. Isaias will be accelerating northward tonight and into the day tomorrow, which should preclude extreme rainfall totals, but widespread 2-3″ amounts look like a good bet, with isolated 6″ amounts not out of the question.

With regard to wind, Isaias should be entering into, or going through, its extratropical transition by the time it reaches Long Island/Connecticut, and thus, the low level wind field will be expanding. Numerical guidance has been pretty bullish on the potential for damaging winds in Southern New England — given the full foliage, I am pretty concerned about the potential for downed trees and power outages tomorrow evening. A broad 50-70kt H85 jet swings across Southern New England, generally between 20Z and 04Z. A warm, very moist and well-mixed boundary layer is expected to be in place, and accordingly, some pretty strong gusts could be realized at the surface. Closer to the south coast, especially in Connecticut and Rhode Island, gusts of 65-75mph could be somewhat common, with maximum gusts tapering to 40-55mph well to the northeast (say, southern New Hampshire and far NE Massachusetts).

In Isaias’ right-front quadrant, as is traditionally favored, conditions could become quite favorable for tornadoes from New Jersey, through the NYC Metro and northward into MA/CT/RI. The low-level jet responsible for the ambient wind gusts of 50mph+ is a component of a well-organized, veering wind profile, which will result in extreme low-level shear magnitudes. An area of 0-3KM SRH of 200-400m2/s2 or greater will advance northward in lockstep with Isaias’ center of circulation, approaching Southern New England by Tuesday afternoon. Within this region, remarkably large, right turning hodographs are modeled, and Critical Angles in the 45-60degree range indicate the presence of significant stream-wise vorticity in the boundary layer.

With the persistent southeasterly advection of very moist boundary layer air, and perhaps even some limited solar insolation, an unstable environment will likely evolve along the leading edge of the most significant low-level shear. It is within this corridor that the risk for tornadoes should maximize. In southern and central New England, the tornado threat will likely be greatest between 20Z and 01Z, and generally evolve from south to north as Isaias advances.

Those in the region where tornado potential is greatest (from DelMarVa/New Jersey in the morning, into the NYC Metro midday, and southern/central New England during the afternoon and evening) should pay close attention to the weather tomorrow. Given the magnitude of low-level shear, several tornadoes appear possible across this region.