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Synopsis: A large low pressure system will bring active weather to much of the eastern seaboard, with a focus in New England, late this weekend into early next week. As the low moves northward from its current position in the northern Gulfof Mexico, a favorable upper-level pattern evolution should allow the system to deepen rapidly to near 980mb at 40°N. Rapid pressure falls should result in a large mass response/impressive low-level jet evolution, increasing the risk for damaging wind gusts and drawing a large plume of moisture northward. Near the surface low, a 10-18 hour period of steady to heavy rain can be expected, with a shorter window of potentially damaging winds along the NewEngland coast during the day on Monday.  

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Discussion: As of 12/17 00Z, the surface low of interest remains over the Gulf of Mexico featuring robust convection and gradually falling pressures, with a current minimum pressure of approximately1007mb.  Aloft, two disturbances ofinterest are apparent.  The first, a vigorous shortwave rounding the southern flank of a closed H5 low, is located over MO/AR and is digging rapidly southeast.  The second disturbance is abroad area of coagulating mid-level vorticity over the northern Gulf of Mexico(associated with the ongoing surface low) which should evolve into a more organized closed low and/or open wave. The two disturbances are forecast to phase over the Carolinas late in the day on Sunday, and in response the surface low should begin to deepen rapidly.  

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From Sunday afternoon onward, the upper-level pattern evolves favorably for further intensification of the surface low. On the eastern fringe of a long wave trough centered over the GreatLakes, an approximately 150kt jet streak will evolve over southern/centralQuebec and lift northward, with a large right-entrance region of favorable large-scale ascent positioned over the northeastern United States.  Concurrently, a more subtle 100kt jet streak may evolve east of NC, also positioning the mid-Atlantic/northeast states in the favorable left-front quadrant.  All of this is to say, the model depiction of a rapidly deepening low pressure system at the surface appears very reasonable given the strong upper-level support.  

As the surface low deepens rapidly on Sunday, a plume of deep and rich moisture will be drawn northward. Throughout its evolution, the surface low appears likely to draw tropical moisture from the vicinity of the Bahamas up to the eastern seaboard via a vigorous warm air conveyor.  On the northwestern fringe of this moist conveyor, which, in this case, is co-located with favorable orographic ascent, precipitation totals should maximize in a corridor from near Washington, DC northward into upstate NY and the White/Green Mountains.  In this corridor, localized areas of flash and/or river flooding appear possible and the Weather Prediction Center has issued a Slight Risk of Excessive Rainfall.

By 12Z Sunday at the latest, robust low-level jet (LLJ)intensification should be underway near southeast GA in response to the deepening low pressure.   The low-level jet is forecast to continue intensifying through Monday evening/overnight, at which point it would be exiting Downeast ME and moving into the Canadian Maritimes.  As the LLJ moves over southern and easternNew England, the risk of damaging winds appears to increase relative to areas further south.  

Guidance indicates an impressive LLJ across coastal southern NewEngland during the daylight hours on Monday – perhaps on the order of 100-115kts.  In most cases, a southeasterly wind direction is unfavorable for transferring momentum from LLJs to the surface due to the advection of cool maritime air onshore and resulting low-level thermal inversion.  With that said, there are some elements that appear to work in favor of stronger surface gusts in this setup.  First, vigorous southeasterly flow (>50kt at H85) should commence 12 or more hours before the jet core moves overhead at a given location, a substantial window of time to modify the cool and stable antecedent airmass.  Second, there is some signal that weak convective elements could be embedded within the broader rain shield, supported by a ribbon of weak MUCAPE in the 100-400j/kg range, which could support some transfer of winds from the lower atmosphere to the surface.  Finally, the diurnal timing could also result in a more favorable situation for strong winds at the surface, as warm sector temperatures nearing 60°F could help to erode the otherwise strong low-level inversion.  

In summary, confidence is quite high in damaging winds along the immediate coast. Though the potential exists for damaging winds further inland as well, it will be strongly dependent on the evolution of the thermal profile.