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*** Nor'easter develops off Mid-Atlantic Coast Sunday with impacts in the Northeast through Monday ***
Complex phasing evolution between a strong n/s shortwave and s/s disturbance migrating east out of the Rockies will generate a significant SLP maturation along the Atlantic seaboard with appreciable impacts poised for a large portion of the Mid-Atlantic and Northeast. Trends the last 24hrs have favored a cleaner phase scenario with the 500mb height pattern amplifying once east of the Mississippi, negatively tilting as the pattern evolves downstream into the Ohio Valley and east coast. Model discrepancies are beginning to shrink with the majority of ensemble members now showing a more "tucked" SLP solution, albeit location of primary coastal is still being worked out as it reaches the latitude of MD/DE which would imply fairly robust implications in terms of the positioning of the CCB development and accompanying norlun (inverted) trough axis that will likely develop due to the phasing pattern and 500mb ULL progression across the central and southern Mid Atlantic. Sensible weather pattern will likely begin early Sunday morning with a light precip field entering the central and southern Mid Atlantic, encroaching a relatively benign antecedent airmass with the lower PBL likely to be marginal or just too warm for snowfall until you get further north closer to the Mason Dixon and northwest of the fall line in the Piedmont where wetbulb temps are forecast to be near freezing at precip onset. Further south off the Carolina coast, surface cyclogenesis will materialize within a strengthening diffluent axis ahead of the amplifying mean trough, and within the LER of a strong upper jet rounding the trough base across the Southern Mid Atlantic. As this evolution materializes over the course of Sunday, expectation is for the precip field to blossom with height falls encroaching the region allowing for reputable diabatic cooling to aid in lower PBL temperatures to cool, thus changing the primary hydrometeors from a mixture of liquid/solid, to all solid, implying a shift to snowfall with a favored collapse of the rain/snow line from northwest to southeast. As this occurs, expectation is for rates to begin picking up under the influence of an ULL passage across VA leading to heightened ascent north of the closed 500mb progression with increasing coverage of banding structures as the dynamical processes begin to mature. Cross- sections from most of the numerical suite indicate appreciable omega across the central Mid Atlantic over to the Delmarva and points northeast as the system continues to evolve and the cyclone deepens rapidly off the Mid Atlantic coast. Areas furthest west from the primary surface low will, at the very least be under a threat from a maturing norlun trough that is pretty well-defined within several of the main NWP outputs, including the global deterministic. This area will be a relatively narrow corridor of heavy precip with significant ascent allowing for a band of heavy snow to develop and slowly progress east- northeast through the storms life cycle. Some guidance goes as far as a full "capture" at the mid-levels which allows for the surface low off the Atlantic coast to slow considerably and tuck closer to the coast allowing for a more pronounced coastal enhancement to be thrown back further west with a robust QPF distribution within a defined CCB axis that will develop to the west of the surface reflection. These outputs are generating the more significant snowfall accumulations across the Mid Atlantic to Southern New England in NWP output. That is only one potential outcome however, as current cluster analysis indicates two other viable scenarios that still deliver solid snowfall totals, but not as pronounced due to a lower influence directly from the coastal. Inverted trough axis is most likely to benefit areas west of the Chesapeake Bay up into PA with the coastal impacts likely to be felt from coastal Delmarva up through coastal NJ, Eastern LI, and Southeast New England. These areas have seen an appreciable uptick in the prob fields for all reputable totals, but especially >6", which is up to 40-80% across the aforementioned areas. The areas with more question marks are further west and northwest away from the coastal areas just due to potentially missing the coastal enhancement and losing some of the precip to the marginal airmass presence. That said, this setup is forecast to be very dynamic with appreciable ascent within the column with a favorable 850-500mb evolution capable of producing snowfall rates >1"/hr for hours, even within that norlun trough axis that will develop.
