Here is some of CTP’s latest Forecast discussion:
”LONG TERM /MONDAY THROUGH SATURDAY/...
KEY MESSAGES: * Increasing potential for plowable to possibly heavy snow Monday night into early Tuesday (February 12-13th) *
A Winter Storm Watch has been issued for portions of central Pennsylvania Monday evening through Tuesday afternoon.
* Some uncertainty still exists on the western fringe and across southern PA, where the placement of the low could allow for longer periods of rain, lowering snowfall totals. All medium range guidance phases a potent southern stream shortwave with the northern branch of the jet early Tuesday, as if lifts northeast from the Miss Valley. The bulk of model guidance tracks the associated deepening surface low just south of PA and off the Delmarva Peninsula by early Tuesday morning, placing much of Central PA in the bullseye for the heaviest precipitation associated with strong warm advection and upper level diffluence north of the low track. A dual jet streak structure favors a region of enhanced fgen forcing in this region north of the low track. Model cross section analyses are also indicating a good instability signature between 06Z-12Z Tuesday, with a layer of negative EPV located in the layer just above the frontogenesis. This scenario supports the potential of intense banding somewhere over Central or Southern PA late Monday night. As for ptype, model consensus currently points to an initial rain/snow mix changing to snow late Monday night in association with dynamic cooling initially, then cold advection on the northwest periphery of the exiting low. Although this scenario definitely supports the possibility for a period of heavy snow across a large portion of Central PA, there remains some significant model spread in the exact low track, which could alter exactly where the heaviest precipitation falls. The 18Z GEPS for instance tracks the surface low considerably further south, which could result in the heaviest precipitation falling over extreme Southeast PA with little over the northern half of the state. The current Winter Storm Watch highlights where model consensus currently supports the best chance of a 6+ inch snowfall. Locations to the north and west of the main swath will get less precip and thus less snow overall, while areas further south (closer to the low track) will have to contend with a longer period of rain before any changeover. Timing has become more clear with recent model guidance, suggesting the heaviest snow will fall during the predawn hours of Tuesday, with the most significant travel impacts expected for the Tuesday morning commute. Please stay tuned for updates to the forecast, as details continue to be ironed out in upcoming forecast cycles.”