Excellent forecast discussion this afternoon from CTP concerning tomorrows snow event and Mondays. Very transparent and honest regarding the difficulty in both events. They like 1-2” tomorrow and likely 3-5” Monday with a current 25% chance of a widespread 5”+ amounts Monday “.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... Main conversation and consternation by the dayshift was centered around the possible/expected precip and what impacts it will have on the area on Friday. Newest guidance from meso models and ensembles has brought some clarity, but only for the expected precip/snowfall across the southern third of the area Fri aftn. The late aftn and very early evening hours (rush) are the most concerning of all. HRRR and fine mesh NAM output make a burst of heavy precip, most likely snow at the intensities they are predicting, for places mainly S of the PA Turnpike. However, the temps will be marginal at first in the valleys of the Lower Susq for accums to occur during mid-late day. The interesting hours appear to be 3-6PM when the low-mid level frontogenesis increase and some meager CAPE bubbles up. The SNSQ parameter is low during this time frame. It may be more of a CSI-banded feature that occurs as the strongest forcing moves thru - the sfc pres drops ~9mb as it crosses far srn PA on Fri. Whether it be SQ or CSI band, the precip appears as a relatively thin band (40mi?) oriented N-S. This could spell a very brief (30-45 min) period of heavier precip which would likely be snow as the intensity peaks, even where the temps are initially above freezing. This temp fluctuation/drop could spell more of a worry for the roads to ice up as the melted snow freezes up with a cooldown below freezing during the heaviest snow rates. While accums may only be an inch or so, and generally sub-advy (advy amounts are generally >=2" for the SErn cos), the possibility of a quick inch and the temp drop may spell commute/travel trouble for the metro areas, esp along and S of Route 30 and east of I-81. At this point, we`ll hold off on an advy due to the lack of 2"+ area-wide signals, and expected brevity of the heaviest snow. This may be better handled by SQWs than a broad advy. But, we`ll continue to weigh these factors and may issue an advy in due time. The precip/snow should be off to the east around 6-7 PM.” “Attention then turns to a larger low pressure system moving east out of the Four Corners region of the Western US, set to impact parts of the Eastern US Sunday night into Monday. Latest model consensus tracks the low across the Carolinas, certainly putting central PA on the cold side of the storm, but the question will be how far north moisture will make it. It is worth noting that most long range models keep the wave pretty flat as it approaches the coast, with little or no strengthening. Ensemble guidance is relatively tightly clustered, given this event is still out on Day 5. Cluster analysis reveals that the most likely scenario, represented by 75% of ensemble members, keeps the wave between the trough approaching from Mississippi Valley, the ridge over New England, and the deeper trough over the Canadian Maritimes (which has origins in Friday`s clipper system) on the flatter side, yielding less moisture advection and lower snow totals, with accums ranging from an inch or 2 along I-80 to 3-5 inches along the southern tier. The worst case (snowiest) scenario is shown by about 25% of ensemble members (mostly GFS/Canadian members with very little representation from the European ensemble system). In the snowier solution, the wave packet is more amplified, allowing moisture to advect farther northward and jet forcing to be stronger. This could force the heaviest snow axis to land within our CWA. In this case, most places south of I-80 would pick up 5+ inches of snow. Remember, according to the ensemble guidance, the lower- snow scenario is 3x as likely! All things considered, PoPs (all snow) were raised Sunday night into early Monday, with snow likely winding down through the latter half of Monday. This storm is still completely outside the range of deterministic QPF and snow forecasts.” .
