... Dry and seasonable weather is expected this weekend for the Upper Ohio Valley and Allegheny Mountains. A strong low pressure system will then bring rain on Monday, followed by possibly accumulating snow from Monday night through Tuesday, along with much colder temperatures.
NEAR TERM /THROUGH SUNDAY/... Cloud cover with the occasional sprinkle persists across western PA and parts of northern WV this morning. After dawn today, high pressure will build in wake of the trough, quickly eradicating any area sprinkles. Subsidence and much drier air above 900mb will likely result in a mostly sunny day. However, the NAM Nest model continues to indicate a cloud layer persisting beneath a subsidence inversion near 900mb, while virtually all other guidance suggests otherwise. Highs today will largely depend on the degree of sunshine. Full sunshine through peak heating will lead to highs in the mid/upper 40s, while the NAM Nest cloudy solution would result in lower values. Have used blended guidance that is closer to the former, resulting in highs in the mid/upper 40s, give or take a few degrees.
Model guidance is in better agreement for the overnight period, with temperatures falling in the lower 30s and upper 20s for overnight lows underneath a mostly clear sky and absence of wind.
Dry and warmer conditions can be expected Sunday as the ridge amplifies in response to deepening systems across the central CONUS. Sunday will be a sunny day, perhaps the last in quite a while, though some higher clouds may eventually overspread the area by the latter half of the day as the subtropical jet inches northward.
SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/... After a rather meteorologically-benign weekend, a low pressure system is expected to bring a mix of rain and snow to the Upper Ohio Valley and Allegheny Mountains.
Models continue to indicate the phasing of a northern stream shortwave dropping out of the Upper Midwest and southern stream system ejecting out of the ArkLaTex region. The resultant surface cyclone will deepen and move northward along or just west of the spine of Appalachian mountains. As the low approaches the Upper Ohio Valley, precipitation associated with the warm front shield on the north side of the comma low will overspread the area south to north under highly diffluent flow aloft. At precipitation onset and through the first half of Monday, precip-type will very likely be rain for all locations across the forecast area as boundary layer temperatures remain quite warm.
Much colder air will then cascade southward and wrap around the low, eventually resulting in a transition from rain to snow across eastern Ohio and perhaps NW PA by sunset or soon thereafter, followed by a transition across the remainder of the area through the overnight hours. The deformation band in the trowal sector of the low may lead to a brief period of heavy snow on the northwest side of the low, which could end up as far south as somewhere from perhaps New Philadelphia OH to Mercer PA, but confidence in longevity of band over one particular location, position, and transition time all remain low.
Snow showers will likely continue through much of the day Tuesday and into Tuesday on the backside of the departing low thanks to wrap around moisture and divergent flow in conjunction with some lake enhancement. However, it doesn't appear there'll be any strong mesoscale features resulting in abundantly high snow rates on Tuesday.
Regarding snow accumulations, models have come in to much closer alignment with the lows position and progression, which yields a higher confidence snow forecast... though the heavy snow possibility mentioned above is still questionable. Ensemble spread has been greatly reduced with the mean snow total accumulation for much of the Upper Ohio Valley, including Pittsburgh, somewhere in the 1 to 3 inch range... which definitely seems like a realistic solution. As mentioned above, New Philly to Mercer may receive a bit more if they're close enough to a mesoscale band, along with the higher elevations in the Allegheny Mountains which will receive an upslope component towards the end of the event. These locations may very well receive snow in the 2 to 5 inch range once the event wraps on Wednesday.
As for temperatures, highs will generally be in the 40s to around 50 on Monday afternoon followed by a sharp drop to the upper 20s and low 30s by Tuesday morning once cold air ushers in. Tuesday highs will likely occur at midnight, with afternoon temperatures remaining steady in the upper 20s and low 30s.
LONG TERM /WEDNESDAY THROUGH FRIDAY/... Cold northwesterly flow in the wake of the departing system will support continued chance for intermittent snow showers through Wednesday. Thursday will be dry before another set of northern and southern stream troughs transition across the region. The southern stream cyclogenesis will be a bit more potent and may result in a little rain across the southern forecast area (/northern WV) on Friday.