Pittsburgh (PBZ) — the National Weather Service forecast in plain English · Plaincast

Messages

Lake effect showers continue periodically through the weekend.

Dry and quiet weather returns for next week.

Discussion

KEY MESSAGE 1. Deep upper troughing lingers across the Northeast through the weekend while expansive surface high pressure gradually slides east out of the Plains. The resultant setup for us locally will favor periodic lake effect rain and snow showers, most prevalent in the daytime hours, supported by rounds of vorticity lobes streaming through and lake to air temperature delta-T exceeding favorable difference. The primary uncertainty is where exactly any heavier showers or localized bands set up. Ensemble clusters exhibit little disagreement in the overall pattern which should suggest that low-level flow holds on to a more northerly direction on Friday before backing northwesterly into the weekend which should shift higher coverage of showers from Eastern OH on Friday into Western PA for the weekend.

First, for the lower elevation, REFS probability hones in an area of (initially) wet snow showers falling into marginal surface temperatures in the pre-dawn hours of Friday morning from Erie County down into Mercer County and pivoting east through the morning. This should mostly turn over to rain as temperatures warm into the upper 30s to low 40s, though evaporative cooling with precipitation falling into a surface with near-freezing dew points could continue to mix in some wet snowflakes with the rain in any more persistent, stronger bands. Could envision perhaps some white patches on the grass and elevated surfaces, but no notable accumulation. Elsewhere, expect diurnally-driven coverage of scattered rain showers each day through the weekend with modest daytime heating steepening low-level lapse rates. Some snowflakes could mix in as far south as Pittsburgh metro early Saturday morning with no accumulation expected. Overall coverage is favored to be greater for Eastern OH on Friday and for Western PA on Saturday and Sunday as low-level flow backs more northwesterly and directs coverage farther east. All told, probability for >0.5" of snow reaches about 50-60% along I-80, though think that will be periodic accumulation in the periods favoring colder temperatures (overnight and early morning) with melting in between.

Where it likely does remain cold enough for snow to be more favored will be the PA and WV ridges, though the most favorable low-level flow direction for upslope snow won't come until Saturday. Some light snow showers are possible on Friday, but coverage should be at its most on Saturday. Even with that, moisture depth remains trapped relatively shallow beneath a sinking subsidence inversion barely allowing the depth to reach the DGZ at times and resulting in less favorable snow growth. Probability for >1" of snow focuses on the higher elevation Preston and Tucker at 45-55%. Overall limited liquid-equivalent and lack of deep DGZ moisture looks to lend a low chance of exceeding much more than that (10-20% of >2").

KEY MESSAGE 2. Upper troughing exits on Monday as the surface high eventually becomes the dominant feature. Increasing subsidence and flow backing more westerly will cut off the moisture fetch and mixing will gradually erode lingering stratocu coverage. Dry weather returns across the board into at least the middle of next week with temperatures eventually moderating back to above-normal values as the airmass modifies in weak southerly flow.