Area Forecast Discussion
Issued by NWS Pittsburgh, PA

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000 FXUS61 KPBZ 181823 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 223 PM EDT Mon Mar 18 2024 .SYNOPSIS... Scattered snow showers are expected on and off through Wednesday with a crossing upper trough. Dry weather is then expected to return as temperatures gradually warm through the end of the week. && .NEAR TERM /THROUGH TONIGHT/...
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KEY MESSAGES: - Convective snow showers/graupel today. - Minor accumulation on grassy/elevated surfaces north of Pittsburgh. - 1-2" is possible with a localized 3 inch total in the ridges. ------------------------------------------------------------------- Expect the axis of an upper trough to continue to pivot through the area today with cold advection in northwest flow dropping 850 mb temperatures to as cold as -11C and 500 mb heights fall. Shallow instability will develop into this evening as the coldest air moves overhead and sufficient diurnal heating from an increasing mid- March sun angle resulting in convective snow showers intensifying into this evening. Hi res soundings show depth of the moisture reaches to the top of the dendritic growth zone as the lapse rates steepen which will allow for heavy bursts of snow/graupel. Coverage will be reinforced in the early overnight hours despite the loss of instability as a surface trough slides south across the area. This may be the best shot for some minor accumulation up to an inch to two, especially in any areas where snow showers persist, north of Pittsburgh as the surface cools. Probabilities are showing roughly 50% to 70% for an inch of accumulation in the counties along I-80 and in the higher terrain. Overall, outside of the ridges, accumulation will be highly variable and dependent on where the snow showers develop, but with a warm ground at this time of year and unfavorable afternoon timing, expect that accumulations will be confined to grassy/elevated surfaces and bridges and overpasses. In the ridges, 1-2" is expected with aid from upslope flow. Given the instability and upslope component, they may be an isolated 3 inch amount in the higher terrain from this snow shower event.
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&& .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
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KEY MESSAGES: - Scattered snow showers Tuesday and Wednesday with best chances for I-80 corridor. - Light accumulation possible. - Potential for strong wind gusts Tuesday night - Wednesday. - Low RH and elevated winds possible for Tuesday. ------------------------------------------------------------------- The upper trough axis gets an assist from a trailing shortwave and pushes out of the area on Tuesday allowing surface ridging to briefly build from the south. Another upper wave dives out of the Great Lakes later Tuesday and associated low pressure will slowly meander by to our north Tuesday into Wednesday. The best forcing and moisture will remain off to our north with low end snow shower chances mainly north of I-80 as ridging keeps the rest of the area dry through the daytime hours. A surface trough will sag through the area Tuesday night with vorticity advection streaming along with it and bring snow shower chances further south into the PA and WV ridges overnight with the probability for measurable snowfall highest in the WV ridges at 50% to 60%. This stands to be the best area with snowfall chances. In addition, outside of areas that get the heavier snow showers such as the lower elevations and south of I-80, expect drier air to infiltrate the areas in OH and WV and into some of SW PA. This will lead to low RH and elevated winds. An SPS will likely be needed for Tuesday afternoon. Colder air aloft moves in on Wednesday as the upper trough sags south with several shortwaves rounding its base. Low level lapse rates will steepen as a result and with the passage of a cold front, we`ll see snow chances continue on Wednesday again primarily for the I-80 corridor with the best moisture. Forecast soundings show some marginal instability with ensemble probability of >50 J/kg around 50% to 70% there, so the potential for some heavier banded snow showers is there and supported. Given the unfavorable diurnal timing and higher March sun angle, would be tough to accumulate efficiently on surfaces and result in more of a visibility concern, but any showers with higher rates may overcome the warmer ground and put down a quick accumulation. That said, the higher end of the distribution for snowfall totals seems reasonable for Wednesday, but it will be a day where the higher totals are localized to areas that see heavier snow showers, most likely north of Pittsburgh. Wind gusts will also increase with the passage of the front as the gradient tightens with highest gusts expected on Wednesday. Ensemble probability of exceeding 30 mph gusts is 80-90% areawide and will have to monitor the WV ridges for potential wind headlines with probabilities of advisory criteria increasing for Wednesday where gusts to 40-50 mph are possible.
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&& .LONG TERM /THURSDAY THROUGH MONDAY/...
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KEY MESSAGES: - Below average temperatures Thursday rebounding to near normal for the weekend. - Rain/snow chances return Friday. ---------------------------------------------------------------- By Thursday, the upper trough pivots out through Nova Scotia as a brief bout of surface ridging noses in from the north. This should shut off any lingering lake effect precipitation Thursday morning and return dry weather for a day. A few ensemble members linger the trough a bit longer which keeps low probability snow shower chances around into the late morning on Thursday, but any additional accumulation would be minimal. Things then become a bit more uncertain with timing and strength of a flat shortwave sliding through the Great Lakes on Friday in relatively zonal flow returning the chance for snow showers mainly for the I-80 corridor and north. Lack of injection of cold air ahead of it with the progressive nature of the wave should keep any accumulation on the lower side. Going into the weekend, a southern stream shortwave slides through the Southeast and attempts to phase with the northern wave. Low pressure looks to develop across the Southeast and turn up the coast. The most likely ensemble solution at this time is a phasing offshore of New England keeping the low out of our hair, though a low probability chance exists for quicker phasing which would result in higher precipitation chances continuing from Friday into the first half of Saturday. Models and ensembles seem to also provide a bit of consistency to the Sunday and Monday forecast bringing in high pressure to the area with the potential of a few upslope showers.
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&& .AVIATION /18Z MONDAY THROUGH SATURDAY/... Ceilings remain VFR during the early afternoon, although stratocumulus has overspread the region as a shortwave trough crosses from the northwest. Low-level dry air should keep ceilings above the MVFR threshold through the afternoon into the evening. The main issue into the evening will be scattered to numerous snow showers, which have increased in coverage with the arrival of the shortwave and with a modest increase in instability due to daytime heating. While terminals will experience VFR visibility for the majority of the period, a few drops to IFR will be possible in heavier snow showers. Included TEMPOs into late afternoon at several terminals to cover this. Wind gusts of up to 20 knots can be expected with the mixing as well as a decent surface pressure gradient. Snow shower intensity will drop off as daytime heating is lost, although some modest accumulation will become more likely after the sun sets. Ceilings should drop slightly into the MVFR range overnight as the boundary layer cools and moistens a bit, and as the inversion level drops. A few snow showers may linger north of I-80 past 12Z Tuesday, with dry conditions otherwise. Wind will be gustier by midday, with peak values of up to 25 knots out the west-southwest. .Outlook... MVFR conditions may linger into Wednesday with a new shortwave and burst of colder air from the north. VFR returns Thursday as high pressure builds over Ohio. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...Shallenberger NEAR TERM...Shallenberger SHORT TERM...MLB/Shallenberger LONG TERM...MLB AVIATION...CL

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