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FXUS61 KPBZ 041913 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 213 PM EST Mon Dec 4 2023

SYNOPSIS
Isolated rain and snow showers are expected north of I- 80 today with a weak upper disturbance. More widespread, high- probability rain and snow will occur tomorrow, with some accumulation likely for higher elevations. A warming trend with mostly dry conditions is expected through the next passing disturbance on Sunday.

NEAR TERM /UNTIL 6 AM TUESDAY MORNING/
The region remains wedged between two surface lows, one centered over New England and the other passing over the southern Appalachians. A weak surface trough between the two systems is expected to pass during the day today. Precipitation mentions remain confined to northwest Pennsylvania with limited moisture above 850mb. As flow modulates more west-northwesterly with the trough passage, moisture from Lake Erie will increase precipitation chances north of I-80 this afternoon and evening. Though surface temperatures are expected to remain above freezing, dry adiabatic lapse rates drop off temperatures readily with height. Therefore, a mix of rain and snow is forecast for the northernmost zones, with conditional snow chances increasing after sunset. With surface temperature above freezing, any accumulation will be difficult with this weak disturbance. If there are any accumulations, they will likely remain light and after sunset. NBM 90th percentiles cap any totals at a couple tenths of an inch at best. High temperatures today are expected to remain close to normal for this time of year.

Into tonight, forecast sounding maintain shallow instability into a saturated layer around 900mb to 850mb. Saturated layer depth increases to the north. Further, precipitation chances remain possible through the first half of the night, increasing towards the I-80 corridor. Towards the early morning hours, a brief bout ridging and mid-level subsidence is forecast to reduce saturated layer depth and decrease precipitation chances through daybreak. Lows are expected to remain seasonable.

SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/
The next chance of precipitation will arrive during the afternoon hours on Tuesday as the next system makes its way into the area.
There will be two main aspects of this system. The initial forcing for precipitation will be a shortwave passage. This is expected to encourage ascent. Environmental lapse rates will still remain steep and saturated through around 900mb due to existing low level cold air. This will bring snow mentions for most of the area into the day. There is some uncertainty in surface temperatures depending largely on the amount of low- level cold air. This will govern precipitation type in the lowlands through 00Z. Current thoughts reflect that highs will remain towards the lower end of guidance with cold 900mb temperatures and minor influences of wet bulbing with precipitation onset. Nonetheless, there is high confidence that lowland maximum daytime temperatures remain above freezing, limiting accumulations below 2000'. Snow as the predominant precipitation type or any light accumulations remain more likely the farther north with lower ambient temperatures.

Above 2000', near-to sub- freezing temperatures are more likely. Further, accumulations are more likely during the daytime with roughly 50% to 70% probability. Low level winds are expected to remain light and westerly throughout the day; this will not utilize optimal orographic lift, but enhancement is likely nonetheless in low- level marginal instability. Some ascent and saturation in the DGZ mean that higher end scenarios could not be disregarded in the ridges through 00Z despite low probabilities of exceeding an inch in current NBM guidance.

The parent trough and affiliated surface low are expected to pass during the overnight hours, cooling temperature aloft and veering winds north-northwesterly. This will act to do two things; enhance lake-moisture influences over much of the area and cool temperatures aloft. This will reduce stability in the low levels and allow orographic influences to pick up. Dendritic growth remains possible ahead of the trough, but will reduce after the its passage.

For the lowlands, a light coating is possible overnight as temperatures near the freezing mark. The lack of daytime heating will allow for accumulations with higher probability. For the higher elevations, all factors will come together and the current forecast tends toward the higher end of guidance. There is still considerable spread in ensembles with struggles in boundary layer moisture and uncertainly in the extent of orographic influences. NBM suggests a 60% to 85% chance of exceeding an inch in the Laurel Mountains and ridges of West Virginia through Wednesday afternoon. Greater than 50% probabilities of exceeding two inches are confined to the ridges of West Virginia. For eastern Tucker County, there is a 25% chance of warning criteria snow of 6". All things considered, these probabilities should be considered with caution, as the high end NBM guidance under-mentions amounts above the 90th percentile. Opted to go with a Winter Weather Advisory for eastern Tucker County for now with totals between 3 to 5 inches forecast. An upgrade to warning may be necessary if the forecast trends upwards. The issuance of an advisory for the ridges of Preston and the highlands of Westmoreland and Fayette could also not be ruled out at this time and will be considered in nearer the event time.

Through the rest of the day on Wednesday and into the overnight hours, northwest flow will be maintained over the area, and a shallow saturated layer is forecast with low-level instability below a weak subsidence inversion. Given limited moisture availability and synoptic forcing, additional snow accumulations seem unlikely save a few tenths in the ridges. Temperatures are forecast to remain 5 to 10 degrees below average.

LONG TERM /THURSDAY THROUGH MONDAY/
A rebound in temperature is expected through the end of the week with high confidence in a building east coast ridge. The corresponding trend towards upper subsidence and low-level southwest flow with no signals of synoptic ascent will suppress precipitation chances as well.

By Saturday, southwest flow will accelerate ahead of the next passing trough. This will bring warm advection back into the picture. There is the chance that many areas could see the 60s by Saturday, with higher end probabilistic guidance topping out 20 degrees above climatological normals.

The next trough passage on Sunday is expected to be potent, with our area in the warm, moist sector of a deepening Great Lakes low.
There is high confidence in precipitation with this system, with only rain mentioned in the forecast with temperatures well above freezing. Probabilistic QPF is between 0.25" on the low end and around 2" on the higher end at this time. Cooler temperatures are likely early next week after the trough passage in cold advection.

AVIATION /19Z MONDAY THROUGH SATURDAY/
Cold air advection will maintain MVFR stratocu cigs through the day. A crossing shortwave will increase chances for light snow mainly north of KPIT later this evening, but impacts will be minimal. Probabilities of IFR cigs are less than 20%.

OUTLOOK
Restrictions will continue through Wednesday as sfc low pressure brings more widespread (but light) snow to the region. IFR chances are greater than 50%.

PBZ WATCHES/WARNINGS/ADVISORIES
PA...None.
OH...None.
WV...Winter Weather Advisory from 4 PM Tuesday to 7 PM EST Wednesday for WVZ514.

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