Area Forecast Discussion
Issued by NWS Pittsburgh, PA

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000 FXUS61 KPBZ 170331 AAA AFDPBZ Area Forecast Discussion...UPDATED National Weather Service Pittsburgh PA 1131 PM EDT Sat Mar 16 2024 .SYNOPSIS... The first in a series of cold fronts will cross the region tonight into Sunday morning, creating a line of mainly light rain showers. Additional cold fronts and upper disturbances will create periodic precipitation chances again Sunday night through Tuesday morning that may lead to limited snowfall accumulation. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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KEY MESSAGES: - A cold front arrives tonight with gusty wind, rain showers and introduction of a cooler air mass. ------------------------------------------------------------------- Update... Dry air was in place in the lower levels based on recent surface obs, and upper air analysis. A low level jet was also across OH and western PA, with 850 mb wind speeds around 45 kt. Some of this wind is mixing to the ground on the leading edge of a band of showers, as evaporation occurs. Updated the forecast to include wind gusts up to 40 mph as the rain begins. Previous... The passage of an upper level trough and surface low into Quebec will push a well-defined cold front southeast through the region tonight. Rain showers ahead of the boundary are likely to be light (most locations see less than 20% chance of exceeding 0.25" total), with decreasing expected amounts from NE to SW due to farther removal from stronger upper forcing. Ensembles favor fairly rapid drying post-frontal passage as the dry westerlies develop aloft with the main upper trough axis hanging to the NW.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... KEY MESSAGES: - Cold advection plus weak lake fetch will support isolated to scattered showers favoring northwest PA and the higher terrain. - Precipitation type will be a mix of rain and snow during daylight hours before switching to snow. ------------------------------------------------------------------- In the post-frontal environment, the Upper Ohio River Valley will be positioned within general westerly flow as the upper trough axis remains over the Western Great Lakes. There is some discrepancies among hi-res and global models on angle of the boundary layer flow, which will play out in one of two scenarios for the afternoon (forecast more closely tied with scenario 1). Scenario 1: A more W-E to WSW-ENE orientation should maintain dry slotting over the region and create little to no fetch of lake moisture to support afternoon showers. Temperature will be near to slightly above average, though gusty wind will keep `feel-like` temperature down. Scenario 2: A more WNW-ESE orientation would allow for some lake moisture influx to the boundary layer, combining with broad ascent to foster isolated/scattered afternoon showers. Cold advection aloft would remain limited and thus still see a mix of rain/snow with no accumulations. Temperature is closer to average while wind remains gusty. Overnight, a shift of the upper trough axis towards Lake Erie will send a secondary cold front towards the region while cooling 850mb temperature. A slight increase in boundary layer moisture while maintain broad ascent (though losing diurnal heating) will enable a continuation of isolated light snow showers. Lack of deeper moisture and stronger forcing will ensure any snowfall accumulations during this period will be light (if any, given warmer grounds). && .LONG TERM /MONDAY THROUGH SATURDAY/... KEY MESSAGES: - Seasonably cool temperature favored through mid week and may persist into the following weekend. - Large variation in snowfall accumulation during early week scattered snow showers. - Uncertainty remains large for next synoptic pattern to end week. ---------------------------------------------------------------- The weather pattern through Tuesday favors persistent upper troughing over the Great Lakes that will feature embedded shortwaves crossing the Upper Ohio River Valley region. This regime features below average temperature and waves of scattered to potentially numerous showers, with the general thermal profiles favoring snow (though a few locations may see rain mix in during the warmer afternoon hours). The largest discrepancy at this time appears to be moisture availability; while not large, 24 hour QPF 75th/25th percentile spreads from ensemble model data suggest range from nothing to 0.10". That shows potential for either little falling snow or localized bands that could see 1 to 2" of accumulation (without accounting for ground impacts, etc.). Current forecast represents a blend of options; at this time, it remains unlikely for much snowfall impact even if higher- end moisture values develop. Rising pressure heights should promote a drying and warming trend mid-week, but once again, ensemble spread becomes large in diagnosing the evolution of weak upper low leaving the Four Corners region and depth/strength of the lower Canadian trough. In general, two favored solutions which suggested either a cooler, drier weekend pattern (deeper Canadian trough that keeps southern wave farther south) or a warmer, wetter pattern (flow becomes mixed and allows for some influx of moisture from southern wave). && .AVIATION /04Z SUNDAY THROUGH THURSDAY/... A cold front will approach from the Upper Great Lakes region tonight. This front will cross the Upper Ohio Valley region tonight around roughly 04Z - 06Z through Sunday morning 12Z, with showers and MVFR restrictions expected. In addition, SW wind gusts to 20-25kt are expected with mixing and a tightening pressure gradient ahead of the front. A WSHFT to the NW, with additional gusts to 20kt are expected after tonight`s FROPA. There will be a period tonight before the approach of the front where some LLWS will be present. Expect this to last through 03Z-04Z until the front brings the stronger winds to the surface. Conditions will be slow top improve heading through the day tomorrow as northwest flow will continue to bring in scattered to broken MVFR cigs through the day on Sunday. Scattered passing showers/snow showers will also be the case on Sunday. Strong northwest flow will continue and winds are still expected to be gusting up to 20 knots at times during the day on Sunday. Most snow shower/rain shower impacts will be confined to the north and in the ridges to the east. .Outlook... Restrictions and scattered snow showers return Monday night and Tuesday with a reinforcing cold front and subsequent upper troughing. VFR returns Wednesday as a ridge of high pressure builds in. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...Frazier NEAR TERM...WM/Frazier SHORT TERM...Frazier LONG TERM...Frazier AVIATION...Shallenberger

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