Area Forecast Discussion
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000 FXUS61 KPBZ 091357 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 857 AM EST Sat Dec 9 2017 .SYNOPSIS...
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Cold weather continues. Most locations will see a period of light snow tonight, early Monday morning, and again on Tuesday. evening.
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&& .NEAR TERM /THROUGH TONIGHT/... A veritable chasm of dry air is encompassing the area sandwiched between the stream of Gulf moisture working up the Atlantic seaboard and a rapidly weakening system diving southeast out of the western Great Lakes. Model RH projections suggest that the clearing currently evident over Indiana, northern Ohio, and Lake Erie will most likely translate toward and then across our area through the next 12 hours or so. The net result with very low dewpoints in place will be a continuation of dry and cold conditions with the only chance of anything other than that being in the higher terrain of Garrett and Tucker counties, where some evidence exists that a bit of the current virga on radar may briefly make it to the ground as a touch of light snow. That said, dewpoints in the terrain currently running in the single digits and lower teens will lend themselves to very poorly formed crystal development and thus little/no potential for accumulation. Clouds associated with the system diving out of the western Great Lakes will eventually make their way into and through our area. As this occurs, the surface low associated with this system will be rapidly filling. Additionally, mid-level energy with this system will start to rapidly transfer to the coast. Given that the upper pattern and jet positioning are largely unfavorable for significant upward motion vis-a-vis our location in the left rather than right entrance region of the upper jet streak as this system drops through this evening and overnight, it would seem our potential for enhanced vertical motion significant enough to overcome our rather dry thermal profile is fairly low. That said, current projections suggest a roughly 3 hour window as the system moves through in which the column should saturate basically everywhere to allow for a short period of light snow in most all locations. Thus, PoPs were drawn up as likelies while qpf/snow were generally reduced from previous forecasts. While deep saturation isn`t very apparently with the large scale lift portion of this system, as it departs to the east and flow becomes upslope into the ridges, deep saturation isn`t really required if the thermal profile is suggestive of efficient crystal development. In this case, even with inversion heights only running around 4 kft, boundary layer moisture co-located with the dendritic layer below 4 kft should allow for snow showers to continue through the night in the upslope ridge areas. That said, qpf still looks rather lean and even heading toward the higher end of the guidance with 20:1 snow to liquid ratios, it seems most locations should stay just below the criteria for a winter weather advisory. While light snow accumulations will be mentioned in the HWO for these areas, advisories will be foregone for this cycle. Fries && .SHORT TERM /SUNDAY THROUGH TUESDAY/... While Sunday looks to be a short period between weak northwesterly waves, yet another very weak wave approaches int northwest flow for Sunday night. Good large scale agreement exists amongst the EC/GFS/NAM on this, however their moisture profiles and hence qpf are all over the map. Given some evidence of warm advection around 850 hPa and ample saturation through the dendritic layer with weak isentropic ascent ahead of this wave, PoPs were increased especially southwest of Pittsburgh and into Ohio as this weak wave translate southeast. However, a quick passage and modest moisture feed mean only very light snow accumulation should be anticipated. Warm advection looks to begin anew on Monday with southwesterly flow increasing toward the area as the next system starts to again drop through the western Great Lakes in the overall cyclonic pattern. Light snow should breakout ahead of the warm front that pushes toward the area Monday evening. The preponderance of guidances suggest the best corridor for ascent with this will be north of Pittsburgh, however the GFS is a notable outlier and draws the swath of heaviest qpf right down the PA Turnpike given a bit colder solution overall in our region and a farther south storm track. The current thinking favors the stronger clustering, however trends will be monitored. Regardless, much colder air moves toward the region as the system passes and a cold front pushes through. Northwest flow over the Great Lakes, a rapidly destabilizing boundary layer, increasing lake to 850 mb delta T values, and the coldest air of the season moving in should start to increase lake effect and upslope snow showers rapidly Tuesday. Fries && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... Conditions for a continuing of heavy lake effect snow showers and squalls as well as upslope snows into the ridges continue to be fairly optimal right through Wednesday. While wind directions are difficult to pin down this far out, it certainly seems as though it will be more likely than not that winter headlines will be in the cards for lake effect/upslope snow. These likelihoods will be covered in the HWO for now, however the snow belt regions should be prepared for accumulating snow. As northwesterly flow starts to ebb by Thursday, another system looks to dive southeastward toward the area. Warm advection ahead of it suggests there is potential for some mixing with rain as precipitation develops, however with this system, the GFS is far warmer than the ECMWF. Behind this system, cold air again filters in the region to end the work week well below normal. Fries && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... VFR conditions are expected through much of the day, with only high clouds present. By evening, a disturbance digging into the Great Lakes will bring lowered ceilings and snow showers to the area. Once the lower cigs and vis reach the region, they will remain through the rest of the forecast period. VIS will be general MVFR and CIGS will be MVFR to occasional IFR, particularly at FKL. Surface flow will generally remain out of the south at 10kts or less until FROPA later tonight. .Outlook... The next chance for widespread restrictions is Sunday night into Monday with increased chances for light snow. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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