Area Forecast Discussion
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000 FXUS61 KPBZ 100801 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 301 AM EST Sun Dec 10 2017 .SYNOPSIS...
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Cold temperatures and periodic snow chances can be expected for the coming week. There is the potential for heavier lake effect snow showers from Tuesday through Wednesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Weak moisture and lift associated with the shearing mid-level system that dove to our south overnight is peeling off to the east slowly this morning. Resulting snow accumulations were generally in line with expectations, expect decidedly lower in the ridges as all of the deepest moisture dove to the south of our CWA. That said, many portions of the CWA outside of the mountains likely saw their first quasi-measurable snow of the year, so the overnight was notable at least for that. As the system from overnight peels off to the east, low level flow will fairly quickly turn toward the southwest by this afternoon. This will result in drying the the column, and some potential for eventual partial breaking up of some cloud cover over the southern half of the area. That said, even as precipitation ends from the southeast to the northeast through the day, cold air will be stubborn to depart even as flow turns southwesterly. Thus, highs generally across the board will have difficulty mustering even freezing. Fries
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&& .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/...
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A very busy short term period is ahead this upcoming week as a succession of waves will allow for a deepening upper trough to settle over our region with the coldest air of the season so far in the cards. That said, with the Great Lakes still fully open for business, this will mean a very strong potential for lake effect snow showers and squalls beyond Monday night, as well. The first wave looks to pass tonight with some low level convergence noted at 850 mb as it passes combined with left exit region upper divergence. This combined with modest mid-level moisture should result in the development of some light snow overnight again, even though model qpf is essentially nil. PoPs were increased for this, however lean qpf should result in almost no accumulation. Warm advection returns to the area for Monday ahead of a much deeper wave that will be the last stand for anything resembling a temperature near freezing. As moisture transport increases ahead of this system on Monday evening, isentropic ascent increases in the 290-300K layer co-located with a minimum in condensation pressure deficits, light snow should develop. The best ascent/moisture overlay is noted to the north of Pittsburgh. This is also were the lifting profile comes the closest to the dendritic layer. Thus, while still not an optimally efficient light snow, a swath of a couple of inches of snow at least will be likely across the northern half of the CWA, while south of there, amounts will drop off rapidly due to insufficient saturation depth and a warmer boundary layer. Things start to get much more interesting in the wake of the Monday night system as a secondary rapidly reinforcing mid-level push of cold air comes down over Lake Erie. This sets cold advection in motion in the entire column and also rapidly trends the wind direction toward a WNW orientation. With cold advection through the column, lapse rates will quickly increase and inversion height summarily shoot through the roof, likely maximizing over 11 kft. This will allow for a long duration of lake effect snow potential across the I-80 corridor beginning Tuesday and continuing into Wednesday. Also of note during the Tuesday to Wednesday period, continued cyclonic flow aloft with additional mid-level energy will be traversing through the trough. That said, while dominant multi- lake bands are likely to setup, mid-level perturbations may cause their positioning to wobble a bit as they pass. This would work to tone down the maximum accumulations, however it would spread them out over a bit wider area. Regardless, with a very long period of heavy snow potential over the snowbelts of NW PA, a winter storm watch for heavy snow will be issued that covers both the system snow from Monday night as well as the quick transition to lake effect Tuesday through Wednesday. While the QPF will likely be front end loaded in this interlude, rapidly cooling through the column and destabilization will overcome the QPF reduction with snow-to-liquid ratios trended toward and above 20:1 easily. Very heavy snow accumulation is possible locally during this period in the I-80 corridor. Farther south, things get a bit more complicated. It would seem as cold advection hits the ridges, there will be a period of enhanced upslope snowfall centered on Tuesday morning there. However, moisture peels off quickly, and flow direction never becomes truly NW south of the lake induced troughing downstream of Lake Erie. That said, several inches of snow during the day Tuesday at least seems possible. For the rest of the area, accumulating snows will be possible Tuesday through Wednesday, however exact downstream banding from Lake Michigan over Ohio and Lake Erie/Huron over PA will be difficult to pin down farther inland from the lakes. These details will need to be hammered out better in the future. Fries
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
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As flow rapidly turns more southwesterly by Wednesday night, lake effect snow should wind down. Likewise, warm advection will increase into the extended forecast and begin the slow wander back toward more normal temperatures. This will result in a weekend closer to normal before it seems the cyclonic flow pattern and below normal conditions again return to the area. Fries
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/...
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Light snow will move through the forecast area overnight, ending at all ports shortly after dawn. MVFR ceilings are expected into the afternoon, with the back edge of the stratocu moving quickly through late in the afternoon, returning all ports to a brief period of VFR. The more intense pockets of snow that will move through overnight, are very isolated, so visibilities could bounce from high IFR to VFR depending on whether the more intense cells go over a port. A SW to W wind shift is expected overnight, with wind gusts picking up to 20kts due to steepening lapse rates in cold advection. .Outlook... Scattered restrictions in light snow are expect tonight into Monday. The next chance for widespread restrictions will come with low pressure progged for Monday night/Tuesday.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES...
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MD...None. OH...None. PA...Winter Storm Watch from Monday evening through Wednesday afternoon for PAZ007>009-015-016. WV...None.
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&& $$

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