Area Forecast Discussion
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000 FXUS61 KPBZ 110932 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 432 AM EST Mon Dec 11 2017 .SYNOPSIS...
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Much colder temperatures will arrive on Tuesday along with snow. Heavy lake effect snow showers will be possible through mid- week. Some warmer temperatures temporarily arrive late in the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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No substantive changes needed overnight, although the gradient in PoPs has been sharpened to focus higher PoPs across the I-80 corridor. Any light snow showers in this vicinity should amount to no more than a light dusting, although most areas are not likely to see any meaningful accumulation. Previous discussion follows... A shortwave trough is moving esewd across the upper Ohio Valley region toward the Mid-Atlantic region. As it departs early this morning, any lingering light snow will dissipate, leaving behind mid clouds and a slow backing of the wind by mid-day. Modest warm-air advection will increase ahead of a second shortwave trough that will advance ewd from the Nrn Plains today. Despite the advection, temperature will remain below the seasonal average, but will be just a bit warmer than those seen yesterday. Any light snow that develops this afternoon in the warm advection pattern will be confined to the far nrn zones, where cold air will linger the longest. Conditions start to get more interesting going into the overnight hours as warm advection isentropic ascent starts to become sufficient to saturate the 290-295K layer such that light snow chances start to increase in the northern zones. Given the height at which the maximum lift occurs, a marginal boundary layer, and a less than convincingly saturated dendritic layer with this activity, though QPF projections suggest a few tenths will be possible, an efficient snow is not expected. As a result, just a couple of inches will be possible tonight over the northern reaches of the CWA in advance of a strong cold front that will be following the first of a few potent short wave disturbances through the area over the next several days. This will cause the column to cool rapidly aloft, dropping the dendritic growth layer toward the surface, destabilize the column, and cause far more efficient snow growth. Thus, snow chances rapidly increase along the front and also snow accumulation quickly become more efficient.
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&& .SHORT TERM /TUESDAY THROUGH THURSDAY NIGHT/...
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The cold front completes its passage through the area by early Tuesday, and much more favorable lapse rates in its wake combined with remnant boundary layer moisture and a dendritic growth zone dropping toward the ridgetops will more than likely mean a short interval of very efficient snowfall across the terrain. Tuesday morning was really hit quite hard in the forecast in the terrain, however given the amplitude of cold advection and an unfavorable fetch for lake moisture to intersect the ridges, while snow will continue there, the lack of moisture will mean quickly decreasing snow amounts beyond Tuesday for the ridges even though light snow showers will probably continue. For the ridges, snow will not necessarily be the biggest issue. In the wake of the front, while most of the area will be likely to see gusts in the range of 30 kts, the potential momentum increases rapidly off the deck, thus in the terrain, gusts in excess of 40 kts will be possible. With new and falling snow, this will lead to issues with blowing and drifting. Furthermore, as 850 mb temperatures fall off a cliff toward -19C by Tuesday and Tuesday night, much colder air temperatures will combine with the wind to cause sub-zero wind chill values across the area and sub -10 degree wind chills in the terrain. The combination of snow, wind chill, blowing snow, and wind potential in the ridges will likely lead to an advisory or potential warning in a subsequent forecast issuance. Elsewhere, a burst of heavy snow seems likely with the frontal passage. For most areas this will be rather brief, however with the dipping dendritic growth layer, it would seem all portions of the area could see at least some light accumulation as the front passes. The biggest impacts seem poised to occur as the lake effect starts to increase behind the front. This will be especially true across the areas north of I-80. While the immediate post-front environmental fetch doesn`t turn horrible northwesterly over the snowbelts, a secondary wave passage aloft of Tuesday night will give another trend toward more northerly component. This combined with a forecast dual- lake band setup from Lake Huron over Lake Erie and into NW PA will mean snow rates in the snowbelts should rapidly increase Tuesday night as inversion heights rise yet again over 11 kft, flow becomes better aligned through the deepening boundary layer, and frictional convergence increases farther inland as lake induced troughing wanders toward the I-80 corridor Tuesday night. Not only does moisture flow increase into the area while the boundary layer deepens during this time, however, but so too does crystal efficiency. Thus, snow-to-liquid ratios rapidly increase toward 22 or higher to 1. With all of these factors in mind, the winter storm watch for the Lake Erie snowbelts remains in effect from Monday night through Wednesday, though the heaviest lake effect potential during the interval is Tuesday night into early Wednesday. Further, while no headlines will be issued at the moment for the ridges, it would seem winter weather advisories at least will be needed early Tuesday for snow, possibly extending thereafter for blowing snow or wind chill concerns. Please refer to future statements from the National Weather Service regarding this. Elsewhere, depending upon how one fetch downwind from southern Lake Michigan into Ohio and a second fetch that extends across Lake Superior/northern Lake Michigan/western Lake Erie into western PA evolve, accumulations from additional less easily forecastable snow bands will be possible. As higher resolution datasets start to resolve this period, we will likely get a better idea as to how these potential bands may develop and evolve with some refining of the snow forecast likely centering around these bands. Everything should start to wind down by late Wednesday afternoon as flow turns more westerly and eventually southwesterly. Warmer air will quickly start to move into the region on Thursday, which given how cold the air it will be displacing is, the chance of light snow will increase yet again. At the moment, qpf looks fairly meager, so amounts of snow should be fairly light on Thursday with this as temperatures rebound toward freezing. Fries
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&& .LONG TERM /FRIDAY THROUGH SUNDAY/...
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The cold upper trough that has been in residence in the ern CONUS will be ejected newd and supplanted by progressive flow aloft, leading to the passage of a series of shortwave troughs for much of the medium-range forecast period. Return flow around Gulf Coast high pressure will lead to temperature moderation in the Sat-Mon/Tue timeframe, with maxima and minima around climatological average and periodic chances for precipitation. Thereafter, models suggest that a return to a colder pattern may be in the cards, as a H5 ridge builds again into Alaska, and a shortwave trough dislodges cold air into the ern CONUS.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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Residual light snow this morning in association with a now- departing shortwave trough will come to an end, returning all terminals to VFR (or worst-case high-end MVFR) by mid-morning. Highest chance for IFR conditions through early morning will be from KFKL-KDUJ. Warm air advection will strengthen by this afternoon as a second trough approaches from the Nrn Plains. Cloud height will lower gradually this evening, with restrictions and light, accumulating snow expected late tonight as a cold front approaches and crosses the region 12/0800-1200 UTC timeframe. .Outlook... A prolonged period of lake-effect snow showers is expected in the region, especially in the vicinity of KFKL/KDUJ. Widespread restrictions can be expected Tue-Wed before improving Thu.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES...
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MD...None. OH...None. PA...Winter Storm Watch from this evening through Wednesday afternoon for PAZ007>009-015-016. WV...None.
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&& $$ Kramar/Fries

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