Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 150203 AFDCTP Area Forecast Discussion National Weather Service State College PA 903 PM EST Thu Dec 14 2017 .SYNOPSIS... A weak ridge of high pressure will slide across the region tonight with cold, but mainly dry conditions. Another clipper will move north of the region overnight Friday and Saturday morning followed by high pressure and a brief moderating trend in the temperature Sunday. A series of cold fronts will move across the area late Sunday into Wednesday. Windy and colder temperatures will follow. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Radar showing a few light lake effect snow showers/flurries across the Warren/Mckean counties early this evening. However, boundary layer flow is progged to back to the southwest shortly, shifting remaining lake effect activity north of the border by midnight. Otherwise, fair and chilly conditions overnight, as surface high drifts over the state. Light wind and fresh snow cover have allowed temps to fall quickly this evening where skies are clear. Some of the normally colder valleys within the central mountains are already down into the single digits at 01Z. However, increasing high clouds in advance of approaching shortwave should put the breaks on the radiational cooling later tonight, with some locations actually seeing rising temps before dawn. In general, min temps are expected to range from the upper single digits across the northwest half of the forecast area, to the upper teens in Lower Susq Valley. However, wouldn`t doubt that a few valley locations get close to zero.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
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Thickening cloud cover ahead of approaching shortwave won`t allow temps to recover much from the cold start Friday. Highs only expected to range from the low 20s over the northern mountains, to around freezing in the extreme southeast. WAA ahead of shortwave could produce a few afternoon flurries across the southeast half of the forecast area, while a developing low off the Mid Atlantic coast could potentially graze the extreme southeast corner of the forecast area during the afternoon, so have included low POPs down there for a period of light snow in the afternoon. Have kept the forecast dry for much of Friday across the northwest mountains, where models place a ribbon of low PWAT air. However, increasing moisture works in late in the day ahead of approaching cold front, bringing scattered snow showers toward evening.
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&& .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
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The period starts out with below normal 500 hPa heights and below normal 850 hPa temperatures. The heights and temperatures rise to above normal values over the northeastern United States Sunday as a strong ridge retrogrades to our south. The really warm air should stay to our southeast. The heights and temperatures settle back toward normal Tuesday and Wednesday at 500 hPa and 850 hPa respectively. Models indicate a developing band of low level fgen forcing across northern Pa Saturday within left exit region of jet streak. Model soundings also indicate a nearly saturated sfc- 700mb layer with upslope flow into the Alleghenies. Thus, have mentioned the chance of spotty light snow across the entire Allegheny Plateau Saturday, with higher POPs across the northwest mountains, where best fgen forcing is indicated. WPC qpf indicate a few inches of accum are likely Fri night- Saturday across Warren/Mckean counties, with minimal amounts likely further south along the Alleghenies. Sunday into Monday a strong shortwave to our north will help bring in warmer air. We may be cold air dammed near the surface so the +1 to -2C air at 850 hPa may not translate to a warm boundary layer. The warm advection should bring snow and snow to rain in the south but QPF amounts are low and EFS based POPS are not very high in central and southern areas. As this northern stream wave zips by to our north it should drag cold air back into the region Tuesday into Wednesday. The northwest flow triggers snow in relatively high POPS in NW PA due to LES. This cold surge should be relatively fleeting and high pressure and a potential surge of warmer air in the NAEFS/GEFS. This suggests relatively warmer on Thursday. The Warm advection and approaching cold front will likely produce increased potential for snow in NW Thursday into Friday. Farther south any precipitation would likely be mixed or rain. A strong frontal boundary will likely push another surge of cold air into the region Friday into Saturday. At this time and recent GEFS/CMCE and combined NAEFS runs keep the strong ridge suppressed to our south. This keeps the warm air from getting here. This keeps us relatively dry with all the significant QPF events suppressed to our south. There is considerable spread in the ensemble members. And we have seen run-to-run variations in the solutions with the strong southwest Atlantic ridge. The NAEFS deeps reforming vortex over east-central Canada and maintains the sharp ridge over northwestern North America. This seems to keep us in or on the edge of cold air intrusions. Thus so far single model runs of big warm ups appear to disappear in the ensembles.
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&& .AVIATION /02Z FRIDAY THROUGH TUESDAY/... * Cloud/Ceiling info will be missing at KBFD until new part can be installed * All Central Pennsylvania airfields now reporting VFR conditions, although appears there are still some patchy MVFR ceilings as depicted on satellite across the northern mountains. Overnight will be clear to partly cloudy and cold with some MVFR ceilings expected to move back into the higher elevation terminals of the west and north. The chance of snow showers will be on the increase Friday, mainly over the north. Outlook... Fri...MVFR with snow showers north and west mountains, mainly VFR elsewhere. Sat...Scattered snow showers and local restrictions NW/Laurels. Sun-Mon...Mainly VFR. Tue...mainly VFR, but possible restrictions in the western and northern mountains in snow showers. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...La Corte NEAR TERM...Fitzgerald SHORT TERM...Fitzgerald/La Corte LONG TERM...Grumm/Fitzgerald AVIATION...Jung

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