Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 062346 AFDCTP Area Forecast Discussion National Weather Service State College PA 646 PM EST Tue Dec 6 2016 .SYNOPSIS... Low pressure over western PA will fill and a coastal low will deepen and move away on Wednesday. High pressure will briefly visit PA before a dry but sharp cold front sweeps southeast across the area on Thursday. A low pressure system is likely to track west of Pennsylvania through the Great Lakes Sunday and Monday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 630 PM update... Temps have been warming steadily at almost all sites except in the NE where it has only just cooled off due to the wet-bulb effect. They will stabilize or even rise a few degs by morning. The back edge of the steady precipitation is into wrn PA with a quickly expanding dry slot working through the SC mtns. Will hold onto the Ice storm warning for just a little while longer - since the temps at all ob sites in the Laurels are not quite yet abouve freezing. The general mix of everything is occurring as expected. However, the snow/sleet totals seem a little high for how fast this stuff is moving and how marginally warm it is. Just have the first few reports of about an inch so far in the nrn tier - in Blossburg. That dry slot will likely end the precip over the srn tier. The northern mountains will probably only have another 2-3hrs that they may have some accumulation of snow/sleet. There will likely be a prolonged period of drizzle and fog/low cigs overnight. Will revisit the need for continuing the advys in another few hours. Prev... Main focus continues to be over the high terrain of the Laurels, where model soundings continue to show some ice accumulations probable and latest JST ob has around a tenth of an inch so far. Based on the near to slightly sub-freezing temperatures and temps warming aloft to just above freezing in a rather deep layer with another swath of moderately heavy precip streaming NE across the region through this evening, we opted to upgrade the Winter Weather Advisory to an Ice Storm warning. Once the mean south to serly flow shifts to the west-southwest later this evening, the precipitation should taper off to just some isolated rain showers/patchy drizzle in the Warning area with temps drifting slowly upward. We issued a Winter Weather Advisory the NW mtns through 12z Wed, where an initial period of mixed precipitation late today could mix with snow at times, yielding light glazing of ice up to around 0.10 of an inch from periods of -FZRA and sleet, and perhaps a coating to inch of snow (with up to 2 or 3 inches of snow across Potter county) based on model blended QPF. Based on coordination with BUF/BGM, these higher end snow amounts should be fairly isolated and given that it will be a rain/snow mix with mild SFC temperatures, have lowered snow amts from WPC guidance and held off on a winter wx advisory across Northern Pa. Deep moisture will exit the area with passage of mid level shortwave late this evening. However, model soundings remain nearly saturated below 850MB, indicating the likelihood of linger drizzle/rain and a snow mix late tonight. Superblend and high res NAM both indicate temps creeping above freezing even across the N Mtns late tonight. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Surface high pressure building into the region should supply a dry Wednesday to Central Pa. However, low level moisture trapped beneath inversion and weak mixing will likely result in stubborn low clouds across the NW Mtns. A downsloping westerly breeze should result in partly to mostly sunny skies east of the Mtns. Temperatures there should respond by rising into the low-mid 40s, while cloud cover holds readings to the mid 30s across the NW Mtns. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... The long term will begin with a brief period of tranquil conditions and deep southwesterly flow, ahead of an arctic front that will be pushing across the state Thursday morning and early Thursday afternoon. Clouds will thicken up Wednesday night and early Thursday with snow showers moving into the far northwest around or shortly after midnight, then spreading SE and possibly mixing with rain showers across the Southeast half of the state Thursday morning. The subsidence inversion base lifts rapidly up to, then well over 7-10 kft agl Thursday afternoon, right through Friday night as the mean low to mid level flow becomes well-aligned from the West-Northwest. The subsidence inversion base will drop gradually down to below 7 kft agl during the day Saturday as the mean 925-700 mb flow backs to more of a west, then swrly direction by around 12Z Sunday. This scenario is very favorable for heavy, to locally excessive Lake effect snow and significant amounts of blowing and drifting snow with occasional wind gusts around 30 mph, and up to several hundred j/kg of cape being transported SE of interstate 90 leading to the potential of some Thundersnow with periods of 2+/hr snowfall rates possible, especially Thursday night through Friday night as quite anomalously cold air of -2 to -3 sigma in the 700-500 mb layer moves overhead. Posted a Winter Storm Watch for Warren and McKean counties from Noon Thursday through 7 am Saturday, and the significant bands of LES could even continue through much of the day Saturday. Snow totals throughout the perennial snowbelt of NW Warren county could top 18 inches by Saturday, with areas of 6-12 inches further inland across the SE half of Warren county and at least the NW half of McKean County. Scattered narrow bands of snow showers will stream SE into the Central Ridge and valley Region occasionally, with a coating to an inch possible in a some locations. Expect rather significant snow totals (in excess of 4 inches) to also occur across the Laurel Highlands, albeit over a more extended period Thursday night into Saturday, and with lighter snowfall rates. A bubble of high pressure at the sfc (and a weak ridge aloft) will bring a brief period of tranquility (though very cold overnight min temps) as it slides east across the forecast area late Saturday into early Sunday. Expect partial clearing (and perhaps mostly clear skies across the SE part of the CWA) for the middle third of the weekend. Deep, mean-layer flow backs to the southwest with a slug of low-mid level warm advection sliding up and over the deep/retreating cold airmass. This will create an area of steady light snow or snow showers across mainly the northwest half of PA Sunday afternoon and Sunday night. The boundary layer could become warm enough by Monday to mix in or changing the scattered light precipitation to rain showers, throughout the Lower Susquehanna Valley. One or more, weak and fast moving short waves in the zonal flow aloft could bring some light mixed precipitation in the Ridge and Valley Region, and scattered snow showers across the Alleghenies. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... 21Z TAFS sent. Large area of rain, with some freezing rain to the west. Earlier discussion below. IFR cigs at JST and through the central mountains should continue through 05Z before the precipitation tapers and cigs should slightly improve. MVFR cigs will lower from south to north tonight especially given the mixed precipitation. Confidence in ptypes is marginal given complex and evolving thermal structure/evaporative cooling in the boundary layer. Elevation and pcpn rates will also be a factor. A period of LLWS is possible at BFD/JST from roughly 6/21z-7/03z. Overall, expect flying conditions to be in the sub- VFR Tuesday night into Wednesday morning before slowly improving after 09Z. Restrictions should lift Wednesday morning. Outlook... Wed...A.M. low cigs becoming mainly VFR. Thu-Fri...Becoming windy with periods of snow showers. Mainly MVFR/IFR west to MVFR/VFR east. Sat...Decreasing winds. Low cigs/-SHSN psbl NW. VFR east. Sun...Some snow possible. Mainly late across the NW. && .EQUIPMENT... Temp/dewpoint sensor at KIPT appears to be malfunctioning. Far too warm compared to meso obs and surrounding METARs. Have used a more-representative 40F for the max for today at KIPT. Techs have been advised and repair is estimated to be done on Wed. && .CTP WATCHES/WARNINGS/ADVISORIES... Lake Effect Snow Watch from Thursday afternoon through Saturday morning for PAZ004-005. Winter Weather Advisory until 7 AM EST Wednesday for PAZ005- 006-010>012-017-018. Ice Storm Warning until midnight EST tonight for PAZ024-033. && $$ SYNOPSIS...Ceru NEAR TERM...Dangelo/Ceru SHORT TERM...Ceru LONG TERM...Lambert AVIATION...Ceru/Martin EQUIPMENT...

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