Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 221943 AFDCTP Area Forecast Discussion National Weather Service State College PA 243 PM EST Sun Jan 22 2017 .SYNOPSIS...
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A multifaceted storm will bring drenching rain, elevation- driven snow and gusty winds to Central Pennsylvania on Monday. High pressure and unseasonably mild conditions will briefly return for midweek before a pattern change brings colder temperatures and lake effect snow to close out the month.
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&& .NEAR TERM /UNTIL 10 PM THIS EVENING/...
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Another day of wildly varying conditions across Central PA ranging from very mild spring-like weather in the Alleghenies where temperatures reached the upper 50s to low 60s, to cloudy, cool and foggy conditions with readings in the 40s from the central ridge and valley region into the lower Susquehanna Valley. The fog has been incredibly stubborn locally. POPS trend upward from south to north later tonight as moisture and lift increases ahead of the strong storm system tracking northeast across the Southern Appalachians. Latest HRRR shows rain spreading north of the Mason-Dixon line between 02-04z.
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&& .SHORT TERM /10 PM THIS EVENING THROUGH 7 AM MONDAY/...
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Rain will increase in coverage and intensity from south to north through daybreak Monday. A tightening pressure gradient between deepening low pressure in the Carolinas and high pressure in eastern Quebec will produce strong/gusty east winds early Monday morning and continue through Monday afternoon. Issued a wind advisory for most of the lower Susquehanna Valley with max wind gusts 45-50mph.
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&& .LONG TERM /MONDAY THROUGH SUNDAY/...
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130PM: Issued winter storm watch for the north-central zones with potential for 6+ inches of heavy wet snowfall from around midday Monday through Monday night. The greatest risk for heavy wet snow is across the higher elevations. Still making minor tweaks to storm total snow accumulations. Main impacts would include hazardous travel during the evening commute and potential for localized utility interruptions. 1130AM update: Minor changes were made to storm total snowfall maps with greater emphasis on snow accumulations in the higher terrain from north-central PA down along the Allegheny Front and into the Laurel Highlands. The overnight HIRES models were rather bullish on several inches of snow accumulation over the higher terrain, but marginal boundary layer thermal profiles and potential warm nose aloft (sleet?) add several layers of uncertainty. The greatest risk for several inches of - heavy wet snow - is over the north-central mountains to the north of I-80. This is also the area that may need a winter weather advisory and will be considering/collaboration this potential hazard this afternoon. A period of heavy rain is possible on Monday but flood risk will be mainly localized and confined to poor drainage areas. The rain will help to ease dryness across east-central PA. Previous... * Surface low taking a very favorable track for winter storm. * Monster easterly jet 60-75 kts forecast. * Number of SREF/GEFS plumes showing snow is on the increase. * Dynamic cooling under intense forcing will bring a rain to snow scenario. * Higher elevations under most threat for significant snow. * Central ridge-valley areas not out of the woods. * ECMWF/GFS develop steep mid level lapse rates...thunder snow? Monday will be the most complicated and active day of the forecast period. In a normal winter with normal cold air in place, the expected track of a deep low up along or just off the coast would be cause for joy among snow crows. But we are abnormally mild ahead of this storm and that complicates the forecast enormously. During the day the models agree in intensifying the easterly low level jet, as strong as 75 kt in some of the guidance. SREF/GEFS show this to exceed 6 STD DEV in the anomaly plots. Not a surprise with such an intense jet, ensembles also show a high likelihood of more than an inch of QPF over much of southern and central PA during the day Monday. The complicating factor will be the amount of dynamic cooling that is forecast to occur at the height of the storm as strong warm advection and frontogenetic forcing support a period intense upward vertical motion Monday morning into the afternoon. Initially we will see rain in all locations, but by mid morning the higher elevations could mix with or change to wet snow that will continue into the afternoon. Forecast soundings suggest the snow could actually become more than just a higher elevation threat by mid afternoon with thermal profiles even as far east as AOO/UNV cooling sufficiently for all snow. If the next run of models continues this idea of explosive dynamic cooling as precip increases under the intense forcing, I would anticipate some sort of headline to become necessary. The threat is for several inches of heavy wet snow, especially favoring the higher elevations from the Laurels northward, but possibly even eastward into the central ridge-valley region of the CWA. Another concern is the potential for flooding. Target area for the heaviest rain is across the south central part of the state, where orographic forcing at nose of powerful easterly low level jet will play a role. Mean QPF from both SREF and GEFS max out around 1.5 inches in this region, while FFG/FFH is around 2 inches/6hr. Have to stay alert for locally higher amounts such as hinted at by the higher resolution NAM. Right now it looks like a relatively fast moving event so will mention the possibility of minor flooding in the HWO for this area late Monday. It`s all going to make for a busy day for sure. The storm will reach our latitude between about 6-12Z Tuesday, an old rule of thumb that dictated when the steady precip tends to taper off. Upper ridging will quickly build into the region later Tuesday into early Wednesday, bringing a short period of fair and mild wx. A pattern shift is advertised for the second half of the week as low pressure takes up residence over eastern Canada, and despite a tenacious ridge over the northern Caribbean and Bahamas, models carve out a long wave trough over the eastern US with a return to seasonable cold by the end of the week into next weekend. The ridge in the west and trough in the east is also usually a good one for winter storminess, but the deterministic ECMWF and GFS don`t show much more than a series of clipper type systems that promise an extended period of clouds and scattered mainly mountain snow showers that should continue through next weekend into the week after.
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&& .AVIATION /19Z SUNDAY THROUGH FRIDAY/...
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Solid IFR-VLIFR continues across the central and eastern airfields while KBFD/KJST remain VFR conditions as of 22/20z. Will continue IFR or lower conditions at KAOO/KUNV/KMDT/KIPT/KLNS through tonight and into Monday. For KBFD/KJST will gradually trend lower tonight into early Monday morning with lowering cigs and rain reaching the southern airfields around 23/03z. Widespread sub-VFR is a near certainty through Monday as rain overspreads the airspace and mixes with/changes to snow in the higher elevations along the Allegheny Front. Strong gusty winds will impact southern airfields especially KMDT/LNS with 30-35+KT gusts early Monday at KMDT/KLNS. Outlook... Tue...Widespread sub-VFR with some improvement by aftn. Rain/snow ending in the morning. Gusty winds from NNW decreasing late. Wed...Low cigs/rain possible western 1/2. Breezy late. Thu-Fri...MVFR/IFR in rain/snow showers west. MVFR to VFR east.
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&& .CTP WATCHES/WARNINGS/ADVISORIES...
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Winter Storm Watch from Monday afternoon through Tuesday morning for PAZ005-006-037-041-042. Wind Advisory from 5 AM to 4 PM EST Monday for PAZ057-059- 064>066.
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&& $$ SYNOPSIS...Steinbugl NEAR TERM...Steinbugl SHORT TERM...Steinbugl LONG TERM...La Corte AVIATION...Steinbugl

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