Area Forecast Discussion
Issued by NWS State College, PA

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290 FXUS61 KCTP 221046 AFDCTP Area Forecast Discussion National Weather Service State College PA 546 AM EST Sun Jan 22 2017 .SYNOPSIS... A storm moving through Dixie today will move off the Mid Atlantic coast Monday to near Long Island Tuesday. High pressure and mild conditions will briefly return for midweek before a pattern change brings more seasonable temperatures to close out the month. && .NEAR TERM /THROUGH TONIGHT/... We extended the fog advisory to cover the late burn off times that tend to be most likely this time of year. Otherwise another bland mainly cloudy day is in store for most of the region with temperatures that are very mild for the traditionally coldest days of winter. Models want to increase the chances of some light rain advancing into my southern zones the deeper into the day we get. I have relatively high Pops but only light amounts of rain. Near term guidance is in good agreement with tracking the upper low out of the southern Miss Valley to NC by sunrise Monday. Moisture will continue to increase ahead of the system and it seems a good bet that most if not all parts of the forecast area will be wet by morning. && .SHORT TERM /MONDAY/... * 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. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... 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. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... Made some moderate changes to 09Z TAFS, some of which will carry into the 12Z TAF package. There was a small window on Saturday where we had some improvement. Starting to see more of a window today, as the system just east and south of our area moves off the coast this morning. Main problem will be late today into early Tuesday. Very deep upper level low will result in a deep low forming well inland across the southeast states today. This system will track northward. At the current time, the low is fcst to move just far enough away from our area, to edge down the chance for strong storms, strong winds, and widespread heavy rain late today into early Tue. However, still expect some gusty winds and some heavy rain, especially across the southeast later Monday into early Tuesday morning. Outlook... Mon...Rain/low CIGs likely. Tue...AM rain/snow and low CIGs possible. Wed...Reduced CIGs/showers possible NW. Thu...MVFR/IFR in rain/snow showers mainly west. && .CTP WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until 10 AM EST this morning for PAZ005-006- 010>012-017>019-025>028-034>037-041-042-045-046-049>053- 056>059-063>066. && $$ SYNOPSIS...La Corte NEAR TERM...La Corte SHORT TERM...La Corte LONG TERM...La Corte AVIATION...Martin is the U.S. government's official web portal to all federal, state and local government web resources and services.