Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 161148 AFDCTP Area Forecast Discussion National Weather Service State College PA 648 AM EST Tue Jan 16 2018 .SYNOPSIS... A prolonged period of overcast skies with light snow will occur today into early Wendesday as a slow moving frontal boundary edges east across the region. A few weak upper-level disturbances will ripple northeast along this boundary, possibly bringing periods of briefly heavier snowfall. A few to several inches of snow will fall over the upcoming 24 to 30 hours. A deep upper level trough will swing through the area later Wednesday, then be replaced by a building east coast ridge next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Update... Moved the onset of the Winter Weather Advisory across our far SE counties up by several hours to the current time, as a batch of light snow with 1 to 1.5SM vsbys has developed a little further south (and will be slightly more extensive and persistent this morning) than depicted by the past few runs of the HRRR/RAP. This light snow will slicken up untreated roads for the morning commute. No other signif changes made to weather/wind/temp grids. Previous Disc... The postive tilt trough extending southwest from the Great Lakes region this morning (and an embedded clipper moving ESE to the Central Miss Valley) will help to form a weak secondary low that will track from the southern Appalachians to the Mid Atlantic coast. Low levels of the atmosphere will gradually moisten up across the CWA today ahead of these features with light snow becoming more widespread, steadier, and perhaps falling at moderate intensity for brief periods across central and SE PA this afternoon into tonight as the thermally direct RE region of a 130-140 kt upper level jet sets up across the Central and SE part of the CWA and persists into the early morning hours of Wednesday bringing persistent weak to moderate UVVEL. Coordinated with KBGM and KPBZ to expand the Winter Weather Advisory several layers of counties to the NW, which encompasses the Central Mtns and Northeast Zones. A blend of the latest 03Z SREF, RAP and 00Z GEFS shows strong clustering of individual model members between 0.20-0.30 of an inch of liq equiv across the Central Ridge and Valley Region with similar clustering (though slightly bi-modal with higher amounts of almost half SREF members into the 0.4-0.5 inch range) over the Susq Valley and points to the east. These Liq Equivalent values and a 12-14:1 Snow/water ratio equates to a general 3 or inches from the Laurel Highlands northeast through KUNV and KIPT and KTHV/KMDT. The higher terrain to the east of the Susq Valley (namely Sullivan and Schuylkill Counties) could see 5-6 inches by daybreak Wednesday. This event will be a marginal Winter Weather Advisory over the Laurels, Central Mtns and Mid Susq valley as the bulk of the approx 3 or 4 inches falls over roughly a 12 to 15 hour period this afternoon and the first half of tonight. Snowfall will be mainly around 0.25 inch/hour, but periods of near moderate snow intensity will increase rates to around or slightly over 0.5 inch/hour. High temps this afternoon will vary from the mid 20s across the higher terrain of the north and west, to the upper 20s in the Central Mtns, and between 32-34F across the Lower Susq Valley. The slightly milder temps across the SE and later onset of the more persistent light snow with vsbys of 1-2Sm will lessen the impact across that region during much of the daylight hours today. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... The aforementioned upper level forcing beneath the RE region of the upper let, and associated light snow will continue through at least the first half of tonight across the Central Ridge and Valley Region, and linger into the day Wednesday over the Lower Susq Valley and far eastern zones. Any snow left across the NW mtns will be spotty and very light and intermittent as the jet axis shifts east of that area with broad mid to upper level subsidence and drying developin. Light WNW flow of colder air will act as the lone factor for some light snow INVOF KBFD and points west. The light snow will gradually taper off from NW to SE during the early to mid morning hours Wednesday, but could linger til around Noon Wednesday to the SE of Interstate 81. A tricky call for low temps at daybreak Wed as the deep-layer moisture and thick-layered cloud cover will be slowly exiting to the east, with the back edge likely near the I-99/Route 220 corridor at 12Z Wednesday. This will mean a moderately sharp NW-SE temp gradient of about 10F within just 10-20 miles where some clearing is possible. Mins will range from the low-mid single digits across the Laurel Highlands and NW Mtns to the low teens in the Central Mtns, and around 20F in the SE. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Mianly dry and quite cold conditions persist Wednesday night under westerly flow - that will gust frequently between 20-30 mph Wednesday night and Thursday as high pressure builds over the Lower Ohio Valley and SE States, while a strong North-South pressure gradient resides across the Midwest, Great Lakes Region, and Upper Ohio Valley into PA. Gusts over the western mtns could be around 40 mph (but probably not reaching Wind Advisory criteria of 46kt gusts). Conditions still look dry for Thursday into Friday. Winds shift more to the west across the northwest on Thursday, thus more of a warm advection set up there. The coldest push of air is down the TN Vly on Wed. Temperatures by the weekend will be well above normal, as the upper lvl ridge builds. Took out rain and snow for Saturday. 00Z EC has next system so strong over the plains that the cold front gets push well east of the upper lvl system by late Monday. Went with a dry fcst aft this. This warm up not fcst to have dewpoints much above the lower 40s, thus not expecting a lot of heavy rain. More information in hydro section below. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... An Alberta Clipper swinging southeast across the mid Atlantic region overnight into tomorrow will continue to bring snow showers through the northwest. IFR has spread through BFD with periods of IFR possible at JST. This will continue to spread eastward tonight into tomorrow. Expect cigs and vsbys to continue to drop as the night continues. A weak secondary low Tuesday through Tuesday night will shift MVFR to IFR restrictions to eastern areas in light snow, with more scattered snow showers (and thus more scattered restrictions) for western areas. As deep upper trough swings through the region Wednesday, ceiling restrictions again shift back to the western higher terrain along with scattered snow showers. Otherwise, primarily VFR conditions will prevail. Thursday looks to be breezy with gusts to 15-25 mph. Outlook... Tue...Restrictions in light snow mainly SE half. Lingering snow showers with intermittent restrictions over the NW half. Wed...VFR SE half, but ceiling reductions and intermittent vsby restrictions across the Northern and Western mtns as a result of isolated snow showers. Thu-Sat..Mainly VFR. && .HYDROLOGY... The threat of ice jamming is diminishing. Several cold days into mid week will form additional ice as stream flow and open channels decrease. Another warm up by the weekend. Dewpoints for this warm up much less than the event last Friday. && .CTP WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 5 AM EST Wednesday for PAZ012- 018-019-024>028-033>035-037-041-042-045-046-049-050. Winter Weather Advisory until 1 PM EST Wednesday for PAZ036- 051>053-056>059-063>066. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...Lambert/Martin AVIATION...Ceru/RXR HYDROLOGY...Lambert is the U.S. government's official web portal to all federal, state and local government web resources and services.