Area Forecast Discussion
Issued by NWS State College, PA

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824 FXUS61 KCTP 201300 AFDCTP Area Forecast Discussion National Weather Service State College PA 800 AM EST Sat Jan 20 2018 .SYNOPSIS... A large area of high pressure centered over the Southeastern States will combine with a potent area of low pressure moving by well to the north of our area across Central Quebec Canada, and lead to fair weather and rather windy conditions across the northwest half of the state. Clouds will increase and thicken- up from the west this afternoon and tonight, bringing the chance for some light rain or drizzle. The first several hours of the precipitation could occur as some mixed light rain, sleet and light snow across the western mountains. Temperatures will climb above normal over the upcoming days. A cold front will move through the area later Monday night and early Tuesday, preceded and accompanied by a few periods of more substantial rain as moisture streams north all the way from the Gulf Coast region. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Fast zonal flow aloft, occurring well to the south of a strong...150 kt upper jet stretched across the northern Great Lakes and SERN Canada, will help to maintain fair weather with moderating temps and a gusty westerly wind today. Variable amounts of cirrus clouds will drift southeast across the region today and remain the dominant (and perhaps the lone) cloud type until at least late this afternoon. Temps early today will be much milder than normal by as much as 15-20 deg F across the central and NW mtns, thanks to a 12-17 kt SW wind across northern and western parts of the CWA...with gusts to around 20 kts. A few locations across the Lower- Mid Susq Valley could see temps hover near to a few deg F above normal (in the low to mid 20s) as a result of mainly clear skies and a decoupled boundary layer with nearly calm air. Elsewhere, min temps early today will range from the mid 20s across much of the Middle and Lower Susq Valley, to the low-mid 30s across the NW half of the CWA. After a mostly Sunny morning in most places, the operational NAM, GFS and RAP show a rather shallow layer of stratocu clouds advecting ESE and into the western zones during the late morning/early afternoon hours, and later in the afternoon or evening across the Susq Valley. The latest National Blend of Models and ensemble MOS pops continue to show a small chance of some light rain...mixed with wet snow flakes...spreading into the NW mtns and Laurel Highlands around sunset this evening. Wet bulb profile below zero by a few deg F through all layers of the Atmosphere favor spotty --SN and/or --PL for the first few to several hours of the precip. Highs today will range from near 40F north to the lower 50s across the south. However, a brisk westerly wind (averaging in the mid to upper teens (KTS) with gusts as high as 30 KTS during the late morning and afternoon Western high terrain and 20-25 kts elsewhere) will make it feel notably cooler than the actual air temp - or in the upper 20s across the higher terrain of the north and west, and upper 30s in the SE. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Moisture takes the circuitous route around high pressure over the Carolinas tonight with low clouds gradually thickening - enough to support some areas of light precipitation as the night wears on. Any precip that falls prior to 09Z Sunday will be almost exclusively across the NW mtns and Laurel Highlands and in the form of light rain, sleet or even some spotty light freezing rain or freezing drizzle as Wet Bulb temps will be generally near to slightly below zero at all levels except for perhaps a shallow layer between 900-925 mb. Wet bulbs warm to a few or several deg C above zero by 12Z Sunday across the Laurel Highlands and by 17Z over the NW mtns of PA, to change any spotty light precip to rain showers. Sunday looks to be primarily a cloudy day with some areas of drizzle or brief showers. Temps may be sluggish to warm out of the 30s across the north, while only reaching the low to mid 40s in the Southern valleys. Winds will be generally light and from the south to southwest. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Another January thaw will bring a period of relatively mild weather to Central PA into early next week. Temperatures are not expected to be as warm as the previous thaw late last week, but departures should reach +10 to 15 degrees above average for mid to late January. The progressive pattern should keep the cool down through the middle part of next week rather brief before temperatures moderate/trend warmer into next weekend. A surge of warm advection ahead of a strengthening Mississippi Valley/Great Lakes cyclone will impinge on a retreating thermal gradient between I-80 and I-90. Forecast soundings indicate moisture will be rather shallow initially with areas of light rain confined mainly to the western and northern parts of the area. Elsewhere expect low clouds to develop with pockets of drizzle/mist and fog. Examination of thermal/moisture profiles specifically sfc temps and maxTw aloft suggest some patchy freezing drizzle is possible over parts of north-central PA Sat night and Sun night. The other concern may be black ice as snow melts during the day and then freezes at night. The risk of ice jam flooding will continue over the weekend and into early next week. See the hydro section for more details. The most likely period for widespread rain is Monday night into early Tuesday along/ahead of south-southwesterly LLJ axis and cold/occluded front. A secondary low may form in the lee of the Appalachians and support a period of enhanced rainfall over the eastern 1/2 of PA Tuesday morning. At this time, the total rainfall forecast for early next week does not appear to be as heavy as Jan. 11-12. Winds look rather blustery from later Tuesday into Wednesday on the backside of the low pressure system and may approach advisory criteria. Snow showers are likely downwind of Lake Erie into Wed night. Dry wx with moderating temperatures Thu-Fri. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Widespread VFR will continue today as high pressure maintains control of the local weather pattern. A MVFR stratocu/stratus deck should advect East into the Western zones tonight. This deck could become IFR at times, especially at BFD. Some very light snow or flurries could develop from this cloud deck tonight over favorable upslope areas of the Alleghenies (KBFD and KJST TAF sites), as the clouds will be 1000 feet or so thicker. This will be watched closely and likely included in TAF updates. Strong winds aloft will slowly weaken as the gradient shifts to the northeast, however LLWS will continue today. Outlook... Sun...Mainly VFR. Mon...Reduced conditions with rain showers associated with an approaching cold front. Tue...MVFR/IFR with snow showers west and north. MVFR/VFR central and eastern terminals. Wed...MVFR with snow showers west and north. VFR central and eastern terminals. && .HYDROLOGY... Main concern is watching water levels rise on Susq downstream on Harrisburg. Marietta gage /MRTP1/ continues its slow rise as ice builds up downstream. Gage above action stage and there is some minor flood concerns in Wrightsville ongoing. The Flood Watch for this area has been extended through the weekend, set to expire Monday afternoon. With milder temperatures there is potential for ice movement, as well as slow ice deterioration/thinning with time. All areas near rivers and streams that have significant ice buildup should monitor water levels closely for the next several days. && .CTP WATCHES/WARNINGS/ADVISORIES... Flood Watch through Monday afternoon for PAZ065-066. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...Lambert/Steinbugl AVIATION...Lambert/Ceru HYDROLOGY...

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