Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 201132 AFDCTP Area Forecast Discussion National Weather Service State College PA 632 AM EST Tue Feb 20 2018 .SYNOPSIS... Areas of dense fog and light wind will occur across much of Central Pennsylvania and the Susquehanna River Valley early today. Unseasonably warm air with near record afternoon high temperatures will push north across the region for today and Wednesday. Temperatures 25 to 35 degrees above average will be common this afternoon, and again on Wednesday, making it feel more like May than February. A strong cold front will move southeast across the commonwealth during the day Wednesday, followed by several weak waves of low pressure bringing bouts of rain for the second half of the week, and even some light freezing precipitation Thursday morning. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Mild air, light wind and relatively high sfc dewpoints (ranging from the 30s in most places, to as much as 50F across the Laurel Highlands) will lead to low visibilities in locally dense fog early today. The best combination of high sfc dewpoints and light wind was occurring across the lower and Middle Susquehanna Valley early today, which is where to expect to see the most widespread and longest lasting dense fog this morning. Clouds and the fog should erode fairly quickly during the midday hours in most places throughout Central and Western PA. However, a shallow but strong llvl inversion anchored by light sfc wind and an persistent, moderately strong 1000 mb Ageostrophic wind could keep, cloudy, cool and foggy conditions intact across the Lower and Middle Susquehanna Valley into this afternoon. Forecast max temps of the low 50s across the NE zones and upper 50s to upper 60s elsewhere were lowered by some 8-10f after being weighted about 80 percent toward the latest 00Z Tue NAM with the rest being the Nat`l Blend of Models (which typically washes out the detail and llvl cool air damming thanks to it`s ensemble average). Persistent and moderately strong LLVL northeasterly Ageostrophic Flow should help to maintain a rather strong inversion within 1KFT AGL SE of the Allegheny Plateau. We could still easily bust on the high side by as much as 10 deg F in some locations scent PA and the Lower Susquehanna Valley. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... *Record-challenging warmth Tuesday night through Wednesday Increasing deep layer south to SW flow will continue to advect unseasonably warm air into the region tonight and Wednesday. Expect a spring-like start to the period with patchy fog tonight into AM Wed. giving way to a mix of clouds and sun with near- record to record high temps across much of portions of Central and Western PA. 925 mb Temp anomalies stay in the 2-3 sigma range during this period with persistent 14-16C air across the CWA. Temps will only fall to the upper 40s to mid 50s tonight, then rebound into the mid 70s SE. The mid to late morning timing of a strong cfropa Wednesday across the NW mtns will likely limit the temp climb to only highs in the mid to upper 50s there, before readings fall quickly during the afternoon and evening. Conditions stay rainfree well ahead of the cfront for tonight. However, rain will develop near and in the wake of the cfropa later Wed morning (NW) and during the afternoon and evening elsewhere across the Central Mountains, then Lower Susq Valley. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... *Periods of rain with a several hour period of icing from freezing rain expected late Wed night through Thur morning. brief interval of mixed precip possible Wed. night- AM Fri. A light accumulation of snow could precede the freezing rain across the Northern Mountains early Thursday. *Pattern favors above average temperatures this weekend into next week The cold front will become quasi stationary near the MD line Wed. night. The wavy frontal boundary will bring periods of rain into the weekend, with several (to as much as 8) hour period of mixed wintry precip likely across mainly the Central and northern mountains Wed. night and Thu morning, thanks to an increasing and moderately strong northerly, 1000 mb Ageostrophic flow that will serve to pump shallow/sub-freezing air south into the region. Confidence is moderate with respect to the exact ptype details (and the location and amount of fzra). However, there is at least some risk for a light snow and/or ice accumulation. We`re in good agreement with WPC`s Winter Weather graphics ATTM, with a coating to 2 inches of snow possible at the onset across parts of northcentral and NW PA, before a changeover to sleet and or Freezing Rain. Another round or two or rain is likely over the weekend. We will need to monitor the potential for flooding especially across south- central PA where the ground is very saturated from previous rain/snowmelt and streamflows are well above normal. && .AVIATION /11Z TUESDAY THROUGH SATURDAY/...
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Areas of dense fog will impact much of the Susquehanna River Valley and parts of the Central Mountains through mid morning, though it may take until late morning until visibilities improve to VFR over the SE. However, a shallow but strong llvl inversion could keep ceiling restrictions ongoing there into this afternoon. The remainder of the CWA is already or will become VFR by 14z. Though BFD could see sub-VFR conditions return this afternoon with a warm front nearby. In addition, LLWS will continue through first half of the day with SW flow just off the deck around 35-45 kts. Patchy fog / low ceilings edge back into the Susq Valley from the east tonight into Wed morning, then give way to VFR conditions across most of the region. Exeption will be over the NW Mtns as a sharp cold front will spread rain and restrictions in from the NW, with restrictions possible in the afternoon/evening into the central mountains. That will be followed by several weak waves of low pressure bringing bouts of rain for the second half of the week, and even some light freezing precipitation Thursday morning. Outlook... Wed...Restrictions likely in rain showers mainly NW half. Thu...Widespread restrictions in rain south/wintry mix north. Fri-Sat...Widespread restrictions in periods of rain.
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&& .HYDROLOGY... Recent rainfall and snowmelt has resulted in very saturated soil and well above normal streamflows across much of the area especially south-central PA. Rainfall of 0.50 to 1.00 inch is forecast between Wednesday and Thursday night, with additional rainfall likely into/over the weekend. River flood potential (including small streams, creeks and tributaries) is above normal and we will continue to monitor the risk in the coming days. && .CLIMATE... Record highs for selected sites for Tuesday and Wednesday February 20th and 21st: MDT: 72/1930;71/1997 IPT: 67/2016;69/1930 BFD: 58/2016;64/1997 AOO: 62/2016;71/1997 && .CTP WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until 10 AM EST this morning for PAZ049>053- 057>059-066. && $$ SYNOPSIS...Lambert NEAR TERM...Dangelo/Lambert SHORT TERM...Lambert LONG TERM...Lambert/Steinbugl AVIATION...RXR HYDROLOGY... CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.