Area Forecast Discussion
Issued by NWS State College, PA

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785 FXUS61 KCTP 200552 AFDCTP Area Forecast Discussion National Weather Service State College PA 152 AM EDT Tue Mar 20 2018 .SYNOPSIS... Big changes to our weather on the way as a storm system will affect the area for the first day of Spring tomorrow. The storm will primarily affect the southern half of the area. It will initially bring a mixture of rain and snow but by Tuesday night and Wednesday primarily snow will fall and significant accumulations are possible across southern Pennsylvania. Cooler than average temperatures will dominate the pattern through the end of the week, with the next opportunity for precipitation coming this weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Near term in good shape with increasing clouds tonight. Latest HRRR/NAM12 shows precip shield moving towards CWA in the pre dawn hours. Latest HREF2 lags by a couple of hours, bringing precip shield into Laurels and South Central mountains after 12z. Siding with the earlier HRRR/NAM12 at this time into the Laurels. Temperatures should be cold enough for snow across the south, but QPF should be light enough to limit much in the way of accumulation initially. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Complicated storm system with snow and potentially some mixed precipitation across southern half of CWA. Initially the temperature profile and intensities of the precip suggest snow will have trouble sticking in valleys and banding will determine where accums are more efficient. Higher elevations will see more accumulating snow. Temperature profiles cool dynamically and this has become more pronounced in the 18z and early look at 00z Tue NAM12 run. This brings a better chance of accumulating snow for valley locales as well. Snow will continue into evening and Wednesday and Winter Storm Watch remains in effect for southern zones for the long duration of potential snow fall. Kept the Watch in place to allow mid shift opportunity to assess latest numbers as we are approaching 12- and 24- hour Warning criteria and trend from 00z runs and CAMs will provide critical information. Besides the typical March complications with snow accumulations (moderate to heavy snowfall rates needed to offset melting as snow falls due to higher sun angles), the duration nature of this system will make achieving Warning criteria difficult, with only 30-40% probs of 6" or more in a 12 hour period in the Watch area currently, and 30-40% probs of 8" or more in a 12 hour period from Tuesday morning through Wednesday afternoon. Latest guidance from the 12z Mon cycle has ramped up QPF/snow totals over southern PA, but this may be in part due to an overzealous NAM package. The current 90th percentile snowfall for Somerset County is in the ridiculoso range (not to be mentioned here), far beyond anything we are reasonably forecasting, so am hoping that the addition of global model members into the pdf suite will yield greater confidence and less spread probabilistically for the overnight forecast tweaks. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... By daybreak Wednesday the storm is expected to be reorganizing off the Mid Atlantic before tracking off south of Nantucket island. The upper trough/low will keep some light precip lingering Tuesday night and Wednesday morning. An additional coating of snow is possible as surface temperatures chill down into the 20s in most areas Tuesday night. By later Wednesday cool and drier air will begin filtering in. Weak high pressure will keep us fair through Friday before the next approach storm brings a chance of rain or snow Saturday night and Sunday. This looks to be a clipper type way which will move through the area quickly with high pressure following for next Monday. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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Main change to 06Z TAFS was to slow things down by an hour or so. Earlier discussion below. VFR will continue through the overnight and into early Tuesday for most of Central PA. The area of concern overnight will be the Laurel Highlands and south central mountains, where an approaching low pressure area will push lowering ceilings in from WV/MD after 09z followed by the onset of snow (mixed with sleet at first) which will continue with IFR restrictions much of the day. The ceiling restrictions look to spread into the Lower Susq Valley by mid morning as low-level easterly jet increases, and reach as far north as KDUJ- KUNV- KSEG by late morning/midday. Precip reaches KMDT-KLNS after 13z, and should fall as mostly snow, but sleet and rain could be mixed in at times. Across the central mountains, snow will arrive midday bringing further reductions. Lower cigs and snow for southern PA continues Tuesday night into Wednesday. Outlook... Tue night and Wed...Snow and restrictions continue. Some improvement late day. Thu-Sat...No sig wx expected.
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&& .CLIMATE... The Spring (Vernal) Equinox will be at 12:15 PM on Tuesday, March 20. Sunrise in State College, PA will be 7:14 AM and sunset at 7:23 PM, for a total day length of just over 12 hours. && .CTP WATCHES/WARNINGS/ADVISORIES...
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Winter Storm Watch from this evening through Wednesday evening for PAZ056>059-063. Winter Storm Watch from 6 AM EDT this morning through Wednesday evening for PAZ033>036-064>066.
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&& $$ SYNOPSIS...Ross NEAR TERM...Ross/DeVoir SHORT TERM...Ross/DeVoir LONG TERM...La Corte/Gartner AVIATION...Martin/RXR CLIMATE...Steinbugl is the U.S. government's official web portal to all federal, state and local government web resources and services.