textproduct: State College

WHAT HAS CHANGED

*Increased rain totals across northeast zones *Added more discussion about snow potential

KEY MESSAGES

1) A deepening trough will result in a strengthening coastal low providing a shot of rain on Thursday with the potential for a transition to snow in the northeastern zones Thursday night.

2) Northwesterly flow brings much cooler temperatures this weekend, with lake-enhanced snow showers continuing Friday into Saturday night.

DISCUSSION

KEY MESSAGE 1: A deepening trough will result in a strengthening coastal low providing a shot of rain on Thursday with the potential for a transition to snow in the northeastern zones Thursday night.

The main game in town across central Pennsylvania over the next week will be the potential for the first accumulating snowfall mainly for the higher elevations. As a low pressure system approaches the region from the Ohio Valley, precipitation chances will begin to ramp up tonight with rain likely breaking out in an area of 850mb fgen forcing over north central PA. That area of fgen forcing will slowly pivot across the northern tier and finally translate eastward later Thursday as the upper level support for the system tracks from west to east across PA. The developing sfc low is progged to be sub 1000 mb as it moves across the Balt-Wash area midday Thursday. The low will increase SE flow and moisture transport from the Mid Atlantic Coast / Chesapeake Bay up the Susquehanna Valley. Orographic effects will likely lead to an enhancement in rainfall totals from the highlands of Schuylkill County northward towards Sullivan County and the Endless Mountains during the daytime hours Thursday. This area has the greatest chance of seeing over 1.00 inch of rain with this system (prob rain > 1.00 inch is 30-40 pct). For much of the rest of central PA, rainfall amounts are projected to average around a 0.25 inch, with higher amounts closer to 0.50-0.75 inch possible in the northern tier near the pivot point of the persistent fgen banding.

Cold air will wrap in on the back side of the storm. While the main area of precip will have already pulled away from much of central PA, fgen forcing may support a more persistent comma head lingering across northeast zones, in particular Sullivan and back into Potter/Tioga Counties, where steady rain could transition to snow around or shortly after sunset Thursday evening. The main questions yet to be ironed out are how quickly temps fall Thursday night, and how long the stratiform precip lingers over those zones, but that area will need to be monitored for a potential coating-2" of snow in the Thursday night - early Friday time frame if the colder/snowier solution pans out.

------------------------------------------------------------------

KEY MESSAGE 2: Northwesterly flow brings much cooler temperatures this weekend, with lake-enhanced snow showers continuing Friday into Saturday night.

Cooler temperatures and breezy northwesterly winds will take hold of the region as the area of low pressure shifts east towards the New England Coast. Wind gusts in the 25-35 mph range are likely from Thursday night through the weekend. By sunrise on Friday, the stratiform precip will be pulling away and north- south oriented lake effect streamer bands will likely be in its wake. Typically streamer bands will trend towards a more cellular mode with daytime heating. With highs on Friday in the 30s and 40s, the dominant ptype will likely be snow up in the north and west, but rain would be more likely towards the southeast and where precip rates are lighter. Even the southeast could get some snowflakes if the rates are high enough. The lake effect and upslope snow on the back side of this system is expected to produce a coating to locally up to 2 inches of snow in the higher elevations of the Laurels and across the northwest mountains.

Chances for precipitation begin to decrease Saturday night into Sunday as a subsidence inversion builds in with approaching high pressure. A slight warming trend emerges into the beginning of next week, with temperatures trending closer to climatological averages by Tuesday.

AVIATION /09Z WEDNESDAY THROUGH SUNDAY/

Update - for the Overnight:

KCCX Radar showing some slightly enhanced inbound velocities drifting SE of a line from KDUJ to KELM over the past 30 minutes, likely depicting some ducting of the beam within the shallow, cool airmass behind a very weak cold front.

This boundary may move another 30-50NM further SE overnight. only a very subtle wind shift to the WNW by 10-20 deg is expected in the wake of the frontal passage.

The main impact we'll see will be the formation of some IFR to LIFR stratus later tonight through early Wed across the NW Mtns (KBFD) where milder air aloft and a slight uptick in Theta-E will drift in from the SW - overrunning this shallow cooler airmass.

The wind should go light for most places overnight as high pressure moves in. There is a 50% chc that fog forms in some of the river valleys tonight. We did not go IFR anywhere due to fog at this point, but do mention VCTY FG at several sites. BFD is the only terminal that is more than likely (90%) to go IFR as low clouds move down from the N. These low clouds may get as far SE as IPT/UNV, but there's only a 30% chc of that.

The low clouds in the N should break up a few hrs after sunrise. Sct VFR cu or altostratus will then lift northward into PA after noon concurrent with high clouds increasing from the N. Low pressure approaches quickly from the W with the storm maturing rapidly, too.

Conditions gradually deteriorate to IFR over the west first on Wed night, with -RA spreading across about half the state by sunrise Thurs. LIFR clouds are possible in the N/W Thursday and some snow may mix in to BFD and JST by sunset. BR/FG also expected in many places as the clouds and precip become widespread but light Thurs. The occluded sfc low will likely jump to the E later in the day, but the passage will veer the wind to the SW then NW before the end of the day. LLJet seems not strong enough or low enough to mention any LLWS. any showers left Thurs night and Fri will be -SHSN in BFD and JST. UNV, AOO, IPT may see some flakes mix in, but not last more than an hour at a time (showery).

Outlook...

Fri PM...-SHSN, ocnl IFR NW. -SHRA, MVFR SE. Gusty NW wind.

Sat...Improving, but still gusty with MVFR clouds and isold -SHSN for BFD/JST. VFR elsewhere.

Sun...VFR, gusts NW 15-25KT.

CTP WATCHES/WARNINGS/ADVISORIES

None.