textproduct: State College

WHAT HAS CHANGED

* Trending warmer tonight and Thursday night with expansive cloud cover and precipitation across the region. * Slight uptick in snowfall amounts across higher elevations of Sullivan County,

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 of central Pennsylvania. As a low pressure system approaches the region from the Ohio Valley, potential for scattered showers increase this evening into tonight. Signals continue for the developing surface low pressure to be sub-1000mb as it moves just south of the region on Thursday. All guidance points towards increasing moisture with southeasterly flow from the Chesapeake Bay into the Susquehanna Valley, which will promote more widespread shower activity. Current thinking is that there will continue to be some orographic enhancement with recent REFS ensemble guidance outlining two six-hour blocks (between 12Z Thursday and 00Z Friday) with low-end probabilities of exceeding one inch of QPF across the Middle Susquehanna Valley where more forcing and orographic lift will be in place compared to the Lower Susquehanna Valley. Not entirely sold on there being over an inch in any six-hour block; however, this signal does outline the heavy rain potential. No flooding concerns will exist with well-below average precipitation across the entire area over the last two months.

Model guidance has also stayed consistent with respect to cold air wrapping around the back side of this system. The mentions of frontogenetic forcing, which will bring about lingering showers across the northeastern zones mentioned in the previous forecast cycle remains fairly consistent. Current forecast has a slight uptick in snowfall across our northeastern zones (Potter/Tioga/Sullivan), where recent REFS total snowfall probabilities have a greater than 60% chance for receiving accumulations over one inch. Some isolated instances of accumulations reaching into the 2-3" range across the higher elevations of Sullivan County have been outlined in the previous two forecast cycles, but there's still questions on how long precipitation lingers across the region.

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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. At this time, snowfall appears to be the dominant precipitation type across the northern/western halves of Pennsylvania with rain prevailing elsewhere. That being said cannot completely rule out a rain/snow mix further south/east if precipitation rates trend higher. The lake effect and upslope snow on the back side of this system is expected to produce a coating to one inch of snow across the Laurels and the northwest mountains with localized amounts pushing closer to two inches in the highest elevations.

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 /18Z WEDNESDAY THROUGH MONDAY/

Developing cu over the S should start to lift to the north as warm advection kicks in. -RA may begin as it does so, but model data has it lifting to UNV/MDT before starting to make measurable rainfall. These showers then continue to lift to the NE while the low center to our west moves closer tonight. Wind continues to back around to the E then N in most places tonight. Late tonight/early AM, SHRA and lowered cigs get into JST and sweep eastward. Perhaps (40% chc) only JST and BFD will be IFR in the morning, with everyone else MVFR or better (60% chc). The SE (LNS/MDT) do seem to have several hours in the morning where they can heat up and develop some instability. SPC has indicated some thunder is possible there (and maybe IPT, too). So, I added a mention of TSRA for them late in the TAF period -- may not happen until after 18Z, though.

Temp profile gets cold enough around sunset in BFD for snow to mix and then become the dominant precip type. It will turn and a little later for the other northern and western counties (incl JST), and finally late at night for UNV/AOO/IPT. However, those latter 3 terminals will likely get into a lull in the SHRA/SN as the flow will be predominantly from the NE then, and not favorable for lake effect just yet - and the synoptic precip will be to the east. Wind will be picking up as the low deepens and moves east/off-shore.

Outlook...

Fri...Blustery NW wind. Sct IFR in SHSN for BFD/JST. VFR to MVFR clouds elsewhere.

Fri Night...-SHSN and ocnl IFR NW. -SHRA and 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.

Mon...VFR, lighter NW wind.

CTP WATCHES/WARNINGS/ADVISORIES

None.