textproduct: State College

WHAT HAS CHANGED

* No significant changes

KEY MESSAGES

1) A deepening trough will result in a strengthening coastal low providing a shot of rain today with a transition to snow in the northeastern zones tonight.

2) Northwesterly flow brings much cooler temperatures this weekend, with lake-enhanced snow showers continuing through the weekend.

DISCUSSION

KEY MESSAGE 1: A deepening trough will result in a strengthening coastal low providing a shot of rain today with a transition to snow in the northeastern zones tonight.

As a low pressure system approaches the region from the Ohio Valley, rain will continue to overspread the Commonwealth. At 5 AM this morning, much of the rain falling in central PA was associated with a band of 850 mb fgen forcing across the northern tier. Radar rainfall estimates are already near 0.50 inch locally within this band. Another spoke of stratiform rainfall was pivoting into southwestern PA, closer to the primary low pressure system. Over the next 6 hours, the low pressure will accelerate eastward and deepen to become sub-1000 hPa as it passes through the Baltimore-Washington region. As it does so, 925 hPa flow from the south across our area will back to become southeasterly, increasing moisture transport from the Chesapeake Bay into the Susquehanna Valley. This will result in an uptick in coverage and intensity of rain showers along and ahead of the existing stratiform area of rainfall. MUCAPE of 100-400 J/kg will be enough to support a few rumbles of thunder and heavier downpours across the Lower Susq Valley between the mid-morning and afternoon hours. Orographic enhancement will likely result in rainfall totals exceeding 1 inch between Schuylkill County and Sullivan County. No flooding concerns will exist with well-below-average precipitation across the entire area over the past two months. SPC does highlight southeastern PA with a MRGL risk of severe wind and 2 pct tornado risk, just to the east of Lancaster County, where hodographs are a bit more curved.

Cold air will wrap in on the back side of the storm this evening. While the main area of precip will have already pulled away from much of central PA, fgen forcing supports a more persistent comma head lingering across northeast zones, in particular Sullivan and Tioga Counties, where steady rain is likely to transition to snow around or shortly after sunset. The most likely scenario at this point is a coating-2" of snow tonight into Friday, with locally higher amounts of 2.0-3.5" possible in the higher terrain of eastern Sullivan and eastern Tioga counties. Ensemble guidance also suggests a 1-in-10 chance of snowfall amounts exceeding 3 inches in the higher terrain of Schuylkill County, including along I-81, so this potential will have to be monitored. Given the antecedent warm ground, accumulations are likely to be slushy with higher amounts on grassy surfaces. Motorists are urged to use extra caution as this is the first snowfall of the season.

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KEY MESSAGE 2: Northwesterly flow brings much cooler temperatures this weekend, with lake-enhanced snow showers continuing through the weekend.

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 tonight through the weekend, strongest tonight into Friday. By sunrise on Friday, most guidance shows stratiform precip pulling away and north-south oriented lake effect streamer bands will likely be in its wake. It should be noted, however, that a few model runs do show the stratiform comma head snowfall lingering longer across eastern PA, but rates would be light enough and sfc temps warm enough to preclude any significant impacts. At this time, snowfall does appear to be the dominant precipitation type across the I-80/I-99 corridor and north/west, with a mix or plain rain to the south and east. 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 and in areas of locally persistent banding.

Chances for precipitation begin to decrease later 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 /10Z THURSDAY THROUGH MONDAY/

A low pressure system to our west is beginning to move closer tonight. Wind continues to back around to the E then N in most places. 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 tomorrow 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.