textproduct: State College

WHAT HAS CHANGED

* Added discussion of weak severe threat today

KEY MESSAGES

1) Weak cold front today, then a stronger system moves across PA on Thursday night and Friday

2) Windy and wet end to the week with first flakes in the higher terrain

3) Cold weather may end the growing season for many places Sat and Sun mornings.

DISCUSSION

KEY MESSAGE 1: Weak cold front today, then a stronger system moves across PA on Thursday night and Friday

The front moving over central PA today has stalled out. Isolated showers are present along the PA turnpike and the southern half of the state, but heavy cloud cover and morning convection is keeping instability limited for additional storm development. The environment is not ideal for any additional storms that do manage to initiate this afternoon. Bulk shear is progged >40kt at JST, but tops are only expected to be 20-25kft. Pretty rare that we get any severe gusts with CAPE only in the 200-500J/kg range and no favorable thermodynamic environment to allow winds to accelerate completely down to the sfc.

QPF for today is low with just 0.50" as the top end in the Laurels. Any TSRA should be moving along steadily. So, we should have no risk for flooding rains today.

The next storm system will be driven by a sharp upper trough with a strong sfc high behind it. PWAT does increase to 1.8" in the S. The showers and any storms will be moving very quickly Thursday night and Friday. 1hr FFG is currently 0.8" and higher - but could decrease very slightly due to any rain today. Storm total QPF in the 0.1-0.2" range in the SC-SE and near 0.75" in the NW (where FFG is higher) should not lead to flooding.

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KEY MESSAGE 2: Windy end to the week with first flakes in the higher terrain

Wind gusts 30-45 mph are forecast on the backside of the cold front and will deliver much colder air downwind of the Great Lakes this weekend. Latest NBM probs hold only 10pct chc for advy gusts - and only on the very highest elevations. JST BUFKIT profile from GFS does nudge 60KT winds down to 5kft AGL. So, it will be windy, and worth monitoring for an advy at some point - but only for the highest elevations - probably not for 90pct of the CWA.

The shot of colder air will allow for rain to mix or changeover to snow showers Friday night into Saturday morning across the higher terrain along the Allegheny Mtns. Any minor accum (<0.5") would have to occur overnight and be limited to non paved surfaces.

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KEY MESSAGE 3: Cold weather may end the growing season for many places Sun morning.

Big sfc high moves in for the weekend, supplying dry but cold weather for Sat and Sun. Sat will hold the coldest daytime temps with temps getting just above 40F in the NE. Sat AM could be sub-freezing for the nrn tier. So far, only McKean Co has been taken off the list of zones to need any further frost/freeze products. But, we could add to that list Sat AM - and especially on Sun AM. Sunday morning lows expected to be in the upper 20s to mid 30s may kill the growing season for a significant portion of the CWA. We'll be considering appropriate frost/freeze products, but it is still too early to be that confident on details. Pretty cold again Monday AM, but not quite as cold - and some of the area may not need any more frost/freeze products by then.

AVIATION /06Z THURSDAY THROUGH MONDAY/

Winds have shifted northerly in the wake of yesterday's weak frontal passage, allowing for drier air to filter in across the Commonwealth. A subsidence inversion at around 1500 ft is contributing to periods of MVFR ceilings across the Allegheny Plateau and Laurels (KBFD/KJST/KAOO) as low-level moisture remains trapped beneath the inversion. While the central mountain terminals (KUNV/KIPT) are favored to remain VFR through Thursday evening, intermittent MVFR ceilings will be possible (20-30% chance) into the morning hours on the periphery of greater low-level moisture to the north and west. Farther southeast (KMDT/KLNS), restrictions are less likely through Thursday evening due to lower relative humidity values at the inversion height (RH to around 80% vs. 90-100% farther northwest).

As the weak front slides southward across the Mid-Atlantic, southerly return flow will build out ahead of a more potent low pressure system approaching the southern Great Lakes, allowing for the boundary to lift northward as a warm front this afternoon and evening. Winds of 5 to 10 kts will veer throughout today, shifting from N-NW in the morning to S-SE in the afternoon. Scattered showers and lowering ceilings will accompany the warm front later today, with IFR ceilings likely (50-70% chance) to overspread central Pennsylvania from west to east between 00-06Z Friday as warm air advection drives moisture back in across the region.

Moreover, nocturnal LLWS is likely (50-70% chance) over the western airfields (KBFD/KJST) Thursday night as an LLJ ahead of the system brings southwesterly winds at 925 mb to 35-40 kts. LLWS is less likely with points eastward, farther from the low pressure center moving over the southern Great Lakes.

Outlook...

Fri...Higher chances of SHRA with isolated TSRA. Restrictions increasingly likely. Gusty S-SW winds likely ahead of front, with gusty NW winds behind front.

Sat...Lingering SHRA across N/W PA with some -SHSN mixing in early Saturday morning. Restrictions likely at airfields that mix over to SHSN. Precipitation tapers off by sunrise, VFR prevails with gusty winds.

Sun-Mon...AM fog possible; otherwise, VFR conditions prevail.

CTP WATCHES/WARNINGS/ADVISORIES

None.