textproduct: State College

WHAT HAS CHANGED

* No significant changes. Severe threat remains highest across the Eastern half of PA this afternoon and early this evening

KEY MESSAGES

1) A potent cold front preceded and accompanied by an anomalously strong low-level WSW jet of 45-55 kts will move SE across PA today with a severe threat focused this aftn/evening.

2) Windy with the first snowflakes of the season possible on the higher terrain of the north tonight/Sat morning

3) Cold weather may end the growing season for many places this weekend, with Sun morning the coldest.

DISCUSSION

KEY MESSAGE 1: A potent cold front preceded and accompanied by an anomalously strong low-level WSW jet of 45-55 kts will move SE across PA today with a severe threat focused this aftn/evening.

A surface warm front extending from KELM to KUNV and KAOO/KCBE at 08Z will drift northeast across the Susq Valley and Eastern PA through the mid morning hours. This weather feature will bring a few periods of moderate to briefly heavy rain showers before a several hour lull in the rainfall occurs for the late morning and early afternoon hours.

The rather optimal timing of the break in the showers for much of the CWA during the late morning/early afternoon will allow for at least some air mass destabilization before a strong cold front begins to approach from the west.

The HREF shows surface-based CAPE approaching 1000 J/kg for areas east of I-99 which will be more than sufficient for widespread thunderstorm development along the front in the high shear/relatively low CAPE envir.

With 925-850 mb West-Southwesterly winds averaging 45-55 kt and 0-6 km bulk shear expected to be in the 40 to 60 knot range and oriented nearly parallel to the surface boundary, initial discrete cells should quickly merge into a low-topped, fast- moving squall line containing mini-bows, capable of producing strong to severe wind gusts. SPC continues the wind-driven slight risk of severe TSRA over the approx eastern half of the CWA to highlight this threat.

We will also need to watch the leading edge of the line for brief QLCS tornadoes (mainly over the Susquehanna Valley and points east) as model soundings show between 20 and 30 knots of 0-1 km shear and over 100 J/kg of 0-3 km CAPE that may aid in stretching any vorticity. The threat will likely be maximized (although a much lower chance than the potential for damaging wind gusts) during the afternoon over southeast PA, and this is where SPC has included a 2 percent tornado contour.

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KEY MESSAGE 2: Windy with the first snowflakes of the season possible on the higher terrain of the north tonight/Sat morning

The post-frontal pressure gradient is pretty tight with 10mb of drop across the state tonight and Saturday morning. 50+KT winds behind the cold front will stay AOA 6-7kft and soundings from both the RAP and the NAM show NW winds at the top of the mixed layer remaining below 40 knots. This should prevent wind gusts from getting any stronger than 35 or 40 mph and a Wind Advisory continues to be a low probability.

850 mb temps over Lake Erie will fall to around -5C by Saturday morning as a much colder air mass moves in. This is certainly cold enough to support lake effect clouds with lake surface temps still in the low 60s, but the potential for widespread rain or snow showers continues to decrease.

Northerly flow will result in a very short fetch across the lakes, so moisture will be greatly limited. Even though model RH profiles do show saturation in the DGZ, significantly drier air below that level should prevent much of the precipitation from reaching the ground. That being said, most models do still show a few light rain or snow showers developing over the northern tier. The window of opportunity for these showers will be rather short as drier air moves in by mid-morning. ------------------------------------------------------------

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

A large surface high moves in for the weekend, supplying dry but cold weather for Saturday and Sunday. Saturday will hold the coldest daytime temps with temps getting just above 40F in the NE and 52F in the SE. Those maxes will be 20-30F lower than Friday's maxes.

The current forecast show temperature dropping below freezing Saturday morning over parts of northwest PA, but there is still some uncertainty regarding just how low temperatures get since it should stay pretty cloudy for much of the night. If confidence continues to increase in parts of the region dipping below 32, a Freeze Warning may be needed for some zones across north-central PA. McKean County would be left out of any freeze products since their growing season has already ended. Frost is less of a concern Friday night/Saturday morning due to the aforementioned cloud cover as well as the fact that winds likely stay pretty gusty.

Sunday morning will be the most-likely time for sub-freezing temps across much of the CWA. This could end the growing season for a significant portion of the forecast area. A widespread frost is almost a certainty as winds decrease and go mainly calm Sun AM. We'll be considering appropriate frost/freeze products, but it is still too early to be that confident on details. Sunday night will likely also feature fairly widespread frost, but the location of any frost/freeze headlines will depend on where the growing season remains active after the previous night's freeze.

High pressure is expected to remain in control for at least the first half of next week. This will result in mainly dry weather with high temperatures returning to the upper 50s and 60s.

AVIATION /08Z FRIDAY THROUGH TUESDAY/

Low cigs (anywhere from IFR to VLIFR) early today will occur in advance and likely for a few hours in the wake of a warm front moving through the Central Mtns and Susq Valley through about daybreak. Upward vertical motion over this boundary will bring a few, brief periods of moderate to heavy rain showers to roughly the NE quadrant of the state through 12Z. Stronger 40+ DBZ cores of radar reflectivity should remain below the -10C level for the most part, so the prob of thunder is low early today and much of the wording for TSRA through the mid-morning was removed.

LLWS is likely to be widespread across the CWA prior to the Cold Frontal passage this morning/through mid afternoon as we see the base of 35-45 kt south to southwesterly winds down to between 1000-1500 ft AGL.

A break in the precipitation will likely occur during the late morning for most airfields between the warm and the cold front, but ceilings will likely stay at or near IFR conditions throughout the day. Winds will likely be most gusty over the western terminals of JST and AOO with guts of 35kts possible.

During the 15-17Z period today, the cold front will arrive over northwestern PA, bringing a growing line of heavy showers and thunderstorms that will gradually strengthen and progress southeastward - crossing the Central Mtns during the mid afternoon hours and Mid-Lower Susq Valley respectively, late this afternoon/early this evening.

Brief wind gusts over 40 kts from the WSW-W, vsbys down to 1-2SM and CIGS under 1 KFT AGL are likely as the cold front and line of heaviest SHRA/TSRA moves through.

Outlook...

Late today and Sat...Gusty NW winds of 30-35 KTS behind front late today into early 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-Tue...AM fog possible; otherwise, VFR conditions prevail.

CTP WATCHES/WARNINGS/ADVISORIES

None.