textproduct: State College

WHAT HAS CHANGED

* Severe threat has increased for Friday afternoon and early evening

KEY MESSAGES

1) Potent storm system moves across PA tonight and Fri with a severe threat Fri aftn/eve.

2) Windy with first flakes on the higher terrain of the north Fri night/Sat morning

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

DISCUSSION

KEY MESSAGE 1: Potent storm system moves across PA tonight and Fri with a severe threat Fri aftn/eve.

A surface low moving through the Midwest will cross the Great Lakes tonight and Friday. Showers and possibly a thunderstorm or two will develop tonight ahead of a warm front as it lifts northward into the region. Most guidance shows a break in the showers for much of the CWA during the late morning/early afternoon which will allow for at least some air mass destabilization before a strong cold front begins to approach from the west. The HREF shows surface CAPE approaching 1000 J/kg for areas east of I-99 which will be more than sufficient for widespread thunderstorm development along the front. With 0-6 km bulk shear expected to be in the 40 to 60 knot range and oriented nearly parallel to the surface boundary, initial storms should quickly merge into a low-topped squall line capable of producing strong to severe wind gusts. SPC has a wind driven slight risk of severe weather over the southeastern half of the area to highlight this threat.

We will also need to watch the leading edge of the line for brief QLCS tornadoes 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 first flakes possible on the higher terrain of the north Fri night/Sat morning

The post-frontal pressure gradient is pretty tight with 10mb of drop across the state Friday night and Saturday morning. 50+KT winds behind the front look like they will stay AOA 6-7kft and soundings from both the RAP and the NAM show 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 appears much less likely than it did a few days ago.

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 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 /20Z THURSDAY THROUGH TUESDAY/

The main change for the 00Z TAF package, was to add more groups after 18Z Friday, with thunder at IPT, MDT, and LNS.

For this evening, lower CIGS into the southern sites like AOO and JST, and now just south of the office here, as warm advection sets in. Cloud bases outside the office very wavy in nature. Very dry air aloft had mixed down a bit earlier today, but a rather strong dewpoint gradient to the southwest now not far off. Showers limited at this point.

More detail below.

Few to scattered clouds are present over most of the region to begin this afternoon. Building low clouds over JST and AOO are keeping ceilings in the west MVFR ahead of an approaching warm front and low pressure system. Scattered showers and lowering ceilings will accompany the warm front tonight, with IFR ceilings likely (50-70% chance) to overspread all of central Pennsylvania from west to east between 04-08Z FridCTPWRKTAFay as warm air advection drives moisture back in the region. Nocturnal LLWS is also likely (50-70% chance) over the western airfields (KBFD/KJST) tonight 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.

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. After 18Z Friday the cold front will arrive over northwestern PA, bringing a line of strong showers and thunderstorms that will slowly progress southeastward into the early evening hours on Friday.

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-Tue...AM fog possible; otherwise, VFR conditions prevail.

CTP WATCHES/WARNINGS/ADVISORIES

None.