textproduct: State College

WHAT HAS CHANGED

* Slight Risk of severe weather expanded to west of Altoona on Saturday afternoon. * Increasing confidence in damaging frost/freeze Monday & Tuesday morning.

KEY MESSAGES

1) Warmer than average temperatures and dry weather expected through tomorrow.

2) A cold front will bring showers and thunderstorms (some could be severe in western PA) and a break from summer heat late this weekend.

3) A damaging frost/freeze is likely Monday and Tuesday morning.

DISCUSSION

KEY MESSAGE 1: Warmer than average temperatures and dry weather expected through tomorrow mid day.

Mid and upper level ridging has built in today with a surface slowly moving east through the region will keep temperatures above average and no chances of precipitation through Saturday evening. Today was cooler than the last few days but as the surface high moves to the east tonight with SW flow developing aloft once again, 850 temperatures get back to 5-10C above average. Day time highs will be back into the upper 70s and low 80s with few clouds. The exception is east of the high terrain in central PA as a marine layer moves in, keeping temperatures a little cooler with low stratus.

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KEY MESSAGE 2: A cold front will bring showers and thunderstorms (some could be severe in western PA) and a break from summer heat late this weekend.

A strong late season cold front moves in late in the day Saturday bringing a threat of showers and thunderstorms in the afternoon and overnight. Timing of the front moving through PA is early evening through the overnight hours from west to east. Given the later timing of the front, western PA has the better chance of seeing stronger thunderstorms with better instability from day time heating. SPC has highlighted a marginal risk (1/5) for State College and west, with the slight risk (2/5) expanded to include almost everywhere west of Altoona.

Looking at forecast soundings in western PA, dry air in the low levels helps lead to high cloud bases with an inverted V sounding below the LCL heights. CAPE is tall and skinny but with some curvature to the hodographs, some isolated supercells are possible if cells can remain discrete which will be tough given the strength of the cold front so some hail is possible but will be isolated. There is a slight risk (2%) of tornadoes in NW PA where there is better low level moisture and better surface based instability but LCLs are a little high. Strong wind gusts will be the primary concern with the high LCLs, steep low level lapse rates, as well as some dry air in the mid levels that can entrain into updrafts and lead to stronger downdrafts. The threat for severe thunderstorms falls off quick after dark as those low level lapse rates go away with overnight cooling. Temperatures fall off overnight and into Sunday as a much colder air mass moves into place for early next week.

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KEY MESSAGE 3: A damaging frost/freeze is likely Monday and Tuesday morning.

The significant cold front ushers in much, much colder air beginning Saturday night. In fact, there could even be some snow showers on Sunday as 850 mb temperatures fall to near -10C. While it will be getting near freezing in the NW half of the CWA Sunday night/Mon AM, Sunday night may be too windy for frost to form. Some places will get down to freezing, which will necessitate a Freeze Warning being issued at some point. The most certain time frame for sub- freezing temps is Monday night/Tuesday AM. On Monday morning, freezing temperatures are most likely along, north, and west of the I-99/I-80 corridor. On Tuesday morning, freezing conditions are favored down to the I-81 corridor with frost conditions southeast of there.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Cigs have scattered out this afternoon with VFR tafs at all terminals. With high pressure in place, VFR conditions are expected to continue for most sites over the next 24 hours. A marine layer is moving in from the east tonight that could bring some MVFR cigs to ITP, MDT, and LNS after 9Z to a few hours after sunrise at 14Z or so before scattering out. AOO had some uncertainty with fog formation overnight given the clear skies and some low level moisture. No fog was introduced to the tafs at this time.

Outlook...

Sat eve-Sun...Restrictions in SHRA possible, transition to SHSN across higher elevations overnight Sunday night into Monday morning.

Mon...Lingering SHSN possible early across N PA with restrictions possible; otherwise, dry conditions with VFR prevailing.

Tue...Dry with VFR conditions prevailing early, rain showers move south-to-north across the area during the afternoon and evening hours.

Wed...NW flow with potential MVFR or worse cigs at BFD, but otherwise VFR.

CTP WATCHES/WARNINGS/ADVISORIES

None.