textproduct: State College

WHAT HAS CHANGED

* Increasing confidence in damaging frost/freeze Monday & Tuesday morning.

KEY MESSAGES

1) Warmer than average temperatures and dry weather expected through at least early afternoon Saturday.

2) A cold front will bring showers and thunderstorms (some could be severe in western PA) later Saturday

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

DISCUSSION

KEY MESSAGE 1: Warmer than average temperatures and dry weather expected through at least early afternoon Saturday.

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. At the sfc, winds will veer from N early tonight to E and S by daybreak. Easterly flow will advect a marine layer into the Lower Susq Valley, with a stratus deck likely to begin the day Saturday. However, much of the guidance has this stratus deck burning off by the afternoon. High temps on Sat range from the upper 60s to low 70s in the northeast to the low 80s in the southwest.

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KEY MESSAGE 2: A cold front will bring showers and thunderstorms (some could be severe in western PA) later Saturday

A strong 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 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 likely time frame for sub-freezing temps areawide 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 /06Z SATURDAY THROUGH WEDNESDAY/

All sites VFR 06z Saturday. A marine layer moving in from the east late tonight could bring some MVFR or IFR cigs to MDT, LNS, and even IPT after 9Z and lasting until 15-17z before scattering out. Otherwise to the west of the marine layer, clouds are expected to be scattered with high bases. Showers and thunderstorms will start to impact western airfields by 20-23Z. Lightning activity will wane as storms continue moving east after 00Z Sun. However periods of showers will continue into Sat night. MVFR ceilings are favored areawide Saturday night with some IFR possible as well. Winds will shift and become NW later Sat night with gusts to 20-30 kts.

Outlook...

Sun...Restrictions in -SHRA possible mainly N/W, transition to SHSN across higher elevations overnight Sunday night into Monday morning.

Mon...Lingering -SHSN possible early N/W; otherwise, dry conditions with VFR prevailing.

Tue...Dry with VFR conditions prevailing early, rain showers possible during the afternoon and evening hours.

Wed...NW flow with potential restrictions and a chc of -SHRA N/W

CTP WATCHES/WARNINGS/ADVISORIES

None.