Marine Weather and Tide Forecast for Chambersburg, PA

KEY MESSAGES
1) Slightly cooler but still much warmer than average today and Saturday.

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: Slightly cooler but still much warmer than average today and Saturday.

A moisture-deprived cold front will cross the region today and knock temps down 5-10 degrees compared to yesterday. Even still, temperatures will be +10 to +15 compared to average. Today will be rain-free for most with a mix of sun and clouds. Temperatures will trend a bit warmer again on Saturday as southerly flow increases ahead of an approaching cold front. South winds could gust 20 to 30mph ahead of the front on Saturday afternoon.

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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.

The next round of showers and storms will come ahead of a cold front Saturday afternoon and evening. SPC has introduced a SLGT severe weather outlook for eastern OH and western PA Saturday, with the MRGL risk area extending east to the I-99 corridor. A mid-level trough will drift across the Great Lakes and help generate sufficient instability and shear for severe weather.
There remains some question as to how much moisture will be in place ahead of the front, especially east of the Allegheny Front. If dewpoints trend above 60F, the risk for severe weather could further increase. Damaging straight line wind gusts appear to be the primary threat, but if any discrete cells can form and avoid upscale growth, the risk for tornadoes and/or hail would be locally enhanced.

After the initial batch of storms moves through, a secondary area of stratiform rain showers is likely along the surface front overnight Saturday. A few snow flakes could even mix in along the northern tier as colder air races in. The cold front should pretty much clear southeast PA by 8AM Sunday morning, but lingering showers are expected through at least midday Sunday for southeast PA.

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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. 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 /07Z FRIDAY THROUGH TUESDAY/
A line of scattered showers and thunderstorms will track eastward across central PA through 03Z Friday as a shortwave trough moves over the region. Instability/moisture diminishes with eastward extent, where clearing skies and dry antecedent conditions have allowed for low-level moisture to mix out more efficiently, leading to a transition to -TSRA at KBFD to primarily -SHRA across all other terminals. TEMPO groups for KIPT/KMDT/KLNS account for brief WNW wind gusts to around 25 kts associated with this band of weakening showers/storms, with an inverted-V profile helping to perpetuate these downdraft wind gusts even as the band weakens. Scattered showers are moderately likely (40-50% chance) to linger across KBFD through 07Z Friday due to lake-enhanced moisture, and across KJST through 11Z due to upslope.

Northwesterly offshore flow in the wake of this evening's showers/storms will transport low-level moisture from Lake Erie to the Northern Tier (KBFD) and Laurel Highlands (KJST) later tonight. At KBFD, the LAMP suggests a moderate to high (60-80%)
chance of MVFR ceilings through 18Z, with IFR being moderately likely (30-50% chance) between 07Z-15Z. Moreover, there is a low chance (10-20%) of LIFR per the LAMP, though upstream observations will need to be monitored to increase confidence.
Further south at KJST, MVFR ceilings are moderately likely (40-60% chance) primarily between 11Z-17Z as winds shift northwesterly. A SCT025 layer was added between 02Z-11Z is to hint at a low chance (10-20%) of MVFR ceilings due to upslope showers, though certainty is too low to prevail MVFR.

In the morning and early afternoon on Friday, a 850 mb temperature minimum associated with the trough will be passing overhead, which when combined with daytime heating and sufficient moisture with dew points in the 50s, spotty showers are possible. This possibility was reflected by PROB30s at KAOO/KUNV/KMDT/KLNS where the HRRR had 30% PoPs between 14Z-15Z and 18Z. After 18Z, VFR is highly likely (80-90% chance) to prevail due to subsidence on the backside of this shortwave, with drier air filtering in from the north.

Outlook...

Sat-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.

CLIMATE
DATE 4/15 4/16 LOW HIGH LOW HIGH Harrisburg 87= State College 61 Williamsport 58 87 59 90=

=Temperature ties the previous record

Record high daily mins are possible again today.

Here's a look at the records as they stand right now, with a * indicating that the forecast is equal to or greater than the record.

Record High Daily Max / Daily Min...
For Harrisburg: April 17: 91 (2002) / 60* (2002)

For Williamsport: April 17: 91 (2002) / 58* (1896)

CTP WATCHES/WARNINGS/ADVISORIES
None.