Marine Weather and Tide Forecast for Chambersburg, PA

KEY MESSAGES
1) Big temperature swing ahead with record challenging warmth followed by major cooldown this weekend

2) Windy and wet end to the week with first flakes in the higher terrain

DISCUSSION
KEY MESSAGE 1: Big temperature swing ahead with record challenging warmth followed by major cooldown this weekend

Abundant sunshine and gusty southwest breeze fueling record challenging warmth this afternoon across central PA. It's an OcTOASTer pattern for sure. The unseasonably mild temperatures continue tonight with min temps +20 to +25 degree above climo in the upper 50s and lower 60s.

In stark contrast to the summerlike conditions this afternoon, a major cooldown is looming for the end of the week into the weekend. The temperature difference between highs today and minimum wind chills Saturday morning will be 50 to 60 degrees! For example, the most extreme case would be around Bradford, PA with a drop from 78F this afternoon to a wind chill of 18F Saturday morning.

Friday night could be a wind-driven, advective cold freeze situation (no frost due to gusty winds) across parts of the Alleghenies, with a more traditional frost/freeze pattern setting up for Saturday and Sunday nights.

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KEY MESSAGE 2: Windy and wet end to the week with first flakes

Latest HRRR/REFS bring some rain showers from northern Ohio across the southwest part of the forecast area on Wednesday.
Spotty rain chances should be largely confined to the southern tier Wednesday night into early Thursday. Total QPF through 00Z Friday ranges from a couple of hundreths across the Endless Mtns/NEPA to 0.25-0.50" in the Laurel Highlands.

An amplified upper trough will cross the Great Lakes Fri-Sat and spin up a deep sfc low near the Canadian Maritimes. This system will bring widespread rain/showers Thursday night into Friday before transitioning to a lake effect/enhanced precip pattern Friday night into early Saturday. Total QPF from 00Z Friday to 00Z Saturday ranges from around 0.75" in the NW mtns downwind of Lake Erie to <0.10" in the southern tier valleys to the east of I-99 and south of the PA Turnpike.

Gusty winds 35-45 mph are forecast on the backside of the deep low and will deliver much colder air downwind of the Great Lakes this weekend. The shot of colder air will allow for rain to mix or changeover to snow showers Friday night into Saturday morning across the higher terrain along the Allegheny Mtns. Any minor accum (<0.5") would have to occur overnight and be limited to non paved surfaces.

AVIATION /18Z TUESDAY THROUGH SUNDAY/
VFR prevails across central Pennsylvania this afternoon as AM fog and low stratus have mixed out. Upper-level suppression under the right-exit region of a jet streak will keep conditions dry across our area, with more numerous showers and thunderstorms over Upstate NY as a progressive low pressure system passes to our north.

LLWS will overspread the southern 2/3rds of the Commonwealth later this evening and into the overnight hours due to a nocturnal LLJ developing ahead of an approaching cold front.
Lower confidence at this time in LLWS criteria being met across northern PA (KBFD/KIPT) as the pressure gradient looks to be maximized south and east.

Meanwhile, low-level moisture will increase from NW to SE across the Commonwealth later tonight and into Wednesday as the cold front begins to move through. The lowest ceilings (mainly IFR) are expected across the Northern Tier (KBFD) as the moisture moves through overnight and into Wednesday morning, with the highest ceilings (leaning VFR, though MVFR still possible) across the Lower Susquehanna Valley (KMDT/KLNS). As moisture increases ahead of a front, pre-frontal showers and perhaps a few thunderstorms may move across the area Wednesday morning and into the afternoon. While instability may not be sufficient for thunder across the northern terminals (KBFD/KIPT), MUCAPE upwards of around 500 J/kg may support lightning activity over southern and central PA. Nevertheless, confidence is low regarding the timing/placement of these showers or thunderstorms, which will ultimately depend on the upstream evolution of overnight convection.

Outlook...

Thu...A period of VFR is favored behind the front, with a chance for restrictions and -RA later on Thursday as the front oscillates back northward. AM fog possible.

Fri...Scattered -SHRA and isolated -TSRA could bring restrictions across the region with cold FROPA. W-NW winds may gust up to around 25-35 kts behind the front.

Sat...Localized restrictions possible early due to post-frontal stratocumulus and lake-effect -SHRA/-SHSN, primarily across the Northern Tier. Otherwise, VFR prevailing with NW-NNW wind gusts up to around 25-35 kts possible regionwide.

Sun...VFR likely. AM fog/frost possible.

CLIMATE
Daily record high temperatures for October 13th and 14th:

MDT 85/1954 | 85/1975 IPT 83/1954 | 84/1975 AOO 82/2024 | 80/2021 BFD 78/1969 | 79/1975

STC 82/1930 | 84/1897 State College COOP (7AM-7AM)

CTP WATCHES/WARNINGS/ADVISORIES
None.