Marine Weather and Tide Forecast for Chambersburg, PA

KEY MESSAGES
1) Rain from possible thunderstorms late this afternoon and evening could be locally very heavy over the Laurel Highlands and South- Central Mountains. There is still potential for some heavy rain to fall over the Lower Susquehanna Valley and Central Ridge-and-Valley region.

2) High pressure yields a short dry window for late Monday afternoon through Wednesday morning, before more storms arrive Wednesday afternoon into Thursday.

DISCUSSION
KEY MESSAGE 1: Rain from possible thunderstorms late this afternoon and evening could be locally very heavy over the Laurel Highlands and South- Central Mountains. There is still potential for some heavy rain to fall over the Lower Susquehanna Valley and Central Ridge-and-Valley region.

11 AM Update...
Basically, nothing has changed in the forecast for the next 24 hours. Clouds remain in place overhead as the morning slug of rain moves off to the east. While we did not get the heaviest rainfall possible as we lacked deep convection this morning. We still picked up 0.25" or so in the watch area, priming the ground, but probably not making any rises in the creeks and streams yet. We will (still) need to get rid of the clouds to get enough heating for the convection currently in far wrn OH to survive into Central PA, and even to continue to move through the region. Clearing is already up to the PA/OH border at 11 AM.
Thunderstorms are most-possible (likely) in the western counties where clearing is most possible. The storms are generally expected to move along steadily, but some training/multiple storms are possible, still. So, we'll continue the watch. SPC outlook has not changed at all, either, with the possibility of severe winds (and even a tiny chc of a tornado)
there in the SW part of the CWA, too.

Previous...
An area of moderate to briefly heavy rain from weakening Ohio Valley TSRA was approaching the Laurel Highlands at 0830Z. MRMS 3 hour rainfall estimates indicate that this area of rain will amount to 0.3-0.5 of an inch across the Laurel Highlands, north to Clearfield County and similar or slightly lower amounts by the time it reaches Fulton and Franklin counties in Scent PA.

North of the Interstate 80 Corridor across Central PA, the Middle Susq Valley and its West Branch Valley, rainfall this morning will likely be between 0.10 and 0.25.

After a several hour or more break in the significant rainfall (~13Z-18Z when only around 0.10-0.15 of an inch of rain is expected) peak instability and a llvl Theta-E convergence max near a NE advancing warm front across far SW PA and the Laurel Highlands will be the primary ingredients for two or more rounds of TSRA, which could lead to a quick 1-2 inches of rainfall.

Previous, still valid disc points follow below -

PWATs get above 2" as the brunt of the morning convection dives into MD. The first forecast issue is just how far north the heaviest precip will track. Some recent guidance does indicate a swath of heavy rain being possible north of the first (southern) tier of counties in the morning. FFG over much of Somerset and Cambria Counties is close to 1"/hr. While a little more drying is expected before the rain arrives there, the FFG will likely still be close to being that low when it starts raining. The initial slug of thunderstorms will cross the southern tier by 10 or 11 AM. While this first slug may not be enough straw to break the camel's back, the piece that may do it and make flash flooding may arrive in the afternoon. WPC ERO SLGT risk is well into the CWA, even up almost to Route 6 across the nrn tier.

As alluded to...there will _probably_ be a second surge of instability and lift during the late afternoon and evening. The certainty of this second round of storms is dependent on the erosion of a stable layer which will likely form after the first batch of rain moves through in the morning. If we do break out and get the necessary sun/heating, we should see more tall and wet storms. However, the NAM and GFS paint a layer of sfc/llvl stability/CIN until very late in the day at KUNV as an example and over much of the CWA The FFG is also a bit higher, and ground able to handle a little more rain, on the whole, over the eastern counties. Thus, in collaboration with our neighboring offices, we have posted a Flash Flood Watch for Sunday. However, we have started a little smaller than what we view as the potentially-eventually threatened area. It was a tough decision, with confidence in both the threat area and timing/redevelopment low. Later shifts will be able to evaluate newer guidance as the time window closes.

The threat for heavy rain ends by midnight. So, we have decided to cover both periods of potentially heavy rain with one watch.
While it does stretch across a span of time in mid- day and early afternoon when no heavy rain should occur, the time span is pretty short in length.

Additionally, the potential for strong to severe storms exists as long as we have unstable conditions. The hodographs in the morning are actually well-curved in the south/southwest, but have just a meager LLJet. Shear is stronger in the late aftn/evening. Wind damage is the most probable severe threat, but a spinning storm or two is possible.

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KEY MESSAGE 2: High pressure yields a short dry window for late Monday afternoon through Wednesday morning, before more storms arrive Wednesday afternoon into Thursday.

A cold front moves through late Sunday night and Monday.
Dewpoints don't drop much at all until later in the day Monday.
So, there will still be plenty of time on Monday for instability to develop and pop showers and perhaps a thunderstorm or two during the daytime. By sunset, these will be gone as dry air and loss of heating kill them off. Dewpoints drop into the 50s for many Tuesday morning. Large scale subsidence should yield a mostly sunny day, then. Return flow Tuesday night and Wed will bring the humidity up and a cold front will approach on Wednesday. Storms are expected mainly in the Wed-Wed night timeframe, but speed of the CFROPA will determine if all/part of Thursday will be dry. If showers linger, they would be over the south.

AVIATION /18Z SUNDAY THROUGH FRIDAY/
The first round of rain associated with a Mesoscale Convective Vortex will be moving out of our eastern zones by 20Z.

All attention then turns to a fast moving LEWP with several distinct bow echoes and likely 65+ kt wind gusts racing east to the PA/OH state line ATTM.

The storms should maintain their strength as the surge east along a llvl instability max with an orthogonal mid-level wind max helping to accentuate wind gust potential across roughly the SW half of the of the CWA

Timing of the likely strong bow echo(es)is 21Z at KJST, 22Z at KAOO and 2220Z at KUNV. Westerly sfc wind gusts of 40 KTS or greater likely.

Instability as measured ne way by Mixed Layer CAPE was very minimal and just a few hundred J/KG at 18Z - to the east of a KBFD to KUNV and KMRB line (where the sfc warm front is ATTM).

The airmass is notably cooler and more stable (temps in the 60s)
across the Susq Valley and points east. Therefore the aforementioned storms will likely lose a decent amount of punch by the time they reach the RT 11/15 corridor.

Areal coverage and intensity of the convection will gradually decrease this evening and the warm front will stay pretty much in place tonight and stay as the focus for briefly heavier rainfall rates.

Late tonight, mainly IFR CIGS and MVFR vsbys will occur along with generally isolated SHRA/TSRA.

Outlook...

Mon-Tue...VFR most of the time. Chance of showers/tstm Monday.

Wed-Thu...Afternoon showers or a TSRA with brief restrictions possible. Otherwise, VFR.

CTP WATCHES/WARNINGS/ADVISORIES
Flood Watch until 10 PM EDT this evening for PAZ024>026- 033>036.