Marine Weather and Tide Forecast for Falmouth, ME

KEY MESSAGES
1. The recent dry stretch of weather ends today with increasing chances for slow moving showers and thunderstorms by Tuesday afternoon and evening.

2. A cut-off low develops over the region Tuesday night and Wednesday and remains in the vicinity through Friday. This will bring multiple rounds of rainfall with locally heavy rainfall possible.

3. Some periods of drier weather this weekend with warmer temperatures, but daily chances for precipitation continue, especially Sunday.

DISCUSSION
KEY MESSAGE 1 DESCRIPTION...

Today opens with a large trough over Hudson Bay with shortwaves rotating around this feature. A robust shortwave will reach the eastern Great Lakes by this morning helping to close off a H5 low by this evening over upstate New York. The result will be building southerly flow to the east of this feature with 1.5" PWAT plume pushing east into NH and far western ME by Tuesday night. Mid level forcing lags that moisture plume so expect best potential for afternoon/evening storms to be over NH. The CAMs show a decent setup for locally heavy rain with narrow CAPE profiles, mbE vectors with less than 10kts net storm motion and deep warm cloud depths in the presence of PWATs locally over 1.75". Current depictions show a decent storm motion given that the llevel jet is not all that strong and only isolated/scattered coverage
so
while there are some ingredients for flash flood potential...agree with the previous shift that given such dry antecedent conditions this event will likely only serve as a primer for the larger event on Wednesday.
Cannot rule out an isolated instance of flash flooding but feel the overall threat is low. Clouds will be on the increase through the day with highs cooler than on Monday...close to 80 in most locations.

KEY MESSAGE 2 DESCRIPTION...

Cutoff low slowly moves from upstate NY on Wednesday morning...to southern New England on Thursday morning to the Gulf of Maine by Friday morning. Most impactful weather period is expected to be Wednesday as surface low should drive a band of moderate to locally heavy rainfall through the region...with additional rainfall developing near the low again Thursday. Given clouds and precipitation...temperatures will be below normal through the period.

For Wednesday...the arriving low will strengthen the southeastern LLJ which may allow for more backbuilding of embedded convective elements in a precip-efficient environment
However
instability looks to be less than the day previous. The guidance suite has a decent amount of spread on the exact timing/orientation of the rainfall...but feel that by late morning and through the afternoon moderate to locally heavy rainfall is possible and there will be a threat of isolated flash flooding if training cells can materialize.
While it will be humid...clouds and rainfall will keep temperatures in check with most locations remaining in the lower 70s...and perhaps some spots not reaching 70. Showers will likely continue through Wednesday night with fog also likely given the moist airmass in place.

Shower activity continues for the day on Thursday though placing the heaviest/most persistent shower activity is not possible at this range with a fair amount of spread in the guidance. With the axis of the upper low overhead...rainfall activity may be very slow moving and while the overall column will have cooled a bit...the threat for flash flooding /now with increasingly saturated soils/ may increase if the activity is organized/widespread enough. Expect that as the llevel flow turns more northeasterly Thursday night that any flooding threat will end but some isolated/scattered showers will likely continue.

Reviewing the ensemble probabilities...2" probabilities are generally restricted to far SW ME and NH along/south of the mountains. With flash flood guidance well above this /6 hour guidance over 3" for nearly all of the area and greater than 4" for the NH mountains...lakes region and SE ME/ it suggests that any possible flash flooding for the Wednesday-Thursday period will be isolated. There is not expected to be any river flooding with this event with plenty of storage given a much drier than normal July to date so far.

The cutoff low finally opens up Friday and begins to pull east which should allow more sunshine...and warmer temperatures though the threat for scattered showers will continue.

KEY MESSAGE 3 DESCRIPTION...

Global models are in surprisingly good agreement on the 500 mb pattern going into next week. Low pressure finally loses its grip on the region by Saturday while essentially dissipating offshore.
Should note there is still some split guidance on how quickly this happens, and given cutoff lows are generally not handed well by global guidance, still a bit uncertain how fast this happens.
Regardless, most guidance keeps Saturday on the dry side, with some subsidence behind the coastal low likely playing a role.

Sunday onward may be a different story, as guidance suggests the Hudson Bay upper-level low reorganizes, lifting another 500 mb trough towards New England by Sunday night, with subtle height falls. With southwest flow ahead of this feature developing, as well as a possible surface low somewhere in the Ohio Valley, signs are it will turn humid and possibly rainy once again, although that is about the most we can say with confidence six days out. So in a nutshell, Saturday is likely the drier of the two days, while Sunday brings increasing chances for showers late, especially across the interior.

AVIATION /12Z TUESDAY THROUGH SATURDAY/
Through tonight...

Summary: Increasing moisture has brought low clouds across southern NH and coastal Maine early this morning with cigs likely lifting later this morning. Shower and thunderstorm chances increase this afternoon and evening for NH terminals.
Low cigs and patchy fog are likely tonight with chances for showers

Restrictions: IFR to LIFR cigs across southern NH and PWM likely mid morning. Increasing moisture will allow clouds to thicken and lower across NH and coastal Maine later this morning into the afternoon. Restrictions are again possible this afternoon and evening due to SHRA/TSRA primarily for NH terminals. IFR to LIFR is likely across all sites in low cigs and visibility restrictions in fog tonight.

Winds: Light winds through day break will strengthen again to 10-15kts late morning...diminishing to less than 10kts tonight.

LLWS: No LLWS is expected through the TAF period and through Tuesday night.

Thunder: Isolated to scattered slow moving /movement generally from south to north/ thunderstorms are possible this afternoon and evening, particularly for NH terminals.

Outlook:

Wednesday - Thursday: Slow moving low pressure will bring multiple rounds of rainfall to the region with a prolonged period of IFR/MVFR restrictions. Winds at or below 10kts.

Friday - Saturday: Expect conditions to slowly improve on Friday as low pressure moves away from the region though scattered showers and a thunderstorm are possible. VFR conditions should dominate Saturday outside of an isolated afternoon shower/storm. Winds at or below 10kts.

MARINE
Conditions remain below SCA levels through at least the first half of Wednesday...though there will be increasing potential for fog over the waters by late tonight and continuing through much of the week. Low pressure passing south of the region Wednesday night and Thursday will bring strengthening easterly winds and building wave heights with SCAs likely necessary Wednesday afternoon and continuing through Thursday. Improving conditions arrive to end the week Friday and persist through the weekend.

GYX WATCHES/WARNINGS/ADVISORIES
ME...None.
NH...None.
MARINE...None.