textproduct: Gray - Portland

WHAT HAS CHANGED

Daytime showers will be most widespread this afternoon into early evening, but another round of showers is possible across New Hampshire overnight.

KEY MESSAGES

1. Showers continue this afternoon, mainly across the interior. A rumble of thunder can't be ruled out in southwestern New Hampshire.

2. A seasonably strong low pressure will cross the St. Lawrence Valley Wednesday into Thursday. Thunderstorms (some could be strong) will be possible ahead of the trailing cold front Wednesday, and gusty northwesterly winds will be possible behind it Thursday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Showers continue across the interior this afternoon into early evening as an embedded wave in cyclonic flow passes overhead. It will also bring breezy conditions across southern ME through the afternoon.

Overnight, gusts slacken but there will remain a breeze as wave completes its passing. There will be the continued chance for showers across much of NH through tonight, but these should be out of the area by Sunday morning.

Sunday and Monday will be drier, but remain breezy in proximity to tight pressure gradient to the east. Less clouds Sunday night brings the possibility for radiational cooling in the valleys, but higher, more exposed locations could see continued breeze that negates further overnight cooling.

With the eastward exit of low pressure into Tuesday, comes a drier continental airmass from Quebec. Temperatures will have warmed from the weekend, but only into the upper 70s to around 80. Lower daytime relative humidity values could allow for a efficient drying day, but the next chance of rain will be advancing towards the area mid to late week.

KEY MESSAGE 2 DESCRIPTION...

Ensemble MSLP has a sub-1000 mb low pressure moving thru Quebec north of the Gaspe Peninsula. A cold front will trail that low pressure, and most of the weather next week will be centered around its passage. A modest southwest LLJ, on the order of 40 to 50 knots, will develop Wed, but conditions at the surface look stable enough that I am not anticipating significant wind gusts from it. The southwest winds will transport more moisture into the region though, so dewpoints will climb into the 50s across the region, and may even push the 60s in parts of the southern forecast area. While overall the forecast soundings are looking pretty stable, this may be enough for some nominal instability by afternoon. That is concern because there will be plenty of shear thanks to the aforementioned LLJ. CSU machine learning severe probabilities are showing a roughly 5 percent severe thunderstorm risk locally Wed. That seems reasonable, not something I plan to highlight in the forecast grids just yet, but worth mentioning in discussions/briefings.

The precip itself will be showery in nature along the cold front. So I do not anticipate widespread significant rainfall. This is yet another high Pop/low QPF type event, which is more or less what the NBM is forecasting at this time.

Finally once the front crosses the area, winds shift to northwesterly as cold air advection commences. On Thu as mixing heights increase that will lead to a gusty day. At this time forecast soundings over much of the area indicate that gusts around 30 mph are likely. However a few spots may push 40 mph across southern NH and coastal western Maine. Once that cooler airmass settles into the region, Fri night and Sat night may be chilly across many spots. Some widespread 40s are likely, and a the northern valley cold spots may be flirting with upper 30s.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Through 12Z Sunday...SHRA continue this afternoon, more isolated in southern Maine. Occasional downpours could bring brief MVFR vis, otherwise cigs/vis VFR with SCT MVFR cig at times. These decrease in coverage this evening, but rounds of SHRA may continue for NH terminals overnight. If clouds begin to thin after midnight, valley fog could develop late for HIE/LEB. VFR Sunday morning, but breezy as NW winds increase with gusts 15 to 20 kts.

Outlook:

Sunday Night: VFR with valley fog possible. Light winds.

Monday: VFR, no sig wx.

Tuesday: VFR conditions expected.

Wednesday: Local MVFR or lower conditions possible in showers and thunderstorms. LLWS possible near the coast.

Thursday: Becoming VFR. Northwest wind gusts at or above 25 kt possible.

Friday-Saturday: VFR conditions expected.

MARINE

North to northwest winds continue through Sunday and much of Monday as broad low pressure influences the region. Gusts should remain below SCA criteria, but some gusts around 25 kts are possible Sunday on the outer waters.

Southwest winds increase Wed and by Wed night SCA conditions will be possible over all waters. Seas will continue building overnight to 5 or 6 feet outside of the bays. Winds shift to northwesterly Thu and remain breezy, so seas will only gradually diminish thru the day. Winds and seas below SCA thresholds are expected Fri into the weekend.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.