textproduct: Gray - Portland

WHAT HAS CHANGED

Have increased chances for showers late tonight into Monday morning across northern NH and the western ME mountains and then during the day Monday over portions of interior western ME.

KEY MESSAGES

1. Showers (and higher elevation snow) return to northern NH and the western ME mountains late tonight into Monday morning. A warming trend then starts Monday with breezy westerly winds and a few showers possible across interior western ME.

2. Tuesday will be warm and breezy with chances for thunderstorms in the evening near the CT Valley, mountains, and Canadian Border.

3. A stalled front will bring chances for beneficial rain Tuesday night through Thursday night with the highest chances centered on Wednesday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The strong and distant coastal low is currently centered just to the SSE of Nova Scotia and will continue to lift north into the Canadian Maritimes the rest of today. The gusty northwest winds of 25 to 35 mph will continue to advect drier air into the area, but instability and a lingering moist layer aloft while the 500mb trough pushes through will still result in mostly cloudy skies this afternoon. We should start seeing a decrease in clouds early this evening as we approach sunset.

Skies should clear out for the first part of tonight with the loss of daytime heating/mixing and shortwave ridging briefly building overhead. This ridge will quickly shift east as another shortwave trough quickly follows, bringing increasing clouds along with a chance of rain showers to western and northern NH as well as the western ME mountains (except snow showers in the highest elevations) late and toward daybreak. Depending on how quickly clouds move in and if winds can drop off enough, there is a window of opportunity for good radiational cooling and patchy frost. The better chance of this looks to be in the normally cooler, sheltered areas of western ME.

As the same shortwave moves through on Monday, the mountains and far north will have an ongoing chance of showers (snow showers in the higher terrain) into the first part of the day, but the moisture profile is also favorable enough to include a chance of showers farther south toward the ME foothills, Augusta and Midcoast regions in the morning and afternoon.

Otherwise and elsewhere, it will be another breezy day with westerly winds gusting up to 25-35 mph and partly to occasionally mostly cloudy skies. With good mixing, warming temperatures aloft of +2C to +5C will support highs in the 60s for most places.

Dry conditions are expected Monday night as high pressure returns with partly cloudy skies. The clouds and light southwesterly flow will temperatures milder and mostly in the 40s for lows.

KEY MESSAGE 2 DESCRIPTION...

Tuesday will be the warmest day of the week as deep southwest flow advects warm air into the region with mostly sunny skies to start the day. Temperatures at 850 mb will climb to +10C to +12C and with mixing up to 800 mb this will bring high temperatures into the upper 70s to around 80 degrees across much of the region. Southwest winds will also limit sea breezes from Casco Bay southwards, so even southern coastal areas here will experience the warmth. BUFKIT profiles show that mean momentum transfer within the mixed layer will be around 30 kts, which would result in southwest winds gusting upwards of 35 mph early Tuesday afternoon. Cloud cover and chances for showers and thunderstorm will increase from a slow moving front that approaches late in the day that could limit temperatures in the mountains and points northward.

Latest NWP guidance, including some CAMs continue to show sufficient instability and shear building over the area Tuesday afternoon that would support thunderstorms. A rough blend of the FV3/NAMnest/RRFS brings around 750-1250 J/kg of MU CAPE juxtaposed with 35 kts of deep layer shear along the CT Valley through the mountains and towards the Canadian Border. Timing of storms will need to be ironed out to get a better idea of intensity. The NAMnest holds storms off until around 00Z while RRFS/FV3 brings storms all the way to western Maine mountains by 20Z. Deep mixing that occurs before storms arrive will help increase dew point depressions that would make strong winds the primary threat with any storms that are able to organize before the loss of heating Tuesday evening.

KEY MESSAGE 3 DESCRIPTION...

A slow moving cold front that approaches the area late Tuesday will stall over northern New England Wednesday into Thursday with waves of low pressure tracking along the front. The chances for thunderstorms will diminish Tuesday night with the loss of heating while shower chances will remain. The latest NBM brings the best chances for widespread rain Wednesday with recent model runs suggesting the front will shift offshore into Thursday morning. As the front shifts southeast the next wave of low pressure will track offshore Thursday resulting in lower rain chances than previous forecasts. Broad troughing will linger over the NOrtheast Friday into the weekend keeping low showers chances in the forecast while there will also likely be periods of dry weather with a mix of sun and clouds.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through Monday...RKD will return to VFR a little later this afternoon while VFR prevails elsewhere. Northwest winds will continue to gust to around 25-30kt into early this evening before easing around 00Z.

Mainly VFR tonight with winds becoming light. The exception may be at HIE where a weak disturbance may bring a brief period of MVFR ceilings and showers. Mostly VFR Monday and breezy with westerly winds gusting as high as 25 to 30 kt. Can't rule out a couple of showers near AUG on Monday so VCSH may need to be added later.

Outlook:

Monday night: VFR expected.

Tuesday-Tuesday Night: During the day mainly VFR with southwest winds gusting 25 to 30 kts. Chances for showers and thunderstorms increase NW to SE Tuesday afternoon, although storms are unlikely from CON to AUG and points south. Chances for showers Tuesday night bring some potential for MVFR.

Wednesday through Friday: MVFR cigs likely with rain showers. Periods of IFR possible Wednesday into Wednesday night. Shower chances low low cigs will bring potential for at least MVFR Thursday with a drying trend Friday improving conditions.

MARINE

Through Monday night...SCA conditions continue over the coastal waters into Monday night as the pressure gradient remains tight and seas build to over 5 ft. The strong low pressure will depart into the Canadian Maritimes tonight with winds becoming westerly, and these will become more south to southwesterly Monday and Monday night as high pressure becomes centered well southeast of the Gulf of Maine.

Persistent southwest flow continues Tuesday through Wednesday with SCAs likely needed and possibly Gales. A cold front sinks through the waters Thursday into Friday shifting winds offshore.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ150-152-154.