textproduct: Gray - Portland

WHAT HAS CHANGED

Forecast rain amounts continue to decrease for the Thursday/Friday system.

KEY MESSAGES

1. A series of fronts bring widespread showers and maybe a few thunderstorms late Thursday and Friday.

2. Dry weather and lower humidity returns for the weekend. Chances for showers return early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An upper-level ridge axis will continue to move across the region today as we transition into southwest flow aloft. This pattern will keep the weather quiet the rest of today and tonight with the exception of some valley fog tonight into Thursday morning. Lows will mainly be in the lower 50s to lower 60s.

Upper-level trough then starts it's approach tomorrow, leading to increasing chances of showers and thunderstorms in the afternoon into the night. As we continue to move further into the arena of the CAMs, the forecast continues to trend drier. As of now, NBM probabilities for at least a half inch of rain only maxes out in the ~40 to 60 percent range and that's mainly confined to the White Mountains. That being said, many of the CAMs show multiple rounds of showers and storms moving through tomorrow afternoon and evening along and ahead of an approaching cold front. With PWATs close to normal or slightly above normal for this time of year, some brief heavy downpours will be possible under the strongest convection but it will be very hit or miss. 12z HRRR soundings are advertising some instability with 500 to 1000 J/kg of MLCAPE but deep layer shear will be marginal in the 25 to 35 knot range. Thus, a few thunderstorms that move into the forecast area could become strong with small hail and gusty winds, mainly in New Hampshire. Storms will be diminishing in the evening as we lose diurnal heating but showers may linger over some locations through the night.

Another wave will interact with the cold front on Friday, potentially leading to some more widespread showers and a few rumbles of thunder but coverage is still a bit uncertain as some of the longer range CAMs are showing minimal activity and some are showing quite a bit. Precipitation chances then diminish rather quickly as move closer to sunset and the front passes through in the evening.

KEY MESSAGE 2 DESCRIPTION... The front continues to trend faster in the latest guidance with it looking like most of the area will be able to clear out early on in the night and get in on radiational cooling. I still have my doubts about southeastern New Hampshire and the immediate coast and have left their low temperatures in the mid to upper 50s, but the rest of the interior should easily be able to realize the low 50s. North of the mountains will be clear the longest and should be able to radiate down into the mid to upper 40s. While dry air is going to be quickly filling in behind the front, this radiational cooling is likely to produce valley fog. High pressure slides eastward across the region through the weekend which should result in mostly sunny skies and high temperatures generally in the upper 70s and low 80s. Clouds thicken late Sunday as high pressure departs and an upper low moving through Quebec makes a close pass. All global models have a good handle on this now and looks like the result will be a front draped along the International Border and Central Maine that would provide forcing for showers through at least Monday. Tuesday is a little more uncertain as we get ridging aloft, but a second upper low is going to be following a track similar to the early week one and will likely drag another front through midweek. The global models have different ideas on the exact track of this second system, but having a chance of showers for the first part of next week seems reasonable given this setup. This will also come with cooler temperatures just in time for the beginning of meteorological Fall.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

High continues to dominate the weather pattern today, leading to VFR conditions everywhere through the rest of the day. Some more patchy fog will be possible tonight over the valleys (LEB and HIE) and around the coast at RKD but should lift by 12z to 14z tomorrow morning, giving way to VFR conditions by late morning/early afternoon. IFR or lower visibilities will be possible at any sites that do see fog tonight into tomorrow morning.

Outlook:

Thursday: VFR generally expected, but isolated to scattered showers and thunderstorms could bring local MVFR or lower conditions to sites across New Hampshire in the afternoon.

Thursday night-Friday: Generally MVFR conditions expected with widespread showers. IFR conditions possible in fog at night.

Friday night: IFR conditions possible in valley fog at night.

Saturday-Sunday: Any early MVFR/IFR conditions should quickly improve to VFR Saturday with VFR the prevailing condition through the weekend.

Sunday night-Monday: MVFR conditions possible as ceilings thicken and lower and showers develop.

MARINE

Winds and seas are expected to remain below SCA thresholds through the morning. Until the upper trough can cross the Gulf of Maine, there will be areas of marine fog/stratus that form, especially at night, but with winds generally southwesterly much of that may remain Downeast.

A period of marginal SCA conditions are possible in southerly flow ahead of a cold front late Thursday into Friday. Marine fog will be possible Thursday night and Friday. High pressure then builds across the waters for the weekend, bringing an extended period of fair conditions into early next week.

GYX WATCHES/WARNINGS/ADVISORIES

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