textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes needed this morning as the forecast remains on track. Just performed the routine freshening of the near term grids and aviation discussion.

KEY MESSAGES

1. It will be mainly dry today and Thursday, but a few mountain showers and storms will be possible each afternoon. The next chance for widespread rain moves in Thursday night.

2. Next chance of thunderstorms arrives Friday with a low pressure system moving through the St. Lawrence Valley. This will be followed by fair and warmer weather.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An upper low will continue to spin over southeastern Quebec today, drifting east northeast into the Canadian Maritimes tonight into Thursday morning. This pattern will lead to some afternoon showers and storms today. CAMs suggest that storms will mainly develop in Canada before moving down and weakening into northern New Hampshire and northwest Maine. RAP soundings do suggest we could see some MUCAPE values up to 1000 J/kg here with marginal shear, so some small hail and gusty winds could be possible under the stronger cores. Showers and storms will diminish quickly with the loss of daytime heating.

Progressive ridging crosses the region on Thursday in advance of the next wave. A weak leading impulse could fire off some more scattered showers and storms on Thursday afternoon, potentially making their way out of the mountains this time. Warm air advection ahead of the main wave will then bring our next chances of widespread precipitation Thursday evening and into the overnight hours.

Highs today will range from the mid to upper 60s across the north country, to the mid/upper 70s south. A few lower 80s will be possible over southeast New Hampshire and coastal areas. Highs on Thursday will be similar but all areas should be just a touch warmer.

KEY MESSAGE 2 DESCRIPTION...

Warm front is forecast to grudgingly lift thru the forecast area Fri morning. The major question will be the degree of clearing behind any morning precip and cloud cover. We will not be looking at anything near the strength and anomaly of the storm last Thu, but the CAPE/shear overlap is forecast to be seasonably strong. This is especially true across NH and southwestern Maine. There is support from the ECMWF EFI, convective machine learning guidance, and NWP trends in speeding up the early day precip exiting the area. I am not too worried about getting details into the grids at this time, but forecast soundings suggest that there will be ample shear for storm organization but hodographs are rather straight. So while I cannot totally rule out an isolated tornado completely, the primary hazards would appear to be wind and then hail. Again this will be highly conditional on there being enough clearing to realize the potential instability. Once the sun begins to set that should see showers and thunderstorms diminish quickly in coverage and intensity.

Beyond the frontal system high pressure will try and build into the area but guidance is trending towards a large ridge building over the central CONUS. This will tend to keep heights lower aloft locally thru at least Mon. That will mean the threat of diurnal shower and thunderstorm activity. So I cannot say completely dry. The NBM currently has scattered to isolated PoP, and that seems fine for messaging purposes at this range. Temps will also begin to creep back up as the ridging starts to build eastward especially. We should see readings getting back well into the 80s, and even a few low 90s possible in the typical warm spots in the Merrimack Valley.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Through 12Z Thursday...Valley fog is lifting this morning and will be giving way to VFR conditions for those affected terminals. Otherwise, VFR prevails through tonight and into the first half of the day Thursday. Scattered thunderstorms will likely lead to some MVFR restrictions or lower Thursday afternoon.

Outlook: Thursday night: Restrictions ranging from MVFR to LIFR are possible as low pressure and a cold front approach the region. A steady rain may develop with fog and low ceilings.

Friday: Low confidence IFR or lower in marine fog/stratus. Otherwise VFR conditions until cold frontal passage. Local MVFR or lower possible in showers/thunderstorms.

Friday night: VFR conditions expected. Patchy valley fog possible, especially in the northern valleys.

Saturday - Monday: VFR conditions expected. Local MVFR or lower possible in showers or thunderstorms.

Monday night: VFR conditions expected. Patchy valley fog possible, especially in the northern valleys.

MARINE

Light and variable winds are generally expected. Winds will mainly be out of the west through Thursday morning. Seas of 2- 4 ft expected through this timeframe. Widespread showers and storms return as a cold front approaches Thursday night into Friday morning.

Winds and seas expected to remain below SCA thresholds. Will not be surprised to see marine fog and stratus ahead of the cold front and that will linger thru much of the day Fri. Behind the front things will return to clear.

GYX WATCHES/WARNINGS/ADVISORIES

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