textproduct: Gray - Portland

WHAT HAS CHANGED

Lowered temperatures for Tuesday with cold air damming expected.

KEY MESSAGES

1. The remnants of Isaias will pass south of the area tonight. Periods of rain are likely south of the mountains.

2. A more widespread rainfall will track across the region Tuesday. In addition to beneficial rains, some light snow is possible around the Moose River Valley.

3. Wednesday and Thursday turn drier and breezy before the next chance for widespread precipitation arrives Friday into Saturday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Moisture from what remains of Isaias continues to battle high pressure building across the north. Dry air has been winning that battle so far today, but will gradually give way to rain thru this evening. The best chance of rain remains south of the mtns, and most of the rainfall confined to southern NH where amounts near 0.5 inches are possible. Otherwise rainfall totals will drop off quickly to the north, generally less than 0.1 inches north of the Lakes Region in NH and Portland in western Maine.

KEY MESSAGE 2 DESCRIPTION...

On Mon a bit of a stationary boundary will remain draped across the forecast area and may provide a focus for isolated showers thru the day. Overall it should be mostly dry across the area.

Mon night a backdoor cold front will drive southwest across the region as a strong high pressure branches into Quebec. This occurs just before the next round of precip moves into the area. An active push of cold air combined with precip should ensure cold air damming remains in place thru Tue. I have taken temps down during the day. The NBM often struggles with CAD situations, and this appears to be no different. A blend of the 10th percentile NBM temps and raw GFS produces a pretty fair representation to start with. Guidance at this time focuses on the northern half to two thirds of the area for a solid beneficial rainfall of at least 0.5 inches. This is somewhat lower confidence however as occasional model runs have been farther south. So I have greater confidence in the PoP than the QPF at this point. Also of note will be very chilly temps in parts of the western Maine mtns. Forecast soundings remain supportive of some wintry precip types. As the system gets into the range of more hi-res guidance the WAA aloft is getting stronger. This may take snow out of the mix more but may leave sleet and freezing rain, especially mid-slope in the mtns, on the table. Precip will begin to wind down Tue night.

KEY MESSAGE 3 DESCRIPTION...

Low pressure begins to lift northeast toward the Canadian Maritimes on Wednesday, sending a cold front through in the morning, which will bring an end to what is left of the widespread rainfall. However, the northwest winds behind the front will result in periods of upslope precipitation in the mountains, which should initially be rain, but these could switch to snow showers Wednesday night into early Thursday as colder air move in aloft.

For the rest of the area on Wednesday, forecast soundings suggest a mix of partly sunny and mostly cloudy skies with a few additional showers as a shortwave pushes through. Otherwise, it will be somewhat breezy with northwest wind gusts to around 20-30 mph, and high temperatures will be mainly in the 60s with some guidance even advertising low 70s for parts of southern NH.

A secondary front crosses Wednesday night or early Thursday, and the tightening gradient between the low over the maritimes and high pressure building in from the west will result in even breezier conditions. Gusts to around 25-30 mph are possible Wednesday night, and then 25 to 35 mph during the day Thursday, perhaps closer to 40 mph at times. Drier air will also move in, so we'll likely see more sunshine with upslope precip in the mountains coming to an end. Cold air advection will limit highs mostly to the 50s.

A cold night is possible Thursday night as high pressure builds briefly overhead, but it quickly exits to the east ahead of the next front and low pressure. There's strong support in the ensembles for most of the area to see measurable precipitation Friday into Saturday, so the high NBM PoPs of 60-80% are justified. This will be rain for most, but some solutions are farther south with the low track, which may be cold enough for light snow accumulations across the north. Guidance favors drier weather returning by Sunday with cooler temperatures moving in for the weekend.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through Monday...VFR conditions will gradually deteriorate thru this evening as rain inches northward. A brief period of MVFR is expected at most terminals, and then widespread IFR CIGs are anticipated. HIE will be the one terminal where MVFR or even VFR may persist tonight. By morning conditions should improve Mon.

Outlook:

Monday night: Gusty northeast winds possible near the coast. Areas of MVFR and IFR developing towards morning.

Tuesday: Areas of IFR or lower in rain. Some LLWS possible near the coast.

Wednesday: MVFR possible in the morning as rain gradually comes to an end from west to east, but a return to VFR is expected for most sites, although can't rule out a few additional showers to cause TEMPO MVFR in the afternoon. The exception will be periods of MVFR and rain showers at HIE. Northwest wind gusts to around 25 kt possible.

Wednesday night: Mainly VFR, except possible MVFR cigs and rain/snow showers at HIE. Northwest winds may remain gusty up to 25 kt.

Thursday: MVFR restrictions possible early in the day at HIE due to rain/snow showers. Otherwise VFR with breezy northwest winds gusting 25-30 kt.

Thursday night: Mainly VFR with lighter winds.

Friday: IFR to MVFR possible with increasing rain chances.

MARINE

Winds and seas expected to remain below SCA thresholds thru Mon. A backdoor cold front will cross the waters Mon night, and it is possible that SCA conditions develop with gusty northeast winds. Winds and seas will diminish briefly and then southerly winds develop and strengthen later Tue. I am less confident that south winds will gust to SCA thresholds, but it cannot be ruled out depending on the strength of the system.

Wednesday-Sunday...SCA conditions may occur Wednesday as a cold front crosses sometime in the morning with winds turning west to northwesterly. SCA level winds become more likely Wednesday night and Thursday, and possibly gales over the outer waters, as a secondary front crosses the waters and the pressure gradient tightens between low pressure over the Canadian Maritimes and high pressure building in from the west. There may be brief improvement Thursday night as the high builds overhead but will potentially be followed by more SCA conditions and possibly gales Friday into the weekend as another low pressure/front approach and cross through.

GYX WATCHES/WARNINGS/ADVISORIES

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