textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast package.

KEY MESSAGES

1. Colder weather is forecast through midweek. Overnight frost will be possible, especially Monday and Tuesday night, along with locally dense valley fog.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High pressure will be in control of the region thru Sun night. That means fair weather and seasonable temps are expected. Tonight will be the coldest night. The northern valleys should drop to or just below freezing, while some upper 30s may sneak down to the coast and parts of southern NH. So I do expect some localized frost possible in the coldest spots for zones still active in the growing season, but not widespread enough for headlines. It will also be an ideal night for valley fog. I anticipate patchy fog possible just about everywhere, with the major valleys more likely to see dense fog.

Sun will see increasing clouds with the approach of the next trof. They will gradually thicken and lower from the south and east Sun night. I cannot rule out a few showers sneaking into both the mtns from the cold front itself and along the coast as moisture is advected north into the approaching trof. Because of the abundance of cloud cover, Sun night should be much warmer than tonight and frost/fog is not expected.

A front begins crossing the area Monday morning and some scattered showers may tag along with it, but the overall position of the guidance hasn't budged. The latest NBM gives just about everyone around a 50 percent chance of seeing measurable rain, but any chances of over a tenth get relegated to the International Border and even then the probabilities are only 50 to 70 percent. The ending time of the precipitation is trending earlier as well, so it looks like any chance for a light snow shower Monday night is going to be confined to the higher elevations. A tightening pressure gradient Monday is going to bring west-northwesterly wind gusts into the 25- 35 mph range. These gusts will be higher at higher elevations, so anyone planning a fall hike should prepare for wintry conditions. Winds remain elevated through Monday night, so radiational cooling won't be an issue, but the cold dry airmass behind the front is still going to allow temperatures to fall into the 30s area wide. This combined with the breezy winds could bring windchills into the upper teens and 20s for the mountains and foothills.

850 mb temperatures overhead Tuesday afternoon only look to end up in the -3 to 0C, so surface temperatures only mix into the upper 40s and low 50s in the north and mid to upper 50s in the south. Winds remain breezy on Tuesday, but are going to be trending down as the day goes on as low pressure continues to depart to the northeast. Tuesday may be the coldest night of the season so far, and if winds do end up calming it certainly will be. The latest NBM already brings near freezing temperatures down into the coastal plain, and if it ends up looking like a favorable radiational cooling night this area could see their first hard freeze. For now it looks like at least Frost Advisories may be needed, but again much of the temperature forecast hinges on the wind activity. If we stay well mixed lows may end up closer to what is observed on Monday night.

High pressure settles over the Eastern Seaboard for the remainder of the week which will largely leave our area dry with near normal temperatures. The exception will be in the mountains as troughing aloft may support upslope showers at times.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Widespread VFR conditions expected during the day thru Sun. Tonight radiation fog is expected, and is included at both LEB and HIE. Given the time of year, fog likely does not mix out until closer to 14z.

Outlook:

Sunday Night: Areas of MVFR CIGs developing, with local IFR possible. Rain showers will become more likely near sunrise.

Monday: Generally VFR with some brief MVFR restrictions possible in light rain, mainly in the mountains. West-northwest winds gusting 25- 30 knots.

Tuesday: VFR conditions expected with gusty west-northwest winds continuing, but weakening as the day goes on.

Wednesday-Thursday: VFR prevails once again as high pressure builds back into the region.

MARINE

Winds and seas are expected to remain below SCA thresholds thru the weekend. We will have to keep an eye on moisture return to see if fog can develop fast enough to advect over the waters by early Mon.

A strong cold front crosses the waters Monday night and will briefly bring wind gusts 25-30 kts into Tuesday. Wave heights will respond, building to around 5 ft, before both diminish by the end of the day Tuesday. Sub-SCA conditions then prevail as high pressure settles over the waters for the remainder of the week.

GYX WATCHES/WARNINGS/ADVISORIES

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