textproduct: Gray - Portland

WHAT HAS CHANGED

Confidence continues to improve for blustery and cool conditions this weekend associated with strengthening low pressure passing to the north.

KEY MESSAGES

1. Rain continues this evening with embedded thunder, before dry and warmer weather returns Wednesday through Thursday.

2. Widespread rain returns on Friday, bringing another beneficial soaking rain.

3. Blustery conditions on Saturday will bring a winter feel along with a limited threat for power outages due to a few downed branches.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A very cold rain continues across northern NH and much of western ME this evening. The main rain shield has filled in quite well to the north, while weak low pressure rides along a north progressing warm front.

Along the coast and interior, onshore flow has kept much of the area socked in with low clouds and fog for the day. This has stifled temperatures, along with NE winds reinforcing an active cold air dam setup. Guidance is within a couple degrees of observations, but few capture the cooler reality of this stubborn air.

Low level SE winds are present, as observed in downslope locations NW of the Whites, but it will take awhile for these to overcome a stout low level inversion. The cool low level temperatures have allowed higher peaks in western ME to see freezing rain and light snow through the event, with MWN also reporting light freezing rain.

The unique setup thickens, as an EML approaches from the west this evening. Stratiform rain should become more showery into the mid to late evening as elevated instability increases. Continued to mention thunder for the overnight period, and can't rule out an isolated instance of small hail should any higher CAPE values materialize.

A cold front will then slide through the area Wednesday, with upslope clouds and light precip continuing, but most cloud cover thinning downwind of the mtns with breezy WNW winds. These will be felt the most in the Whites and western ME mtns where wind chills will continue well below freezing in the brisk NW winds. These continued winter conditions could catch some fall hikers off guard as valley temperatures warm to the 50s to 60s, and those along the coast rebound to the low to mid 70s.

Cooler, drier, and not as breezy into Thursday as temp values fall behind the cold front, but manage to rise into the 50s to around 60 outside of the mountains.

KEY MESSAGE 2 DESCRIPTION...

Low pressure will be strengthening through the Great Lakes Thursday night as a sharp mid level trough digs through Michigan. Regional jet dynamics will be supportive of a low that continues to strengthen through Saturday morning into the Canadian Maritimes.

This should bring another round of beneficial rain to the area, in addition to blustery conditions behind (see KM3). Rain should be underway Friday morning for NH, increasing east into western ME by noon. The storm is progressive, so rain won't linger much more than 8 to 12 hours in one location. Winds quickly shift NW Friday evening, and this will begin a period of CAA and upslope rain/snow showers for the mountains lasting through Saturday.

NBM QPF continues to trend greater for this event, but values remain below that of today's system...around three quarters of an inch. Wintry precip will again be possible with this event, with the first round bound to higher elevations Friday morning. The second round, with more confidence, will be associated with the cooler NW winds arriving Friday night into Saturday. These snow showers could fall more widespread across northern NH and western ME, mainly in the mountains but perhaps some snow showers spill into the foothills given the broad 35 to 40 kt low level wind field.

KEY MESSAGE 3 DESCRIPTION...

Forecast confidence is above average for a blustery weekend with well below normal temperatures for mid-October. A strong surface cold front will push of the coast early Saturday morning, with strong NW flow in its wake as a mid level trough axis swings overhead. Anomalous NW flow from 850mb to the surface will support a gusty day with the potential for power outage impacts, especially in the eastern lee-sides of the mountains in downslope favored areas. Will have to watch this trend through the week, but gusts 40+ mph are possible. In addition to the winds, it will be cold with wind chill temperatures making it feel more like winter than fall. Higher elevations will experience full on winter conditions and will be dangerous above the tree line, fall hiking is not recommended. Cold weather will continue through the weekend with sub-freezing temperatures possible Saturday and Sunday night.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18z Wednesday...IFR conditions continue this evening mainly due to low cloud bases, but also occasional fog/rain that creates lower visibility. Continuous RA will lift north this evening, becoming more showery through the overnight. Can't rule out some embedded TS late. Weak LLWS will also be possible as a strong surface inversion remains overnight. Expect a trend towards MVFR Wednesday morning, but it is likely IFR clings as it slowly exits north amid a shift in wind. Guidance is confident terminals south and east of the mountains clear to VFR by mid to late morning, with WNW winds gusting to 15 to 20 kts. MVFR/IFR ceilings could last NW of the mountains through much of the day.

Outlook...

Wednesday night-Thursday: Mainly VFR, except MVFR remains possible at HIE. Northwest wind gusts to around 25 kt possible Thursday.

Friday-Friday night: Widespread IFR and rain are likely. Rain should taper off Friday night with improvement to MVFR or VFR. HIE may see lingering rain showers switch to snow showers with IFR or MVFR there.

Saturday: Rain and snow showers likely at HIE to bring MVFR/IFR. Elsewhere VFR, although it's possible a few showers make it downwind of the mountains. It also looks very breezy with west to northwest winds of 30 to 40 kt possible.

Sunday: VFR. W winds slacken through the day. No sig wx.

MARINE

A warm front will slowly push north across the waters tonight with winds becoming south. Wind gusts should remain 25 to 30 kts, but can't rule out a few gusts to Gale closer to the waters outside Penobscot Bay this evening. Wave heights 4 to 5 feet linger through Wednesday morning before subsiding below 4 ft.

High pressure moves into the region Wednesday night into Thursday, before another area of low pressure gathers strength over the Great Lakes. This low will be more potent, likely passing inland from the waters. It will likely feature onshore SCA winds Friday, before turning NW Friday night through Saturday with greater magnitude, likely Gale.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 2 PM EDT Wednesday for ANZ150-152- 154. Small Craft Advisory until 2 AM EDT Wednesday for ANZ151-153.