textproduct: Gray - Portland

WHAT HAS CHANGED

Most recent model guidance has trended the forecast colder. The colder trend allowed for the possibility of more snow and ice across Franklin and Somerset counties. Opted to move the forecast in this direction as well due to a historical warm bias in the models during cold air dam setups like this one.

KEY MESSAGES

1. A back door cold front brings colder air tonight with low pressure crossing on Tuesday. Mainly a cold, beneficial rain is expected Tuesday while the mountains and parts of central Somerset County see at least the first snow flakes of the season. Secondary roads branching off from ME-27 or US-201 north of Skowhegan/Farmington may become slick Tuesday afternoon.

2. Drier and breezy conditions return late Wednesday and Thursday with high pressure.

3. Low pressure passes through on Friday, with cold and windy conditions behind the system for the weekend..

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

This evening, a dry backdoor cold front will move south from Canada. A stray shower can not be ruled out north of the mountains. Temperatures look to cool drastically after the frontal passage. For instance, in Kennebec county, temperatures should be in the upper 60s currently. By tomorrow morning, temperatures will be in the upper 30s, with below freezing temperatures likely north of the mountains, and temperatures in the 40s along the coast and southern New Hampshire.

High pressure over New Brunswick will allow for light northeasterly winds through the day Tuesday, and should largely prevent temperatures from warming up through the day. This is meteorological phenomena called a cold air dam. For the population corridor, this likely will not pose any issues, as the cold temperatures will likely stay above freezing. Perhaps more concerning is how temperatures pan out across north-central Maine and how long the cold air dam hangs on to below-freezing or near freezing temperatures across northwestern Maine. Low pressure will move in Tuesday morning, with rain across most locations and freezing rain expected in elevations above 3000ft. For the frozen side of the storm, model guidance today has had a desire to bring more wintry precipitation into North-central Maine. From pattern recognition this makes sense given the guidance is almost always too warm during cold air damming. However, I don't quite see this as translating to more snow, rather longer durations of mixed- precipitation: snow, sleet and freezing rain. If it does snow, A thick isothermic layer in the low- levels should keep SLRs very low, and generally make it very difficult for any periods of all-snow.

By Tuesday night, high pressure over New Brunswick will continue to drift eastward. This will in turn, disrupt the consistent northeasterly winds that form and maintain the cold air dam. Without these northeasterly winds, warm air aloft is much more likely to mix down to the surface and will result in all locations in western Maine changing over to rain late in the evening. By this time most of the storm is over, and will depart the region overnight. Skies may even clear out a little by Wednesday morning.

For wintry accumulations, we're not expecting much. The low SLRs and an eventual changeover to ice and then plain rain should prevent snow from accumulating in this area, but just enough snow and sleet to form a coating to an inch is certainly possible up by Jackman or Moosehead Lake. However, even a trace of icing can make roads slippery so drivers in this part of the state should heed caution.

For rainfall amounts, the bulk of the precipitation is likely to fall across northern NH and Maine, with at over an inch likely in the northern half of NH and locations north of Portland. Southern NH and York county are most likely looking at less, likely closer to a half an inch.

KEY MESSAGE 2 DESCRIPTION...

The low gradually moves away during the daytime on Wednesday. Scattered showers likely linger across the higher terrain much of the day, while areas downwind of the mountains gradually dry out through the day. Temperatures warm once winds shift to westerly behind the system, which will take until late in the day for areas further east. Southern New Hampshire stands the best chance to see temperatures jump into the low to mid 70s in the afternoon, but areas further north and east will be pressed for time before the sunset to actually realize the warmer temperatures forecast by the NBM.

Relatively cooler and drier air settles in for Thursday as high pressure builds across New England. Conditions look seasonable, with highs ranging from the upper 40s across the north, to low 60s along the coast.

KEY MESSAGE 3 DESCRIPTION...

By Friday, low pressure rides eastward into New England. The low deepens as it passes through, and becomes a pretty strong coastal storm as it moves through the Canadian Maritimes. Another round of beneficial rain looks likely during the Friday to Friday Night timeframe, with generally a half inch to an inch of QPF.

As the low deepens while moving through the Canadian Maritimes, a strong northwesterly flow develops across New England. Currently, a small area of modified Arctic air looks to be directed into New England this weekend. There remains the chance that this airmass misses us to the east. However, at this time a cold airmass for mid October looks likely for the weekend, along with gusty winds. Highs in the 30s and 40s look likely for Saturday, along with mountain snow showers. Lows likely dip below freezing all the way to the coastline in most areas, with widespread lows in the 20s. Temperatures look similarly chilly with less wind on Sunday, before temps begin to rebound early next week.

AVIATION /18Z MONDAY THROUGH SATURDAY/

VFR and MVFR CIGs will lower overnight as a low moves in from the west. IFR is expected on Tuesday as a system brings rain and low stratus into the area. While some winter weather is expected, all routine TAF sites should stay plain rain through the entirety of the event. However, a light glaze of ice could occur at Rangeley and Carrabassett Valley Muni Airports, with snow and ice likely in Jackman. Conditions improve Tuesday night as the aforementioned low exits the region.

OUTLOOK:

Wednesday...VFR Expected, MVFR possible in the mountains due to upslope showers.

Thursday...VFR Expected.

Friday...MVFR Expected, with widespread rainfall. Lower CIGs and Visibilities may be possible in rain and snow showers in the mountains.

Saturday...VFR Expected south of the mountains, with IFR in the mountains, with KLEB and KHIE. IFR CIGs are possible here due to the potential for rain and snow showers.

MARINE

Light winds and seas this evening. Overnight winds shift to northeasterlies and strengthen to SCA levels. Seas of 4-6ft expected tonight. Easterly SCA winds continue through the day tomorrow and seas holding steady at 4-6ft through Tuesday night.

Low pressure departs on Wednesday. SCA conditions are possible in westerly winds behind this system into Thursday. High pressure then crosses on Thursday with fair conditions. Low pressure crosses the waters on Friday, with northwesterly gales possible behind this system late Friday and Saturday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory from 8 PM this evening to 2 PM EDT Wednesday for ANZ150-152-154. Gale Warning from 8 PM this evening to 5 AM EDT Tuesday for ANZ180.