textproduct: Gray - Portland

WHAT HAS CHANGED

Lower snow amounts are expected for portions of the Maine Midcoast and southern Kennebec Valley. The bulk of accumulating snow is now expected this evening through tonight. There will be another round of snow that moves northward through the forecast area Monday morning.

KEY MESSAGES

1. Accumulating snow begins late this afternoon for southern New Hampshire, pushing north into coastal and interior Maine tonight. Another round of snow is likely for the Monday morning commute. Drivers should allow extra time for travel overnight and Monday morning.

2. An extended stretch of below normal temperatures begins Tuesday with ambient or feels like temperatures near or below zero through the majority of the week.

3. The next chance for accumulating snow may come midweek with slick travel possible for the Thursday morning commute.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Impressive WV presentation this afternoon as a baroclinic leaf orients itself along the US East Coast. Low pressure will organize off the Mid Atlantic coast this evening, progressing northeast outside of the Gulf of Maine.

As a result, accumulating snow is expected this evening and overnight for the coast, interior, and central/southern New Hampshire. However, forecast snow amounts have decreased for zones away from the coast in southern Maine. Morning guidance trends have continued to decrease QPF amounts further inland, with the axis of greatest QPF now aligned along the coast and into southern NH (where snow totals have varied little since last night). The ongoing Winter Weather Advisory will continue, but has been split between two segments to communicate these changes in snow amounts. 3 to 6 inches of snow is still forecast for southern NH and York County, ME...with 2 to 4 inches of snow northeast up the ME coast and interior. While snowfall criteria may not be met for a portion of this area, impacts Monday morning will still be slick.

There will be two distinctive rounds over the next 24 hours. The first will deliver the most snow to the region overnight. Snow begins this evening, with snowfall rates greatest across southern NH between 8pm and 1 am. Its here that organized lift bisects the DGZ for a period to allow good flake growth and quicker accumulations. This pivots north, but moves off the Maine coast late tonight, missing the ME Midcoast out to sea.

The second round is expected to be lighter, but bring continuing snow for the Monday morning commute. This looks to be a weak low pres or an inverted trough developing in the Gulf of Maine Monday morning. Cyclogenesis could also be spurred on as a secondary low development from low pres approaching from the Great Lakes. While portions of Maine see a break or weakening snow late tonight, organization of this disturbance could continue snow in NH much of the night through to Monday afternoon. This snow then lifts north as the morning progresses, with an additional 1 to 2 inches. Should this feature slow, could see a time extension in the Winter Weather Advisory possible.

KEY MESSAGE 2 DESCRIPTION...

High pressure begins to build in to the area Tuesday making for a dry day, but this also coincides with an arctic airmass entering the region. 850 mb temperatures plummet to around -20C by the afternoon, leaving high temperatures topped out in the upper teens to low 20s south of the mountains and in the low to mid-teens in the north. It is going to feel even colder as a tight pressure gradient supports wind gusts of 20-25 mph. This makes it feel more like single digits in the south and single digits below zero in the north. A shortwave crossing the area adds some clouds to the mix for the afternoon and evening, but heights continue to rise over the the region and clear skies by nightfall. This combined with fresh snow and calming winds should make for a very cold night. I blended in some of the 10th percentile NBM to account for what should be solid radiational cooling resulting in low temperatures bottoming out in the low single digits from the foothills southward and in the low single digits below zero. The pressure gradient looks to relax quite a bit by this point, but enough a breeze remains to add a windchill factor that pushes the entire area below zero.

850 mb temperatures warm midweek as an approaching shortwave turns flow southwesterly, but once it passes arctic air will once again be able to enter the region with very cold temperatures possible again toward the end of the week.

KEY MESSAGE 3 DESCRIPTION...

A shortwave trough approaches the region Wednesday night with low pressure passing to our north, dragging a cold front into the area. All this lift through the column would be enough to support snow with cold air already in place. Ensembles are in fairly good agreement on this low track, but do have some subtle timing differences with the GEFS lagging slightly behind the Euro Ensemble. The other thing Ensembles are in good agreement on is QPF around a tenth of an inch, which with temperatures expected to be in the teens may result in accumulations over an inch on account of the likely higher snow ratios. Even with the subtle timing differences it looks like a slippery Thursday morning commute is in the cards if this occurs.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through 18z Monday...VFR conditions through 00z, but ceilings continue to lower, especially for southern NH. MVFR/IFR trend expected quickly this evening as SN begins south to north. Vis impacts greatest for southern NH terminals where snow rates will be heaviest...1/2SM possible between 1z-6z. ME terminals along the coast will see largest vis impact due to SN. Light snow continues for the forecast area into Monday morning where snow will life across the area with additional period of lower vis. This begins to exit northeast after 18z, with VFR trend into Mon evening.

Outlook

Monday Night: Ocnl SN remains for HIE into the overnight hours, otherwise VFR with gusty WSW winds 15 to 20 kts.

Tuesday: VFR prevails at all terminals. Westerly wind gusts 20-25 kts possible in the afternoon.

Tuesday Night: VFR prevails at all terminals.

Wednesday: Ceilings lower through the day, with most terminals MVFR by the evening.

Wednesday Night-Thursday: MVFR prevails with light snow showers possible. IFR restrictions possible at terminals that see these showers move overhead.

Thursday Night: Ceilings improve to VFR at all terminals.

Friday: VFR prevails at all terminals.

MARINE

SCA conditions continue tonight with a brief lull in criteria possible late tonight. Winds then increase Monday morning as low pressure exits to the northeast and another weak low develops over the Gulf of Maine. Winds will then increase Monday evening towards Gale force into Tuesday.

Potential westerly gales taper off Tuesday night with SCA wind gusts tapering off later Wednesday morning bringing an end to freezing spray as well. Wind gusts and seas ramp right back up Wednesday night as a front approaches and crosses the waters. This may necessitate just leaving SCAs in place. With that being said SCAs may be needed through the end of the week as strong winds return late week, with freezing spray returning as well.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory until 1 PM EST Monday for MEZ018>028. NH...Winter Weather Advisory until 1 PM EST Monday for NHZ010>015. MARINE...Small Craft Advisory until 7 AM EST Monday for ANZ150-152. Gale Watch from Monday evening through late Tuesday night for ANZ150-152-154.