textproduct: Gray - Portland

WHAT HAS CHANGED

The remainder of the Winter Weather Advisories have been taken down. Area roadway conditions have been improving and just light snow showers remain this afternoon.

KEY MESSAGES

1. Briefly heavy snow showers are possible tonight as the frontal boundary pushes through, ushering in an Arctic airmass for Tuesday.

2. Weak system will skirt the northern tier Wed night into Thu and may bring light snow to much of the area.

3.Very strong ensemble signal for significant cold snap late this week into the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Just light snow showers are left out there this afternoon as our system wraps up. The frontal boundary that is going to usher in arctic air for tomorrow is progged to sweep through tonight and may touch off some briefly heavy snow showers as it does. The latest CAMs suggest the area to watch is from the mountains southward and I have added in some PoPs to account for this. Due to the time these are going through, I don't expect them to rise to the level of squalls, but an SPS or two may be needed to highlight low visibility and quick accumulations on the roads. Wind gusts 20-25 mph look to also accompany the front as it comes through Monday night.

Still under the influence of a trough, Arctic air will pour in behind the front Tuesday making for a mostly dry and cold day. The morning looks to start mostly clear as high pressure begins to work its way into the region, but an additional passing shortwave may add some clouds during the afternoon. It is also worth noting that strong lake effect dynamics will be ongoing to our west and the CAMs are beginning to suggest a ribbon may be able to stretch its way across central New Hampshire and even into Southern Maine. I have kept it out of the forecast for now as these can be finicky and it wouldn;t have a large footprint, but it is something to keep an eye on if your evening commute takes you north on 93 or 95 through these areas. 850 mb temperatures drop to -21 to -23C by the afternoon, so high temperatures only top out in the upper teens and low 20s south of the mountains, and in the low to mid-teens in the north. Wind gusts look to remain in the 20-25 mph range through the afternoon so expect it to feel more like single digits above (south) and below (north) zero. High pressure further works its way into the region Tuesday night clearing skies and calming winds. This combined with much of the area receiving some degree of fresh snow in the last 48 hours, led me to blend in some colder MOS guidance to account for efficient cooling. Temperatures in the north drop into the single digits below zero with what little wind that remains making it feel more like 10 to 15 below zero. It should be noted that the higher terrain is going to experience dangerously cold wind chills in the - 20 to -30F range. Extreme caution should be used if venturing up there. Areas south of the mountains may stay just breezy enough to keep ambient temperatures on the positive side, but that means wind chills are going to make it feel below zero.

KEY MESSAGE 2 DESCRIPTION... Wed night the warm advection arrives ahead of an approaching clipper system. PoP is fairly unimpressive from NBM guidance, but forecast models show enough snow growth zone saturation and lower to mid level lift to produce light snow. I adjusted PoP up to at least 25 to 50 percent for all of the forecast area, just so that text products and point and click forecasts will say chance of snow. Overall the current timing looks to be almost entirely overnight and snowfall amounts light, 1 to 2 inches, so unless that changes impacts should be relatively minor.

KEY MESSAGE 3 DESCRIPTION... Much more impactful will be the coming cold behind the clipper. The cold front will cross the region Thu and then Fri thru the weekend will feature some bitterly cold temps. Of course it is the climatologically coldest time of the year, but even then NAEFS guidance suggests a once every 2 to 5 year type of cold air mass. Despite the anomalous cold, the NBM seems to have a fair handle on the forecast. I could see temps needing adjusting a few degrees, maybe more if we can time a surface ridge well enough for radiational cooling, but overall the current guidance is the coldest temps of the season. Both Sat and Sun feature highs in the low teens across southern zones and near 0 in the mtns. Overnight beginning Fri night thru Sun night may see readings below 0 everywhere. The timing of surface ridging previously mentioned and reinforcing cold fronts will determine the state of the wind forecast, but it seems reasonable to me to expect some headlines necessary for cold coming if this forecast holds. While we have had a couple cold weather advisories for the mtns so far this season, what is notable about this cold shot is that the entire forecast area may be in play for at least one of the nights this weekend.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through 18Z Tuesday...MVFR prevails this afternoon as light snow showers come to an end. Some briefly heavy snow showers may impact some terminals tonight as a front sweeps through, but confidence on where these will be is low, so they are not mentioned in the TAFs. Southwesterly wind gusts 15-20kts may also accompany the front tonight. Interior terminals may only see these gusts briefly while coastal terminals have the greater chance of seeing them for a longer duration. Conditions improve to VFR Tuesday.

Outlook...

Tuesday Night: VFR prevails at all terminals.

Wednesday: VFR possibly becoming MVFR as clouds thicken/lower.

Wednesday Night: Light snow is likely for all TAF sites through Thu morning with areas of IFR or lower.

Thursday: Local MVFR or lower possible in snow showers. Otherwise widespread VFR.

Thursday Night: Widespread VFR, with local MVFR possible at HIE.

Friday: Widespread VFR, with local MVFR possible at HIE.

Friday Night: Widespread VFR, with local MVFR possible at HIE.

Saturday: VFR prevails at all terminals.

MARINE

Southwesterly Gales are expected tonight, turning westerly as a front crosses the waters toward daybreak. Seas ramp up to 6-10ft during this time as well. Gales taper off by mid- morning, but SCA wind gusts and seas will continue through at least Tuesday night. These elevated wind gusts and much colder air entering the region will allow for light freezing spray.

Marginal SCA conditions are possible Wed into Wed night in southwest flow ahead of a clipper system. There will also be light snow and reduced visibility below 1 mile, especially closer to the coast. A cold front will cross the waters Thu and then cold advection will allow westerly winds to increase Fri. Gales are possible for all waters with gusty conditions lingering thru the weekend. In addition the very cold temps will bring an increase risk for moderate to occasionally heavy freezing spray this weekend.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Gale Warning from 7 PM this evening to 10 AM EST Tuesday for ANZ150-152-154. Small Craft Advisory from 7 PM this evening to 1 AM EST Wednesday for ANZ151-153.