textproduct: Gray - Portland

SYNOPSIS

Next potential storm arrives for Sunday night and Monday and will likely be more of a mixed bag with all precipitation types possible during the event. Winds ramp up Tuesday in the wake of the system and below normal temperatures continue through the New Year's holiday window.

NEAR TERM /THROUGH TONIGHT/

615am Update: Most of the accumulating snow has moved out of southern New Hampshire this morning. Thus will go ahead and cancel the Winter Weather Advisory just a bit early for Cheshire county. Otherwise, no major changes were needed for this update.

Previously: Low pressure moves away from the region early this morning with snow ending around 12z. Slippery conditions are possible this morning, especially across southern NH.

Thereafter, high pressure ridging moves in from the NW with clearing skies and breezy conditions at times. Highs still well below average, except closer to average in the mountains.

SHORT TERM /SUNDAY/

High pressure settles overhead overnight. With a fresh blanket of snow, clear skies, and light winds, conditions will be favorable for radiational cooling. The NBM looks rather warm given this set up, so we continued to prefer to blend the 10th percentile and MOS guidance to capture the typically cooler locations. This dropped min temps in those areas some 10 degrees, but further refining needed in subsequent forecasts.

The axis of surface high pressure moves overhead Sunday with ridging aloft. This will allow for a sunny start to the day, and with 850 temperatures rising into the +1-5C range, surface temperatures end up in the low to mid 30s south of the mountains, and in the mid to upper 20s north.

LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/

Overnight Update...

Extended forecast largely remains on track and minimal updates were made outside of the Sunday-Monday period. A potentially impactful system moves into the area Sunday night, bringing wintry precipitation.

The system arrives Sunday evening. For the immediate coast and midcoast, this should be almost entirely plain rain. However, weak northeasterly surface flow will prevent cold air from getting overrun at the surface and will create a cold air dam overnight across much of the interior. Unfortunately that will most likely result in widespread freezing rain across the area. Freezing rain may even carry into Monday, though enough warm air should move in by Monday morning to flip southern NH to plain rain. However, freezing rain could remain north of Concord and allow for notable ice accretions. Models have been quite uncertain, with many members suggesting only a tenth or two of ice, while others are suggesting more. It's rare that I find myself leaning into the solution with more freezing rain, but models often tend to struggle with the cold air dam and the warm layer at least across New Hampshire looks to be too strong for prolonged sleet over prolonged icing. So given that and the cold air at the surface sticks a little longer than guidance suggests, there could be some serious impacts to travel on Monday, especially across central and northern New Hampshire. On the bright side, there will not be much QPF to work with as it appears unlikely widespread liquid equivalent totals would be greater than 1 inch, so that will be a limiting factor to how much ice we can get.

About a quarter to a half inch of ice seems plausible across the northern 2/3rds of NH, with a tenth to two tenths possible in southern NH and across most interior areas in Maine. Localized ice totals exceeding a half inch can not be ruled out in central New Hampshire.

Previous Discussion: Pattern Overview: A ridge slides overhead Sunday starting the period with fair weather, but that will change heading into Monday as a potent upper low approaches the region providing the forcing for a wintry storm. The upper low passes to our north Tuesday and then we remain in a broad trough through the end of the period. There is some suggestion of waves rotating through the broader trough, but with lack of frontal forcing, nothing looks overly impactful at this time. Impacts and Key Messages: * A mixed precipitation event is likely to impact post-holiday travel Sunday night and Monday. Details: Clouds begin to thicken Sunday evening as low pressure approaches from the west, but precipitation timing is a little more uncertain. The Euro and Canadian continue to be the quicker outliers with precipitation entering in the midnight hour Sunday night, but the GFS and now the NAM hold off until early Monday morning (more in the 2am-4am window). Low temperatures are going to occur early on in the night Sunday, dropping in to upper teens and low 20s to the south (low to mid-teens north) before we cloud over and warm nose starts pushing in. Monday: Precipitation starts early on Monday and initially it is going to be a mixed bag as a warm nose pushes in aloft, with cold air still in place at the surface. BUFKIT profiles in New Hampshire and far southern Maine are suggesting a period of freezing rain as the surface cold layer is very shallow. Southern zones may switch over to plain rain as early as mid- morning which would limit ice accumulations and travel impacts there. Central and northern New Hampshire may see the greater ice accumulations as a solid cold air damming signature is already showing up on the NAM. BUFKIT profiles heading into interior Maine suggest a stronger cold air dam with a deeper cold layer (~4000 ft) that would suggest to me that sleet would be the more dominant precip type here, rather than the freezing rain the global models are going for. Still not great for travel, but better for keeping accumulating ice down. There is still a lot of uncertainty regarding what the temperature profiles are going to look like later in the day. Models want to scour out the cold air and switch much of the area south of the mountains to rain, but historically the cold air dam always wins and interior zones may see sleet and/or freezing rain persist. There is going to be changes as this system moves into the hi-res model window, but if you are using this information for planning post-holiday travel I would say Sunday is definitely the day to do it. Whatever ends up happening, Monday does not look like the day to be on the road. I continued with mixed precipitation in the weather grids to highlight uncertainty in one type or the other, but also to highlight the unfavorable travel conditions. Tuesday-Friday: Strong cold air advection ramps up winds Tuesday with west-northwesterly gusts reaching the 25-35 mph range. As is usual with this pattern, expect the mountains to see persistent snow showers. This looks to continue through the end of the long term period as a broad trough dominates the weather pattern, but a lack of any frontal forcing suggests light showers, mainly in the mountains, is all we may see as we head into the new year.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Short Term...VFR conditions prevail over the majority of the forecast area starting 12z this morning after any snow ends. VFR conditions will last well into Sunday.

Long Term...VFR conditions prevail through the day Sunday before ceilings begin to lower Sunday night. Overnight Sunday widespread mixed precipitation enters the area making conditions variable, but widespread IFR seems pretty likely into Monday morning. Conditions are most likely to improve back to MVFR at coastal terminals Monday as they stand the best chance of transitioning to light rain at some point. Conditions at inland terminals are more uncertain. We should see VFR conditions return for Tuesday and Wednesday. Westerly wind gusts on Tuesday may reach 25-30 kts.

MARINE

Short Term...Low probability of low-end sca conditions this evening as low pressure moves away but mainly quiet conditions are expected.

Long Term...Sub-SCA criteria conditions are expected on Sunday, but winds and seas will ramp up Monday as a frontal system traverses the area. By Monday afternoon frequent wind gusts 25kts+ are going to be present, potentially reaching gale force Monday night. The stronger wind gusts begin to taper off Wednesday, but won't fall below 25kts until maybe Friday. During this same time period seas build to 8-10 ft, again not falling below 5ft until maybe Friday.

GYX WATCHES/WARNINGS/ADVISORIES

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