textproduct: Gray - Portland

WHAT HAS CHANGED

Winter Weather Advisories have been issued for southeast New Hampshire and coastal York County starting tonight through the day Saturday.

A Cold Weather Watch has been split into a Cold Weather Advisory for much of western Maine and northern New Hampshire, and a Extreme Cold Warning for southwest New Hampshire. The trend has been for Saturday night to be the coldest, when factoring raw temperature and wind, with Sunday night of lesser magnitude.

KEY MESSAGES

1. Periods of moderate to localized heavy snow will bring difficult travel late tonight through Saturday morning along coastal York County and southeast New Hampshire. Mainly light snow is expected across interior far western Maine and remaining areas of New Hampshire.

2. A frigid airmass will push across the Northeast this weekend, resulting in dangerous wind chill values Saturday night and Sunday night. Widespread temperatures below zero overnight are expected, with gusty winds making it feel like the teens below zero from the coast to the foothills, and 20 to 30 below zero for the western third of the forecast area.

3. A warming trend settles into the region for next week with temperatures near normal for early Feb. Will be watching for a potential disturbance to bring wintry precipitation in the mid to late week timeframe, but there remains uncertainty on exact impacts.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Latest water vapor imagery shows one short wave approaching the Mid Atlantic and a trailing short wave over the UP of Michigan. The first wave will spawn an area of low pressure that passes well southeast of Cape Cod Saturday while the trailing short wave will provide synoptic support for an inverted trough extending NW of the low pressure system.

We have experienced a couple of these inverted troughs this winter that have resulted in periods of moderate to heavy snowfall. What makes this event particularly challenging is smaller scale nature of the heavy snowbands that form along areas of convergence between Cape Ann, MA and Cape Porpoise, ME. Recent runs of the HRRR and other HREF members have been suggesting some impressive, but very localized QPF amounts in excess of 0.75 inches within this area of coastline. However, there are some HREF members that keep the heavier QPF offshore or south of NH/MA border. Outside of the Winter Weather Advisory in New Hampshire into far western Maine QPF generally ranges from 0.1 to 0.2 inches with areas east of Portland seeing little to no QPF.

Model soundings continue to strong lift through a moist DGZ for few hours late tonight into Saturday morning across SE New Hampshire and southern York County that will be favorable for snow ratios of around 17:1 and snowfall rates around 1 inch per hour. Closer to the immediate coast, soundings show some low level instability that could help boost snowfall rates to 2 inches per hour. The going forecast has mostly utilized the NBM which has tapered down the localized high QPF and resultant snow amounts. I Have issued a Winter Weather Advisory for portions of central and southern NH and coastal York county for snowfall of 2-6 inches with localized higher amounts possible along the NH Seacoast and southern York County. Outside of the Advisory snowfall amounts of 1-3 inches are forecast in NH and far western Maine with little to now snow accumulation east of Portland. Snowfall will gradually taper off Saturday afternoon.

KEY MESSAGE 2 DESCRIPTION... Forecast is still on track for Saturday and Sunday nights to feature well below normal temperatures and gusty winds, resulting in dangerously cold wind chills.

Saturday night looks to harbor the coldest conditions as mid/upper jet drapes from the Great Lakes to the Mid-Atlantic coast. Low level north to northwest flow will broaden across the Northeast, increasing cold air advection. The core of cold does pass to the south and west of the forecast area, where 850mb temps will near record low values based on 3 week centered climo. Raw overnight temperatures fall into the single digits above and below zero, with values nearing the teens below zero NW of the mountains.

Such a strong push of cold air will be accompanied by winds, and these will be what push conditions to be particularly frigid and dangerous Saturday night. Overnight sustained winds of 10 to 18 mph with frequent gusts 20 to 30 mph will drive wind chill values into the teens below zero or colder for the forecast area. For much of the western third of the forecast area, values of 20 to 30 below zero are forecast. This has prompted the conversion of the Cold Weather Watch into respective areas of Cold Weather Advisory and Extreme Cold Warning.

Values this cold can lead to frostbite in as little as 10 to 20 minutes on exposed skin. Any outdoor activities Saturday night should be accompanied with plans if getting stranded outside is a possibility. This is especially true for higher elevations, particularly the Whites, where wind chill values could fall to 30 to 40 below given stronger winds here.

While the main push of cold air departs southeast into Sunday, a cold air mass remains overhead through the day and into Sunday night. Temps Sunday only rise into the teens before again falling to the single digits above and below zero Sunday night. This sets up another cold night with wind chills in the teens below zero, but overall magnitude of winds will be less. Thus headlines will likely again be needed, but maybe not as widespread or extreme.

KEY MESSAGE 3 DESCRIPTION... As northwest winds depart, the area will see a warming trend through early week. Beginning Tuesday, highs in the upper 20s to around 30 will be possible into mid and late week.

There is one period of precipitation due to a passing disturbance to watch for. This is generally mid week, Wed-Thurs, as moisture or a part of a low pressure system rides across the Great Lakes. This is ahead of broad high pressure moving into the Upper Midwest. Should the low be better developed into Quebec, it will look closer to a overunning event vs. cold clipper. This means the possibility of a warm layer aloft that could bring mixed precip to the surface. For now, surface temps are forecast to sustain snow. Still some uncertainty in forecast models on the duration and total QPF, but this will be a period to watch for the next chance at slick travel conditions across the forecast area.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Through 00Z Sunday: Ceilings lower to MVFR tonight with snow developing over NH and then spreading into far western Maine. Periods of moderate to heavy snow possible locally at KPSM and KMHT. KPWM to AUG will be the edge of MVFR/VFR to the east and IFR to the west in SN through Saturday afternoon. A cold front will begin pushing across the forecast area after noontime. This will result in a quick increase in NW winds gusting 25 to 30 kt and a increase in snow intensity w/ BLSN. At the moment, this looks to impact NH TAF locations the most through the afternoon.

Outlook...

Saturday Night thru Sunday Night: Improvement to VFR Sat evening, with HIE still seeing MVFR cigs. NW gusts to 25 kts, slackening Sunday night. No sig wx.

Monday: VFR. No sig wx. NW winds 10 to 20 kts.

Monday night and Tuesday: VFR. No sig wx.

Tuesday night: VFR, ceilings will be thickening and lowering. Light SN possibly approaching region.

MARINE

Northeast flow prevails over the waters tonight with winds and seas remaining below SCA thresholds. Periods of moderate to heavy snow will be possible over the waters late tonight through Saturday morning that will bring visibility restrictions.

Gale conditions onset Saturday evening as northwesterlies. Gusts to gale will be less frequent in the bays/harbors. Combination of a cold air mass and the winds will result in moderate freezing spray accretion Saturday night through Sunday night. SCA conditions likely persist into early next week as offshore wind direction continues.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Cold Weather Advisory from 6 PM Saturday to 10 AM EST Sunday for MEZ007>009-012-013-018-019-033. Winter Weather Advisory from 1 AM to 6 PM EST Saturday for MEZ023. NH...Cold Weather Advisory from 6 PM Saturday to 10 AM EST Sunday for NHZ001>006-009-010-012-013. Winter Weather Advisory from 1 AM to 6 PM EST Saturday for NHZ008>010-012>015. Extreme Cold Warning from 6 PM Saturday to 10 AM EST Sunday for NHZ007-008-011-015. MARINE...Gale Warning from 2 PM Saturday to 1 PM EST Sunday for ANZ150>154. Freezing Spray Advisory from 7 PM Saturday to 7 AM EST Monday for ANZ150>154.