textproduct: Gray - Portland

WHAT HAS CHANGED

5:40am Update... Forecast has been updated to tighten the rain/snow closer to the coast for this afternoon as a slightly cooler solution looks to be evolving for today's system.

Snowfall amounts have increased slightly which has resulted in expanding the Winter Weather Advisory northeastward through central New Hampshire.

KEY MESSAGES

1. A wave of light snow overspreads the area from the southwest through daybreak. There may be a brief lull in spots this morning before more widespread snow develops mid morning through the afternoon. Slick travel is expected across much of the area with widespread 1-3 inches with 3-5 inches within areas under a Winter Advisory. Plan for added travel time if transiting interior locations, especially to/from the mountains.

2. The potential for a coastal storm remains possible late Sunday into Monday. Confidence continues to be lower than normal due to a large portion of NWP members being out to sea.

3. Below normal temperatures are likely from Tuesday onward next week as a series of Arctic air masses descend upon the Northeast. Several nights of sub-zero ambient or wind chill temperatures are possible.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Latest water vapor imagery shows a broad trough dropping southeast across the Great Lakes with embedded short waves ejecting towards the Northeast. The first wave will result in light snow overspreading the area through day break that will result in a coating to an inch or two of snow mainly across southern and central NH and southwest Maine. Latest hi res guidance suggest that there may be a lull in snowfall after day break.

Better forcing for ascent will overspread the area from mid morning through the afternoon as the right entrance of jet streak slides overhead. This upper forcing will coincide with WAA and Fgen forcing in the lower levels resulting in the potential for a band of moderate snow stretching from SW New Hampshire through the southern White Mountains. The latest WPC snowband tracker is highlighting this signal with the potential for this snow band weakening as it translates into western Maine. QPF and snowfall have increased slightly and have expanded the Winter Weather Advisory northeastward based on higher amounts and the location of where a band of moderate snow may set up. Thermal profiles continue to show cold air aloft while surfaces temperatures may creep above freezing at lower elevations and into the mid 30s along the coast through the lower Merrimack Valley. This will likely result in a notable spread in snowfall amounts based on elevation, especially for areas within the Advisory where the higher terrain may see upwards of 6 inches and the lower CT Valley may be closer to 2 inches. Temperatures into the mid 30s across SE NH and coastal Maine will lead to snow mixing with and changing to rain at times that will keep amounts here around an inch. QPF drops off north of the mountains which will also result in lower amounts of 1-2 inches. Precipitation ends SW to NE late afternoon through Saturday evening.

KEY MESSAGE 2 DESCRIPTION... While there continues to be strong agreement that a coastal storm will develop late Sunday into Monday morning, there remains significant uncertainty on whether it will have impacts through accumulating snowfall across our region. Latest deterministic and ensemble based guidance continues to favor a near miss to our east while the AI ECMWF/GFS are more amplified and show snow reaching the region. On avg, around 20-30% of ensemble members show accumulating snowfall while ~80% of AI members show snow accumulations. Much of this difference seems to be due to uncertainty in the strength of the approaching s/wv along with timing differences. Should the s/wv be located to the NW of the parent low, then this could broaden the precipitation shield and there are some hints it could result in an inverted sfc trof. The greatest potential for a plowable snowfall looks to be across coastal and southern areas with lower chances the further north and west you are located. The WSSI continues to favor minor impacts, which aligns with the general current consensus that should this event impact portions of the region, the snowfall would most likely be advisory level criteria at worst.

KEY MESSAGE 3 DESCRIPTION... There remains strong ensemble support for below avg temperatures Tuesday through much of the remainder of the week and upcoming weekend as trofing persists across the NE CONUS. A few s/wv trofs may cross throughout the week with some snow potential towards the end of the week. Highs will mostly be below freezing with lows into the single digits and teens.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Through 00z Saturday...West winds continue to subside with VFR through at least 09Z. Cigs thicken and lower into daybreak. MVFR cigs begin for NH terminals 09z to 12z, with SHSN or light SN reducing vis. Vis may continue to lower to IFR as snow moves across central/northern NH terminals and into ME by mid morning. SE NH terminals (MHT/PSM) may transition to RASN or RA for better vis. Can't rule out some RA also mixing in at PWM/RKD and other terminals along immediate coast. Precipitation ends between 21Z and 00Z while IFR cigs may linger along the coast and KAUG into Saturday night.

Outlook:

Saturday Night: Cigs lift with VFR likely prevailing into Sunday morning.

Sunday: VFR north while clouds increasing from the south bring MVFR near the coast.

Sunday Night: Potential for MVFR or worse restrictions in possible SN. Forecast confidence is low.

Monday: Any lingering SN will end early morning with VFR conditions improving to VFR as NW flow increases.

Monday Night: VFR prevails at all terminals. West wind gusts 25 kt possible.

Tuesday/Wednesday: VFR prevails at all terminals.

MARINE

SCA conditions continue due to elevated wave heights. Low pressure lifts north of the Great Lakes through today, bringing a southerly wind shift and precipitation to the coastal waters. SCA conditions likely continue through Saturday night with wave heights on the coastal waters 4 to 6 ft.

An area of low pressure will form offshore of the Carolinas Sunday and tracks northeastward. How close to the Gulf of Maine this system tracks remains uncertain and may pass well southeast of Cape Cod. This approaching system will bring increasing northerly winds through Sunday. There will be the threat for snow over the coastal waters and visibility may drop below 1 mile at times Sun night into Mon. Winds will gradually shift around to westerly as storm pulls away and cold advection begins. Gale force westerly winds possible Monday night-Tuesday in the cold air advection with light freezing spray. SCA conditions will likely linger through much of the remainder of the week under CAA.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Winter Weather Advisory until 5 PM EST this afternoon for NHZ005>009-011-015. MARINE...Small Craft Advisory until 8 AM EST Sunday for ANZ150-152-154.