textproduct: Gray - Portland

SYNOPSIS

Icy precipitation will bring impacts to travel Sunday night and Monday morning as a frontal system crosses the region. Depending on how much ice accumulation locations see, some isolated power outages are possible as winds get gusty in the wake of the system. It then looks like light snow shower activity and continued below normal temperatures are how we are going to start the new year.

NEAR TERM /THROUGH TONIGHT/

High pressure will nose into the region tonight. With mostly clear skies, light winds, and fresh snow that should allow temps to fall quickly after dark. NBM guidance tends to underforecast the degree of radiational cooling, so I opted to blend the 25th percentile temps with MOS guidance to arrive at lows.

SHORT TERM /SUNDAY THROUGH MONDAY/

Sun will be quiet as high pressure drifts eastward and allows the next winter storm to approach from the west.

The trends have held with NWP continuing to show some weak confluence over the Canadian Maritimes which will help to reinforce at least a branching high pressure lobe north of the forecast area. I continue to lean on colder guidance as the cold air dam usually wins out locally. I have blended some of the 25th percentile NBM temps into our hourly forecast, especially across southern NH and coastal areas where the mean NBM was warming things too fast in my opinion.

Regardless of those changes there was still widespread freezing rain before any warm up and changeover to straight rain. So I have issued winter weather advisories for all zones. I have segmented between southern NH and coastal zones where some changeover to rain is more likely, and interior zones where there could be more substantial icing. At this time the system looks progressive enough and/or with enough dry slotting locally that QPF is below 1 inch. That makes it hard to get to 0.75 inches flat ice for a true ice storm, but I could see some localized amounts near 0.5 inches. I would not rule out perhaps an upgrade to a winter storm warning for heavier amounts of ice combined with some light snow and sleet.

So travel Mon looks hazardous, especially in the morning. By Mon evening precip will be coming to an end.

LONG TERM /MONDAY NIGHT THROUGH SATURDAY/

Pattern Overview: A potent upper low passes to our north Monday night and Tuesday. After that we are left under a broad trough which will help keep showers in the forecast through the end of the long term period, especially in the mountains. Models are starting to suggest a shortwave rotating through the broader trough Thursday night into Friday which may help to force more widespread snow showers, but uncertainty at that range is high. Impacts and Key Messages: * Gusty winds Monday night and Tuesday may pose an isolated power outage threat depending on how much ice accumulation locations see. Details: Monday Night: Precipitation tapers off Monday night as surface low pressure makes its departure. In the wake of this system, we will end up on the outer fringes of a low level jet rounding the base of a potent upper low centered just to our north. Ensemble means suggest winds over our heads in the 35-40 kt range, with Bufkit profiles suggesting decent enough mixing to get gusts 25- 35 mph to the surface. These higher end gusts look most likely south of the mountains, with areas to the north even more on the fringes of the jet and more likely to see the lower end of the range. Regardless, any locations that end up with decent ice accumulations may see an isolated power outage threat as branches feel some of the extra weight. Ice accumulations are not expected to impact power lines directly. Low temperatures fall into the upper teens and low 20s south of the mountains, and into the low to mid-teens north. Tuesday and New Year's Eve: Gusty winds continue through the day Tuesday. High temperatures should be able to rise into the mid- to upper 20s south of the mountains (upper teens and low 20s north), but windchills will be more in the upper single digits to low teens (single digits below zero north). Skies further clear Tuesday night, but breezy winds will keep most locations from decoupling. The result is still a very cold night with low temperatures dropping into the single digits across much of the area and windchills below zero from the foothills northward. New Year's Eve looks mostly dry during the daylight hours with high temperatures in the mid- to upper 20s in the south, and into the upper teens and low 20s in the north. However, it still looks a bit breezy so expect for it to feel a little colder. You are going to want to bundle up for any New Year's Eve activities as windchills look to end up in the single digits and low teens for most locations. A weak shortwave trough rotating through may also bring some festive flakes for the countdown. New Year's Day-Saturday: We start the New Year off on the cloudier side as a broad trough remains over the eastern United States, with our area being closest to the center of the upper low. Global models are beginning to suggest a stronger shortwave rotates through the broader trough late Thursday which may kick us off with some snow showers as well. This will depend on placement of a surface low, which unsurprisingly is uncertain at this time range, but is something to keep an eye on. We may get into more zonal flow for the first part of next weekend which would give us a break from showers, but continue our stretch of below normal temperatures.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Short Term...VFR conditions expected to prevail thru Sun evening. Precip quickly overspreads the area overnight and areas of IFR will likely develop as warm air rides over the colder near surface. In addition to the low CIGs some patchy fog is possible near the coast, especially where dewpoints can climb towards 40 degrees over the snowpack. During much of Mon near the coast there may be periods of LLWS, with light northerly surface winds and south to southwest flow at 2000 ft. Late Mon afternoon may see some improvement in flight categories as winds turn westerly and allow the low clouds to scatter out.

Long Term...Conditions improve overnight Monday as precipitation begins to taper off and most terminals should be back to VFR by daybreak on Tuesday. As precipitation comes to an end, winds will ramp up with westerly gusts reaching 25-30 kts through the day Tuesday. Wind gusts die down Tuesday night with VFR prevailing through Thursday.

MARINE

Short Term...Winds and seas are expected to remain below SCA thresholds until Sun night. Once the warm front crosses the coastal waters southerly winds will begin to increase. Outside of the bays some 25 kt gusts are possible and an SCA is possible, though westerly gales will be right on the doorstep for Mon night.

Long Term...Westerly wind gusts may ramp up to gale force overnight Monday in the wake of a frontal system. These gusts look to taper off Tuesday night, but remain 25kts+ through Thursday. During this same time frame seas build to 5-8ft. Sub- SCA conditions then prevail into the first part of the weekend.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory from 10 PM Sunday to 7 PM EST Monday for MEZ007>009-012>014-018>028-033. NH...Winter Weather Advisory from 10 PM Sunday to 7 PM EST Monday for NHZ001>015. MARINE...None.