textproduct: Gray - Portland

WHAT HAS CHANGED

A low pressure system that will cross Saturday into Sunday continues to trend colder with wintry precipitation possible to the coast. This may lead to hazardous travel conditions.

KEY MESSAGES

1) Fog and freezing fog development Thursday night into Friday will reduce visibility and add a thin, icy coating to untreated surfaces through the morning commute. Temperatures warm above freezing through the morning, with a period of rain showers through the evening.

2) A period of gusty winds accompany a passing cold front Friday night. The strongest gusts will impact the higher terrain, but some westerly gusts of 30 to 35 mph are possible from the foothills to the coast before daybreak Saturday.

3) Accumulating wintry mix will bring weekend travel impacts to interior and mountain regions of New Hampshire and Maine Saturday evening into Sunday morning.

DISCUSSION

KEY MESSAGE 1 Discussion...

Strengthening high pressure exits the Northeast Thursday night with seasonably strong ridging overhead into Friday morning. Calm winds and mostly clear skies set the stage for radiational cooling Thursday night into Friday morning. As temps fall into 20s, potentially teens, at the surface, a robust low level inversion develops. Modeled temperature profiles rise above freezing perhaps just a few hundred feet off the ground. With little additional advection and a cooling surface, residual moisture trapped below the strong inversion creates a scenario for widespread fog and freezing fog from the foothills to the coast.

Besides reducing visibility for the morning commute Friday, the fog may adhere to cold surfaces and create a slick coating. With no precipitation expected, this may be a surprise on untreated surfaces, especially raised surfaces such as railings or handles.

Freezing fog accretion would be more efficient where temps cool the quickest in the evening. This would be best in prominent interior valleys which go calm quickly. Should fog develop early, this may inhibit overnight radiation and keep temps warmer (but still below freezing). Confidence in fog development is increasing, with SREF and HREF guidance supporting reductions in visibility as the night progresses.

Temperatures warm above freezing into the morning daylight hours Friday as southerly flow increases, quickly reducing the impact of slick surfaces. A batch of rain showers is then expected to move across the area through the evening.

KEY MESSAGE 2 Discussion...

Low pressure passing well north of the forecast area will bring a strong cold front through Friday evening and overnight. Strengthening upper jet around 180kt will aid in further deepening of this low, with mid and low level jets responding. 850mb winds of 50 to 60kt will push across the forecast area overnight, with strong gusts expected for the Whites and other high terrain.

With the wind shifting and cold air advection increasing behind the front, it will be possible for low levels to remain mixed. This would mean occasional gusts of 30 to 35 mph at lower elevations after midnight.

KEY MESSAGE 3 Discussion...

Cold trend continues for the forecast winter storm late Saturday into Sunday. High pressure to the NW will aid in feeding cool air at the surface from the NE through Saturday evening. Still a good signal in guidance for CAD to keep surface temps below freezing for much of southern ME and central/north NH. No change in thinking regarding primary low track out of the Great Lakes. Still some differences in guidance re: transfer to coastal low, and this will have impact on QPF and temperature profile aloft for precip type.

Continue to think foothills northward remain mainly snow until after midnight, in which sleet then mixes in. A mix of snow, sleet, and freezing rain will be possible for points in the foothills and interior, with mostly rain into southern NH and far southern NH. Should cold trend continue, would see wintry mix hold on longer and closer to the ME coast.

Most uncertainty may lie with QPF. Still a wide range of solutions here that will depend on which low has better moisture/structure. Going forecast supports a push of QPF initially with the overrunning, with lighter precip rates later in the storm as dry air may cut precip productivity. Ens means continue to depict total QPF around 0.4 to 0.6 for the forecast area.

In most scenarios, enough moisture remains into Sunday morning/afternoon to continue mention of wintry precip for the interior and mountains, and rain to the southeast.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Through 12z Friday...MVFR CIGs continue to erode downwind of the mtns this morning. Upstream moisture remains abundant, so upslope cloud cover is expected thru the day. The best chance for MVFR CIGs will be at HIE, while farther south at LEB the OVC is more likely to remain VFR. VFR will continue into tonight where clearing skies and high pressure may promote some patchy fog/freezing fog.

Outlook:

Friday and Friday Night: MVFR to IFR vis possible in fog/freezing fog in AM. RA in PM. LLWS likely.

Saturday: Improvement to VFR in AM, cigs lower again in afternoon.

Saturday Night: MVFR to IFR in RA SN and PL. RA likely for southern terminals, with wintry mix up ME coast and interior ME/NH.

Sunday: RASN rates decreasing, with improvement to vis.

Sunday Night: MVFR for HIE and US/CAN border terminals. Trend to VFR elsewhere.

Monday: VFR. No sig wx.

MARINE

Seas outside of the bays of 3-5 ft are expected through early afternoon and therefore an SCA is in effect. Winds will be out of the west at 5-10 kts. Light winds and seas are then expected tonight before winds increase out of the south on Fri with gusts approaching 25 kts and seas building to 3-6 ft outside of the bays.

Periods of SCA gusts will be possible Saturday and Sunday as low pressure develops in the Gulf of Maine. This then pulls northwest into Sunday night with strengthening west winds up to 30 kts.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ150- 152-154.