textproduct: Gray - Portland

WHAT HAS CHANGED

Lowered temps for today with rain moving into a chilly air mass. Cold air damming should hang pretty tough even with a passive dam.

KEY MESSAGES

1. Periods of rain today with a brief thunderstorm possible in New Hampshire this afternoon.

2. A seasonably cold airmass arrives to start the weekend, with wintry conditions across the higher terrain.

3. Mild and dry conditions look to persist from Sunday through much of next week as high pressure dominates over the Northeast.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Low pressure is set to quickly track through the St. Lawrence Valley this afternoon. This will spread rain across the forecast area early this morning.

The best overlap of forcing and moisture is on the front-end, and guidance has most QPF falling between 8am and 2pm. The quick moving low will remain flat and progressive, keeping a slot of dry air in between the warm front in the morning, and the cold front mid to late afternoon. Thus it is likely the more steady rain is focused through the morning, with showery characteristics for the afternoon. Flooding is not anticipated based on forecast amounts, but could see some instances of nuisance/poor drainage flooding where leaves may hinder drain performance.

As the warm front lifts through the area, could see some fog develop amid scattered showers through mid afternoon. The final act of the day will be the cold front's arrival, potentially with some low-topped convection into SW NH. MUCAPE values, around 200 j/kg are much less than evening lightning on Tuesday. Thunder may be limited here, but the vertical mixing will likely promote a rapid wind shift with sudden gusts as the front pushes east.

The front's passage will mark the beginning of NW winds overnight and into Saturday, ushering in cooler temperatures and wind chill.

KEY MESSAGE 2 DESCRIPTION...

As the aforementioned low pressure system departs northeast, broad high pressure builds into the Great Lakes and Ohio Valley. The pressure gradient between these two systems will tighten to produce gusty NW winds Friday night through the day Saturday. There has been some slackening of the forecast gradient the last few forecasts, and this has resulted in wind forecasts not as strong as early week. However, the wind combined with the cooler airmass will certainly leave an impact on the area.

Daytime highs will struggle to make it out of the 40s along the immediate coast Saturday, with widespread 30s and 40s across the mountains and foothills/interior. With frequent gusts 25 to 35 mph, resultant wind chill values may reside in the 30s across most of the area.

The effect is more pronounced for the higher terrain, where winter conditions, including snow, are expected with greater wind and wind chill magnitude. Wind gusts over 100 mph are expected on Mount Washington on Saturday. Any hikers or those above 2500ft should plan for wintertime conditions this weekend.

The coldest night of the season arrives Saturday night with widespread lows in the 20s. Lows below freezing are expected to the coastline across the region.

KEY MESSAGE 3 DESCRIPTION...

High pressure crests over the region on Sunday and looks to stay nosed in through at least the daylight hours of next Thursday. This will bring an extended period of mild daytime temperatures, and likely some pretty chilly nights due to radiational cooling, as well as dry weather.

Sunday will be the transition day with high temperatures in the low to mid-50s with mostly clear skies, but after a raw Saturday this will probably feel comfortable. Afterwards, most days should feature abundant sunshine with high temperatures in the upper 50s and 60s. Periods of clouds are possible with subtle troughing aloft, but the midlevel airmass looks very dry which will preclude any threat for precipitation. Low temperatures look to be generally around normal being in the upper 20s to mid-30s north to south, but of course any night where strong radiational cooling takes place, can end up quite cold. Looking at long range ensembles 850-700 mb temperatures stay right around the 0 C mark through this stretch so certainly some cold nights could come to fruition, but nothing crazy.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Through tonight...Rain moves into the area this morning with conditions quickly dropping from VFR to low MVFR or IFR. CIGs remain there thru the late afternoon or early evening when a cold front will mix out the low cloud cover. A mix of MVFR and VFR CIGs continue thru the overnight as northwesterly winds increase.

Outlook...

Saturday...MVFR/IFR cigs could remain for HIE through the morning with -SHRASN, otherwise VFR expected. The most notable feature for the day will be NW gusts 25 to 30 kts, slackening in the evening.

Sunday-Tuesday...High pressure brings an extended period of VFR conditions with no sig wx. Wind will tend to follow typical diurnal patterns.

MARINE

Southerly winds build today as low pressure passes inland up the St. Lawrence Valley. These winds then become SW and then NW overnight, increasing towards Gale force on the outer waters. Winds and seas will gradually decrease Saturday evening.

Sub-SCA conditions are expected Sunday through much of next week as high pressure settles over the waters.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until midnight EDT Saturday night for ANZ150>154.