textproduct: Gray - Portland

WHAT HAS CHANGED

Snow totals have come down a bit on the eastern extent of the system, but have held steady in the areas that will see enhancement due to an inverted trough. While there is a possibility of locally high amounts nearing 4 inches, the placement is still uncertain so I held off on any advisories.

KEY MESSAGES

1. Accumulating snow tonight through Saturday afternoon will bring about slippery travel in New Hampshire and southwestern Maine. Generally 1-3 inches is expected in these areas, with some locally higher amounts possible on the New Hampshire Seacoast and into coastal York County.

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.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Not a whole lot has changed in regards to forecast thinking for tonight into Saturday. Low pressure develops off the Mid- Atlantic coast and takes a track eastward out to sea. However, a trailing shortwave and surface front move over the region resulting in an inverted trough forming and reaching back into our area. It looks like the bulk of accumulation is going to come from this inverted trough as overall the shortwave and front are going to be moisture starved with PWATs less than 0.25". Temperatures tonight are in the teens and single digits, and during the day Saturday they top out in the 20s so there will be a fluff factor that aids in these accumulation amounts despite the lack of moisture. For this reason, I think the NBM is reasonable with its accumulations although I did smooth the QPF output to get more realistic coverage. Areas across the north and much of Western Maine are going to see a coating to maybe a half inch, with the Kennebec Valley maybe getting blanked completely as the front looks to wash out as it pushes east. Mid-level northwesterly flow should help to increase lift around the terrain so northern New Hampshire could still squeeze out 1-2 inches. Central New Hampshire should see 1-2 inches as well as the inverted trough axis looks to reach back into this area. Southern New Hampshire and Southwestern Maine look to be the locations that end up in the area of best enhancement and likely see totals in the 2-3 inch range. Depending on exactly how the trough sets up, the New Hampshire Seacoast and maybe even coastal York County could see totals pushing the 4 inch mark.

KEY MESSAGE 2 DESCRIPTION... Troughing is expected to continue on Saturday. A cold front is poised to move southeastward through the day, with an arctic airmass following the front. Breezy northwest winds in addition to this arctic air will allow for a very chilly Sunday morning. Wind chills may be as cold as -20F to -30F across western NH and interior Maine. An Extreme Cold Watch remains in effect for these areas. Conditions are likely to stay mixed on Sunday, as a pressure gradient holds in place over the northeast. This will allow for another very chilly start to the day on Monday, where wind chill readings of -20F to -30F are also not out of the question.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

Through 06Z Saturday...Ceilings thicken overnight and lower through this morning. Most terminals will be MVFR or near MVFR after 18Z today. Snow holds off until after the end of the TAF period at most terminals, but LEB could see it a little earlier. It should be light enough at the onset to maintain MVFR conditions.

Outlook:

Tonight-Saturday: Light snow overspreads the region bringing about IFR visibilities through the day Saturday.

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

Monday: NW winds slacken, gusts to 20 kts. VFR. No sig wx.

Tuesday: VFR. No sig wx.

MARINE

Sub-SCA conditions continue over the waters through the day Saturday with northerly winds and light freezing spray.

Northwesterly gale force winds are likely Saturday night, continuing through Sunday. Winds lower to SCA levels by Monday and continue to slacken to below SCA levels by Tuesday morning. Seas of 2-3ft are expected through the entire period in the Bays, with 4-6ft seas in the open waters.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Extreme Cold Watch from Saturday evening through Sunday afternoon for MEZ007>009. NH...Extreme Cold Watch from Saturday evening through Sunday afternoon for NHZ001>003-005-007-011. MARINE...Gale Watch from Saturday afternoon through Sunday afternoon for ANZ150>154.