textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes regarding tonight minor snow event. Still generally expecting a coating to 2 inches for much of the area, with locally higher amounts on the Midcoast.

Extreme Cold Watches were issued across locations north of the mountains. Guidance has trended this weekend's winter system northward, increasing confidence in at least measurable snowfall Sunday night into Monday. The highest snowfall amounts are likely to be across southern New Hampshire.

We continue to monitor the late weekend/early next week period as models are trending northward with a system that could bring plowable snow to at least a portion of the area.

KEY MESSAGES

1. Low pressure passes north of the area tonight bringing a period of generally light snow. Accumulations will also be light and snow is forecast to end before the Thursday morning commute. A few snow showers and potential squalls will be possible with a cold front Thursday late morning and afternoon, especially across New Hampshire.

2. Arctic blast will bring dangerously cold weather to the region this weekend. The coldest air of the season thus far is looking likely with sub-zero wind chills Friday night into Saturday followed by will below zero low temperatures Sunday morning. Frostbite onset will occur quickly for those not dressed properly.

3. A low is anticipated to approach New England Sunday night, bringing measurable snowfall to the region. Higher totals look most likely in southern NH, though a lot of uncertainty remains.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION: Not much has changed in regards to the light snowfall event expected tonight. Snow begins to breakout in Western New Hampshire in the 6 to 7pm hour, continuing to spread eastward through the midnight hour. This system will be battling the dry Arctic air currently in place over the region, so QPF remains less than a tenth of an inch for much of the area. Shortly after midnight much of the area may see snow coming to an end with amounts in the range of a coating to an inch (1-2 inches in northern valleys). The coastal plain sees snow longer and gets a little bit more moisture from the ocean, so Portsmouth up through Brunswick have a better chance of ending up in the 1-2" range before it tapers off in the 2am hour. Lastly, the Midcoast sees snow last through 4am and thusly sees the best chance to end up in the 2-4" range before it's all said and done. The islands likely end up on the lower end of this range, as the warm air advection pushes them above freezing and switches the snow to rain for the last couple hours. All this occuring before the bulk of the Thursday morning commute should keep travel impacts minimal, especially considering the light amounts.

The more impactful weather may actually come during the day Thursday ahead of our next Arctic frontal passage. Warm air advection is going to continue at the surface making Thursday a very warm day compared to what we are going to be dealing with later in the week. Temperatures climb into the upper 30s and low 40s south of the mountains, and into the upper 20s and low 30s to the north. Aloft the cold air is going to start moving in and this temperature difference is going to create some instability. The HREF models a clear theta-e ridge offshore which would support the low level moisture pooling over our area and creating a favorable environment for snow squalls. Based on this anywhere could see a squall develop as the snow squall parameter is above 1 across the area, however Northern New Hampshire and into the Western Maine Mountains has the greatest chance supported by a snow squall parameter upwards of 4. CAPE is less important this time of year, but the HREF suggests 50-60 J/kg develop in the north which would further support the signal up there. The 12Z CAMs currently suggest the late morning through the late afternoon is the window for this activity to occur, so timing will have to be watched closely as these heavy showers or squalls could be around for the Thursday evening commute.

KEY MESSAGE 2 DESCRIPTION: At the end of the week, a negative phase of the North Atlantic Oscillation will allow for a very weak polar vortex across the northern hemisphere. The strongest of this cold aloft will gyrate around eastern Canada this week and make a southerly swing into the Great Lakes and New England for the weekend. Wind chills are likely to be dangerously cold, especially in the mountains. An Extreme Cold Watch is in effect across Northern NH and Northwestern Maine.

KEY MESSAGE 3 DESCRIPTION: A strong mid-latitude cyclone will move across the southeastern US. The low will move eastward, while arctic air is in place across the northeast. This may allow for widespread measurable snowfall across New Hampshire and Western Maine. Models are in solid agreement for at least 1-3 inches of snow Sunday into Monday, though some potential does remain for totals greater than 6 inches across southern NH. Any deviation from here north or south could have substantial impacts to the forecast.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Through 18Z Thursday...VFR ceilings this afternoon trend toward MVFR this evening. Light snow breaks out with a period of IFR visibilities expected overnight as rates are briefly moderate. Conditions slowly improve from IFR to MVFR after 06Z Thursday and then eventually to VFR after 12Z Thursday. Winds gusts pick up near the end of the TAF period with most terminals seeing gusts 20-25kts.

Outlook...

Thursday Afternoon/Night...VFR prevails, but briefly heavy snow showers or squalls may impact some terminals Thursday afternoon and early evening. These would bring about quick wind changes and IFR visibilities.

Friday: Widespread VFR, with local MVFR possible at HIE.

Friday Night: Widespread VFR, with local MVFR possible at HIE.

Saturday: VFR prevails at all terminals.

Sunday: light snow likely across southern TAF terminals by PM

Monday: MVFR becoming VFR as snow pulls away.

MARINE

SCA conditions continue with a few wind gusts near 35 kts possible on the outer waters as a front crosses the waters. Winds shift westerly with a secondary front crossing Thursday leading to potential gales Thursday night into Friday.

(Friday - Onwards) Gale force westerly winds will continue Friday morning through Saturday morning. Winds slacken to SCA levels during the day Saturday, with winds shifting to northerlies. SCA winds continue through early next week. Seas of 5-8ft expected on Friday and Saturday, lowering to 2-4ft by Sunday morning.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Extreme Cold Watch from late Friday night through Saturday morning for MEZ007>009. NH...Extreme Cold Watch from late Friday night through Saturday morning for NHZ001>003-005. MARINE...Small Craft Advisory until 7 AM EST Thursday for ANZ150-152. Gale Watch from Thursday afternoon through Friday morning for ANZ150-152. Small Craft Advisory from 4 PM this afternoon to 7 AM EST Friday for ANZ151-153. Small Craft Advisory until 7 AM EST Friday for ANZ154.