textproduct: Gray - Portland

WHAT HAS CHANGED

Snowfall amounts have decreased slightly for much of the area Wednesday night, but have slightly increased on the Midcoast. Still, snowfall amounts are expected to be light and fall outside of peak travel times.

KEY MESSAGES

1. Wind chills below 0 are expected across much of the area tonight as we get our first round of arctic air.

2. Low pressure passes north of the area Wednesday night bringing a period of accumulating snow. Accumulations will be light and snow is forecast to end before the Thursday morning commute.

3. Coldest stretch of the season is likely late Friday into early next week. Gusty winds Friday night into Saturday will likely warrant Cold Weather Headlines. Low temperatures Saturday night likely dropping several degrees below zero across the entire area with -20F or lower possible within northern valleys.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Radar this afternoon is showing lake effect streamers reaching into central New Hampshire and southern Maine as well as upslope snow showers in the White Mountains. CAMs have been handling the streamers well and I have continued to blend these into the forecast, suggesting snow showers are possible here through the early evening. Even lighter snow may also be seen in the foothills during this time as high froude numbers are allowing some of the upslope showers to spill over.

High pressure builds into the region tonight, clearing skies and calming winds. Have continued to blend in some cooler MOS guidance, as the NBM has started to catch on, but isn't quite grasping the radiational cooling potential tonight. Guidance is holding steady with this Arctic air mass dropping 850 mb temperatures into the -20 to -22C range which should be good for ambient temperatures to drop into the single digits below 0 in the north and single digits above for the south. Wind chill will make it feel more like -10 to -15F in the north and single digits below zero in the south.

KEY MESSAGE 2 DESCRIPTION... Temperatures slightly warm Wednesday as flow aloft turns southwesterly ahead of an approaching shortwave. Temperatures climb into the mid- to upper 20s south of the mountains and into upper teens and low 20s to the north. This sets the stage for a clipper to move in late Wednesday bringing some light accumulating snow to the region, but model timing has been pretty consistent with keeping this between the Wednesday evening commute and the Thursday morning commute so impact to travel should be low. QPF looks to generally be less than a tenth of an inch across the area with the exception of the Midcoast which ends up in the left exit region of the mid and low level jet and may squeeze out closer to 0.15". BUFKIT soundings show a fairly deep DGZ, so with this favorable lift and cold temperatures bumping snow ratios higher than 10:1 much of the area receiving measurable snow is in the cards. Areas of 1-2" are probably going to be confined more to higher terrain and northern valleys, with the Midcoast seeing the best chance of 2-4".

KEY MESSAGE 3 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. While mostly dry conditions are expected during the cold snap, radiational cooling is not likely due to a northwesterly pressure gradient and mostly cloudy skies over the region. These factors should keep air temperatures from dropping further overnight, but any amount of wind over this deep cold airmass could create dangerously cold wind chills.

The most dangerous wind chills are expected on Saturday. Saturday morning low temperatures look to be in the single-digits below zero along the coast, with teens below zero in the mountains. These cold air temperatures will be accompanied with an 8-12mph breeze from the northwest. Given the air temperature and winds, most people can expect to see wind chills colder than at least -10F, and potentially as cold as -40F in the mountains. Wind chills do improve through the day Saturday, but still expect to see below zero wind chills all day on Saturday and then into Sunday morning as well. The cold may remain in the forecast for the start of the work week, through wind chills look to slowly warm (to maybe wind chills in the upper single- digits or so) through the first half of next week.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18Z Wednesday...Some brief MVFR ceilings are possible this afternoon as clouds and light snow showers move through the area, but skies clear out again tonight with VFR prevailing at all terminals through the day Wednesday morning. Ceilings trend toward MVFR Wednesday afternoon.

Outlook...

Wednesday Night... Widespread IFR in light snow expected for most of the night, with a trend toward MVFR as snow comes to an end near day break Thursday.

Thursday: Local MVFR or lower possible in snow showers. Otherwise widespread VFR.

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

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: AM VFR, with lower restrictions likely in the afternoon.

MARINE

SCA conditions persist through Wednesday night. Westerly winds become more southwesterly Wednesday as high pressure crosses east of the Gulf of Maine. Conditions favorable for light freezing spray continue through Wednesday morning.

Gale force westerly winds are expected Thursday through Saturday. Seas of 5-8ft are expected through this timeframe. Conditions improve at least to SCA levels by Saturday night as high pressure moves overhead.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 7 AM EST Thursday for ANZ150-152- 154. Small Craft Advisory until 1 AM EST Wednesday for ANZ151-153. Small Craft Advisory from 4 PM Wednesday to 7 AM EST Thursday for ANZ151-153.