textproduct: Gray - Portland

WHAT HAS CHANGED

Guidance has trended more agressively on the cold air dam setting in for Thursday night's system. This should allow most freezing rain totals to stay south of the region, with more sleet and snow expected instead across southern New Hampshire. Wet and heavy snow still expected elsewhere south of the mountains.

Milder weather is still in the forecast for next week.

KEY MESSAGES

1. A low moves across southern New England, bringing wintry weather to the area Thursday night and into Friday morning.

2. Some mixed precipitation possible Friday night into Saturday.

3. Mild conditions and rain showers will lead to some snow melt through next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A low will move eastward Thursday night. The low will bring warm air aloft with it, creating the possibility for some mixed precipitation. A backdoor cold front will move in from the north, preventing warm air aloft to advect into most of northern New England. This will allow for a period of snow and sleet for most, but some freezing rain can not be ruled out across southwestern New Hampshire. A Winter Weather Advisory is in effect for Cheshire and Hillsborough counties for the potential to see a light glaze of ice. Most ice is expected to accrete late Thursday night, with travel impacts possible on secondary roads only.

For locations north of Concord or east of I-93, the backdoor cold front will really help minimize the penetration of the aforementioned warm air aloft. Precipitation in this area should stay snow or sleet. However, the warm air aloft will still allow for very low snow ratios-making the snow heavy, wet and difficult for the snow to accumulate. Generally about 0.5-2 inches of wet and gloppy snow is expected south of the mountains.

KEY MESSAGE 2 DESCRIPTION...

Cold air damming is forecast to largely hold over the forecast area thru Sat. However there is a lack of strong forcing for ascent at the same time beyond Fri morning. The main driver will be weak upglide due to warm advection. This is more of a drizzle situation than a steady precip, though some showers cannot be ruled out. So the main concern would be freezing drizzle/rain until temps can approach freezing Sat. This threat is pretty clearly separate from the wave passing south of the forecast area Thu into Fri, so the plan right now is to treat the storm total grids as separate as well.

KEY MESSAGE 3 DESCRIPTION...

Above normal temps are forecast to continue thru the middle of next week. At this time any precip is also forecast to remain showery and mostly light. The expectation is for a gradual ripening and melt of snow pack, especially south of the mtns. If the current forecast holds that will mean a general 1 to 2, maybe as much as 3 inches across parts of southern NH, of snow water equivalent melting out by next week. This would be more accelerated if temps and especially dewpoints remain warm overnight, and if more rain falls. With this forecast there is no anticipation of flooding. Streamflows remain low from the beginning of winter, and any ice jam formation as ice breaks up is no guarantee of flooding given the low water levels. In the absence of significant rainfall, the spring flood risk remains lower than normal.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

From 18Z WEDNESDAY THROUGH THURSDAY NIGHT:

Clear skies this afternoon will allow for VFR conditions to prevail through the remainder of the day. Clouds gradually increase overnight, but VFR remains until Thursday afternoon. Thursday evening, snow, sleet and freezing rain are all expected across the area and will cause some restrictions, especially at KMHT, KCON, KASH, and KPSM. North and east of these sites are looking at more exclusively snow with snowfall restrictions likely Thursday night. Lowered restrictions continue into Friday morning as low pressure exits the region.

Outlook:

Friday: Areas of IFR improving to MVFR late the day.

Friday Night: Areas of MVFR or lower possible in low cloud cover. Low confidence in freezing drizzle/rain possible.

Saturday: Areas of MVFR or lower possible in low cloud cover. Low confidence in freezing drizzle/rain possible.

Saturday Night: Areas of MVFR or lower possible with precip changing to rain showers.

Sunday: Areas of MVFR or lower possible improving to VFR.

Sunday Night: VFR conditions expected.

Monday: VFR conditions expected.

MARINE

Sub-SCA conditions prevail this afternoon and continue through daybreak on Thursday. Northeasterly winds pick up, and SCA conditions return Thursday afternoon, continuing into Friday morning as low pressure moves across the Gulf of Maine.

Southwest winds increase ahead of a cold front starting Sat. Southwest winds tend not to mix as well over the cold waters, but some wind gusts near 25 kt are possible. More likely the sustained southwest direction will help build seas above 5 ft outside the bays and that they will remain above SCA levels into early next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Winter Weather Advisory from 7 PM Thursday to 1 PM EST Friday for NHZ011-012-015. MARINE...Small Craft Advisory from 7 PM Thursday to 7 AM EST Friday for ANZ150-152-154.