textproduct: Gray - Portland

WHAT HAS CHANGED

Forecast remains on track for tonights wintry weather. Snow is expected across northern New England, with a light glaze of ice across southern New Hampshire. This evening's commute may be slick due to accumulating snow, but impacts will be minor.

Mostly mild but active weather remains in the forecast beyond Thursday.

KEY MESSAGES

1. Some slick travel is likely late this afternoon through this evening as light snow overspreads south-central New Hampshire into far southwest Maine. Light snow expands northeastward tonight with snow ending as a light wintry mix across southern New Hampshire.

2. Wintry mix possible for parts of the area Thursday night into Friday.

3. Mild conditions and rain will lead to some snow melt this weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

As of 1PM EST, snow has just arrived across southwestern NH, with Keene and Jaffrey starting to report light snow. Snow will continue to move northeastward through the day, expanding across much of New Hampshire and Western Maine. Travel for the evening commute may be slick across New Hampshire as snow starts to accumulate. 2-4 inches of snow is expected across most of the area, with lighter totals north of the mountains and T-2 inches expected in southern New Hampshire.

Enough warm air aloft is expected to penetrate into southern New Hampshire, with a changeover to some periods of sleet and freezing rain expected later on in the evening. The snowfall expected prior to the wintry mix will help minimize icing impacts, as most primary roads will have been treated by this time. Icing will be light, with the highest totals expected across the Seacoast. No more than a tenth of an inch of ice is expected anywhere, and most people should only see a trace to a few hundredths of an inch if any.

Precipitation should be tapering off by early Wednesday morning. Warm air looks to continue moving into the area, and highs in the 40s are expected on Wednesday.

KEY MESSAGE 2 DESCRIPTION...

Beginning Thu night there is potential for some wintry mix and snow across parts of the forecast area but there is a fairly high degree of uncertainty. One is in regards to temps across the area. Warmer air is trying to surge north but high pressure centered across Quebec into the Maritimes will tend to support cold air damming. This is something the NBM is pretty consistently poor at handling, so I opted to go with 25th percentile temps instead thru Fri. The other thing reducing confidence is storm track. It remains possible that the storm slides south of the forecast area entirely or only brushes southern zones. The NBM median QPF is lower than the mean, and both are higher than the long range ensemble consensus. Given that and the low confidence I prefer using the lower WPC QPF for this precip event to limit the coverage of wintry precip at this time. The best chances for ice will be overnight Thu into Fri morning and from around 1000 to 2000 ft where the cold air hangs on the longest.

KEY MESSAGE 3 DESCRIPTION...

A warm front is forecast to try and push thru New England for the weekend. While temps will likely be warmer than normal for this time of year, I am somewhat skeptical of the NBM push of warmer air. The upper air pattern is one supportive of high pressure centered to our north or northeast, which would promote onshore winds. Given how cold the ocean currently is, it will be difficult to realize temps into the 50s near the coast. The preference is to hedge away from NBM max temps with some of the cooler guidance. However the warmer than normal temps will introduce both rain and snow melt to the forecast. At this time neither look to cause us trouble on area rivers. The amount of runoff into the rivers will be small, and given how dry things were headed into winter the streamflow was already quite low. So we have some capacity to handle any increase in flow. There may be some river ice break up and movement especially over southern areas, but again given how low rivers were to begin with the formation of an ice jam does not necessarily mean flooding.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 12Z Thursday: Snow overspreads from SW to NE towards the end of the day, with conditions deteriorating to at least IFR between 21Z and 00Z. A brief period of sleet or freezing rain cannot be ruled out along and south of a line from KMHT to KPSM. Conditions will then improve back to VFR after 06Z Wednesday as precipitation ends. VFR expected through Thursday morning.

Outlook:

Thursday: Becoming overcast. VFR conditions expected.

Thursday Night: Areas of IFR possible, especially across southern zones.

Friday: Some improvement in cloud cover, but areas of MVFR or lower remain possible.

Friday Night: Areas of MVFR or lower possible in warm advection aloft.

Saturday: Areas of MVFR or lower possible along with rain showers ahead of a cold front.

Saturday Night: Areas of MVFR or lower possible along with rain showers ahead of a cold front.

Sunday: Return to VFR conditions expected.

MARINE

SCA conditions are expected to continue this evening, with winds out of the south. Conditions improve to sub-SCA levels by early Wednesday morning, with variable winds. Sub-SCA conditions continue through Thursday morning.

Low pressure passing south of the waters will allow northeast winds to freshen Thu into Fri. This will be the best chance for SCA conditions, as northeast winds tend to be stronger than model guidance. Then fairly persistent southerly flow thru the weekend may allow seas outside of the bays to build above 5 ft along with some marginal SCA gusts ahead of a cold front set to cross the waters Sun.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory until 4 AM EST Wednesday for MEZ018>020-023>025. NH...Winter Weather Advisory until 4 AM EST Wednesday for NHZ003>015. MARINE...Small Craft Advisory until 1 AM EST Wednesday for ANZ150>154.