textproduct: Boston / Norton

WHAT HAS CHANGED

Chances for accumulating snowfall increased Friday night into Saturday despite lower-than-normal forecast confidence. Winter Weather Advisory issued for tomorrow night into Saturday morning for Hartford and western Hampden counties. Gaining confidence in the risk for a period of freezing rain Sunday night into Monday.

KEY MESSAGES

- Gusty winds move in briefly this evening following a cold front and temperatures fall into the teens and single digits. Wind chill values fall to 0F and below overnight.

- Accumulating snow possible Friday night into Saturday morning. Uncertainty remains, but the highest chances for any plowable snow remain in western CT/MA.

- Storm system around Sunday night into Monday could bring accumulating wintry mix (sleet/freezing rain) to rain.

DISCUSSION

KEY MESSAGE 1...Gusty winds move in briefly this evening following a cold front and temperatures fall into the teens and single digits. Wind chill values fall to 0F and below overnight.

A cold front passing through today will leave gusty NW winds in its wake this evening, along with temperatures below freezing that can be expected around 6 pm tonight. 925 mb temperatures overnight will dip to around -15C and lower as the colder air mass settles in post- front. Wind chill values will continue to fall as well, falling to around 0F and lower in some areas across southern New England. Winds this evening into early tonight may gust between 25-35 MPH for most, with the Cape and Islands having a brief chance at some gusts to 40- 45 MPH. Lows tonight will fall into the low teens and single digits across the region. Ahead of the next system, high pressure briefly moves over the region during the day Friday as winds drop off. Highs Friday will only climb into the 20s with a cold airmass aloft.

KEY MESSAGE 2...Accumulating snow possible Friday night into Saturday morning. Uncertainty remains, but the highest chances for plowable snow remain in western CT/MA.

The next disturbance comes in the form of a positively tilted shortwave and its surface low approaching the region Friday night. Snow would start to fall over the region after 7-8 PM tomorrow, continuing into Saturday morning.

The latest high-resolution guidance has shifted the highest amounts and probabilities for more significant snowfall totals to the southwest once more. This is in line with the latest global guidance as well, which has trended the higher amounts towards NYC, SW CT, and across central NY. Probabilities for +2" of snow in Hartford and western Hampden counties are a little over 60 percent when looking at the NBM 5.0, though they drop off to around 20 percent or lower when considering totals at 4" or more. The NAM and Canadian deterministic models have totals around 3" in that area as well. Considering HREF guidance as well and the general track of this low pressure and shortwave aloft, keeping totals in Hartford and western Hampden counties between 3-4" seems to be the most reasonable approach at this time. Some uncertainty does remain, though, as the gradient between the higher and lower totals in this system is quite tight. So, any subtle changes in its track would alter expected snow totals. Stay tuned for future updates.

In terms of snow totals outside of western MA and into CT, the rest of southern New England can expect between a coating and 2" of snow. Totals decrease as you move further east; central MA is more likely to get closer to 1-2" of snow, while eastern MA is more likely to be on the lower side.

KEY MESSAGE 3...Storm system around Sunday night into Monday could bring accumulating wintry mix (sleet/freezing rain) to rain.

We continue to monitor trends with a storm expected to impact southern New England some time Sunday night into Monday night. The track of this low pressure still appears to be well to our north and west, favoring a more wet-than-white forecast. Model soundings continue to forecast a significant warm nose on the order of +4-6C. The question then becomes what the surface temperatures will be below this warm layer. Such strong warm air advection is likely to lead to rising overnight temperatures Sunday night. It remains uncertain what the depth and extent of freezing air will be by the time the atmospheric column saturates enough to result in precipitation.

At this time, favoring freezing rain over sleet, but there could be a brief period of that as well at the onset. The duration of freezing rain before fully transitioning to rain is also a source of uncertainty. Probabilities favor less than one tenth of flat ice accumulation. Still too early to pinpoint ice accumulations beyond that statement. The areas most at risk for freezing rain will be across western MA and along and north of Route 2 in MA.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Today...High Confidence

VFR. SW-ly winds gusting 20-25kt shift NW after 20-21Z with 25-30 kt gusts likely.

Tonight...High Confidence

VFR. Gusty northwest winds.

Friday...High Confidence

VFR. Gradually decreasing northerly winds.

KBOS Terminal...High confidence

VFR. Winds shift NW 20-21Z and gust up to 30 knots.

KBDL Terminal...High confidence in TAF.

VFR. Winds shift NW 19-20Z and gust up to 25 knots

Outlook /Saturday through Tuesday/...

Saturday: Mainly VFR, with local MVFR possible. Breezy. Chance SN, chance FZRA.

Saturday Night: VFR.

Sunday: VFR. Slight chance FZRA.

Sunday Night: Mainly VFR, with local IFR possible. RA, chance FZRA.

Monday: Mainly MVFR, with areas IFR possible. Breezy. RA, FZRA.

Monday Night: Mainly VFR, with local MVFR possible. Windy with gusts up to 35 kt. Chance RA, slight chance SN.

Tuesday: VFR. Windy with gusts up to 35 kt.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Christmas Day through Friday

NW-ly gusts 25-35 knots lasting until daybreak Friday. Seas 3-5 feet near the coast and 7-10 feet offshore. Winds on Friday become light NW-ly at 10-15 knots, with seas subsiding to 2-5 feet.

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of freezing rain, slight chance of snow.

Saturday Night through Sunday: Winds less than 25 kt. Seas locally approaching 5 ft.

Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Rain, chance of freezing rain.

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain.

Monday Night: Strong winds with gusts up to 45 kt. Rough seas up to 16 ft. Rain likely.

Tuesday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 15 ft. Slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 7 PM Friday to 10 AM EST Saturday for CTZ002. MA...Winter Weather Advisory from 7 PM Friday to 10 AM EST Saturday for MAZ009. RI...None. MARINE...Small Craft Advisory until 7 AM EST Friday for ANZ230-233>237. Gale Warning until 7 AM EST Friday for ANZ231-232-250-251- 254>256.