textproduct: Boston / Norton

WHAT HAS CHANGED

Minor snow accumulations for portions of the south shore and Cape Cod this afternoon.

Potential for accumulating snow for Friday into part of Saturday mainly for western MA and northern CT but still a low- confidence forecast. Another storm Sunday night into Monday could bring an accumulating wintry mix to rain.

KEY MESSAGES

- Minor snow accumulations for portions of the south shore and Cape Cod this afternoon.

- Breezy with a few rain/snow showers Christmas Day. Temperatures fall quickly below freezing on Christmas Night with wind chill values in the single digits.

- Accumulating snow possible Fri night into early Sat for northern CT/western MA, with lighter snow accums (if any) north and east. Still a lower-confidence forecast.

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

DISCUSSION

KEY MESSAGE 1...Minor snow accumulations for portions of the south shore and Cape Cod this afternoon.

Ocean effect snow showers associated with an inverted trough offshore can be seen on radar for much of eastern MA east of I-95. Some minor accumulations up to an inch may be possible, especially on The Cape where temperatures have dropped to near freezing. Expect showers to taper off an hour or two after sunset.

KEY MESSSAGE 2...- Breezy with a few rain/snow showers Christmas Day. Temperatures fall quickly below freezing on Christmas Night with wind chill values in the single digits.

Additional rain/snow showers possible across eastern MA early- mid morning on Christmas Day. First half of the day is somewhat pleasant by December standards with seasonable tempeatures and light winds. During the afternoon however, a cold front will be pushing in from the northwest resulting in a wind shift to the northwest. This front will be accompanied by bitter cold temperatures and gusty northwest winds between 25-35 mph tomorrow afternoon into tomorrow night. Low temps in the single digits on Friday morning with sub-zero wind chill temperatures.

KEY MESSAGE 3...Accumulating snow possible Fri night into early Sat for northern CT/western MA with lighter accums (if any) north and east. Still a low-confidence forecast.

We continue to monitor the Fri evening to Sat timeframe for possible accumulating wintry weather, as a weak-amplitude mid-level shortwave disturbance and sfc low pressure treks from the Gt Lakes southeastward through the mid-Atlc region or as far north as interior SNE. Seems to be a stronger consensus per AI-based ensembles of accumulating snow potential mainly south and west of a rough ORE-ORH-PVD line Fri night/overnight, tapering off early into Sat. Besides continued storm track uncertainties which could shape how far north and east accumulating snow could develop, we'll also be coming out of a frigid/very dry airmass mainly for eastern MA/RI, which on the flipside could blunt or even keep the northern end of the precip shield from making it into SNE at all. Though the airmass is plenty cold, most of the modest vertical motion that there is is not collocated within the snow-growth region, so accumulation rates also currently don't look substantial. Weighing the above, it unfortunately remains a challenging, lower-confidence forecast and the prospect of a very sharp SW to NE precip cutoff is in the cards, way more sharp than available global models and AI-based ensembles presently depict. It also wouldn't take much of a storm track adjustment northward to put some of our far western/southwestern counties into receiving more substantial amounts of snow (which currently looked more primed for the lower Hudson Valley into the PA/NJ/NY area), which in turn would also increase the potential for accumulating snow further north and east.

Ensemble 24-hr snow probs show moderate (30-50%) probs of accums of at least 3 or more inches in the Berkshires/western MA/Hartford and Tolland Counties, with nil to low (0-20%) further north and east, and there are nil probs for 24-hr snows in the 6 or more inch range. Probabilistically-based Winter Storm Severity Index (WSSI-P) data suggests high probs of a minor winter weather with low probs of a moderate winter weather event for our northern CT and western MA counties. At current, it looks like the best chance at adverse travel impacts possibly needing winter weather headlines would be for these locations; but as mentioned above, there are too many uncertainties that need to be ironed out first before those are considered.

KEY MESSAGE 4...Storm system around Sunday night into Monday could bring accumulating wintry mix (sleet/freezing rain) to rain.

In the wake of the Fri night/Sat system, the lower-level airmass warms substantially and by Sun night brings 850 mb temps in the +4 to +6C range. Models show a more potent and moisture-laden low pressure embedded in SWly flow aloft which looks to track to our north and west Sun night, with some potential for secondary low development based on some bagginess in sea-level isobars Mon. The strength of the lower-level warmer air as mentioned above would instead favor sleet/freezing rain at least at outset in most areas Sun night, and then gradually transition to plain rain; timing of whch varies depending on how quickly surface temps from Sunday night can modify. We typically keep precip types simple (rain or snow) at this extended range, but felt there was enough confidence in the warm nose aloft to indicate wintry mix in the forecast. Too early for specifics on accums at this range but the potential for an accumulating ice event is possible in most areas, but especially so for interior SNE N/W of I-95 which often struggles to scour out colder air as quickly as blended models depict.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18Z TAF Update

Through 00Z...Moderate Confidence

Mainly VFR with mid-level cloud decks between 3000-6000 feet. Some patchy MVFR ceilings possible at times, especially along the east coast of MA. Gusty northwest winds gradually diminish below 20 knots by 00Z. Some light -RASN at BOS and over The Cape/Islands terminals which may be accompanied by periods of IFR/MVFR vsbys.

Tonight...High Confidence

VFR with lights winds. Some MVFR ceilings possible over The Cape/Islands terminals.

Tomorrow...High Confidence

Generally VFR. Slight chacne for some rain/snow showers over BOS and Cape/Islands tomorrow morning. Showers may be accompanied by brief periods of MVFR vsbys/cigs. Best chance would be 12-18Z. Southwest winds to start, but shifting to northwest after 18Z with 25-30 knots gusts likely.

Tomorrow Night...High Confidence

VFR. Gusty northwest winds.

KBOS Terminal...Moderate confidence in TAF.

-SN/-RASN continues through 19-20Z with periods of MVFR civs/vsbys. VFR thereafer with gusty northwest winds diminsihing this evening. Slight chance for additional rain/snow showers tomorrow morning which may bring MVFR cigs/vsbys, but low confidence where rain/snow showers setup.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Friday: VFR. Windy with local gusts up to 30 kt.

Friday Night: Mainly VFR, with local IFR possible. Chance SN.

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

Saturday Night: VFR.

Sunday: VFR. Chance FZRA, slight chance RA.

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

Monday: Mainly MVFR, with local IFR possible. Windy with local gusts up to 30 kt. RA likely, FZRA likely.

MARINE

Forecaster Confidence Levels...

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

Today through Christmas Day

A gale warning remains in effect today as a cold front brings gusty NNW winds of 30-40 knots. Rough seas are anticipated today with 7- 10 foot waves offshore, and 4-6 foot waves near shore and in the sounds. Brief reprieve from the gusty winds tonight before stronger winds arrive again for Christmas Day and night. Gale force winds from the NNW once again look possible starting in the late afternoon to early evening.

Outlook /Friday through Monday/...

Friday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Freezing spray, slight chance of snow.

Friday Night: Winds less than 25 kt. Local rough seas. Chance of snow. Local visibility 1 to 3 nm.

Saturday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow.

Saturday Night: Winds less than 25 kt. Seas up to 5 ft.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain.

Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Rain.

Monday: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft. Rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Wind Advisory until 3 PM EST this afternoon for MAZ022>024. RI...None. MARINE...Gale Warning until 3 PM EST this afternoon for ANZ230>237-251. Gale Warning until 5 PM EST this afternoon for ANZ250-254>256.