textproduct: Boston / Norton

WHAT HAS CHANGED

Confidence increasing in light snow accumulations across mainly the interior for late Friday night into Saturday. Uncertainty remains with how close a coastal storm will track to southern New England. Accumulating snow is possible with best chance near the coast. High confidence in a shot of arctic air later Monday into Wednesday with bitterly cold wind chills.

KEY MESSAGES

- Becoming blustery and turning much colder tonight and Friday

- 1 to 3 inches of snow possible late Fri night into Sat north and west of I-95

- Coastal system remains possible Sunday into Sunday night. Uncertainty in the track remains uncertain; however, a closer track would favor plowable snow amounts to a portion of the region.

- Shot of arctic air late Monday into Tuesday bringing below normal temperatures and bitter wind chills, especially Tuesday. Freezing spray/gale force winds possible over the waters.

DISCUSSION

KEY MESSAGE 1...Becoming blustery and turning much colder tonight and Friday

Cold front will move across SNE through early evening and will be followed by much colder air pouring into the region as 850 mb temps bottom out at -15 to -17C late tonight. Temps in the 40s and lower 50s this afternoon will plummet tonight with lows reaching the teens and lower 20s by Fri morning. The cold air will be accompanied by gusty winds at times tonight with the wind peaking late tonight into Friday with gusts 25-40 mph. Wind chills will bottom out in the single numbers Fri morning, except zero to 5 below over the Berkshires and Worcester Hills.

Upper low and sharp trough axis moves through New Eng tonight and may be accompanied by a few snow showers, especially interior. But moisture is limited so not expecting much if any impact. The trough moves to the east late tonight and Friday with rapidly rising heights and shortwave ridging moving into the region so expect abundant sunshine Friday. The core of the coldest air aloft moves out Friday allowing temps to recover into the lower 30s, except mid- upper 20s higher elevations with winds gradually subsiding mid to late afternoon.

KEY MESSAGE 2...1 to 3 inches of snow possible late Fri night into Sat north and west of I-95

Amplifying upper trough sets up across the Gt Lakes Fri night into Sat with multiple shortwaves lifting NE towards New Eng. One shortwave passes to the north and west with shot of overrunning light snow developing late Fri night into early Sat. A coating to 2 inches is possible, mainly north and west of I-95. Along and southeast of I-95 temps will be warming into upper 30s with lower 40s possible near the coast. So any light snow early will likely change to rain showers with little or no accumulation.

There will likely by a lull in the precip by late Sat morning, then a second and stronger shortwave will move in from the west Saturday afternoon. In fact, guidance is indicating a mid level front moving into New Eng with area of mid level frontogenesis which will support another period of snow and rain. The snow will be mostly confined northwest of I-95 as a warmer boundary layer will result in mostly rain in the coastal plain. Additional accum of an inch or 2 is possible in the interior Sat afternoon with total accumulation from late Fri night through Sat of 1-3 inches, much of this focused across central/W MA and northern CT. It is possible rain flips to snow near I-95 before precip ends but not expecting more than a slushy coating here.

KEY MESSAGE 3...Coastal system remains possible Sunday into Sunday night. Uncertainty in the track remains uncertain; however, a closer track would favor plowable snow amounts to a portion of the region.

We continue to monitor the potential for a coastal system to brush portions of the southern New England Sunday/Sunday night into early Monday bringing the risk for accumulation snow.

Model guidance remains in general consensus regarding a continued active pattern, with a few pieces of shortwave energy moving through the flow. However, models continue to struggle with the phasing of a northern stream and southern stream energy as a piece of southern stream energy also tracks up the coast with a surface low pressure. In the scenario of better phasing, this would result in an amplified pattern supporting a low track closer to the coast. This would allow ample moisture advection and for the potential for plowable snow to parts of the region. With less phasing, the system will track further offshore with limited moisture available. This would result in minimal (< 2") accumulations and likely confined further southeast.

Due to these phasing uncertainties, there is still considerable variance in low tracks and snow totals among deterministic and ensemble guidance as well as run to run shifts. This indicates that this remains a low confidence event, and it is still too early to lock in on a single scenario. Latest guidance has leaned toward an offshore track resulting in a range from a trace of snow further west to an inch or two in southeast MA. It is worth noting that the Canadian, AIGFS, and AI-ECMWF ensembles suggest higher QPF. Bottom line, we are not locking in on a specific outcome yet, light snowfall remains possible, especially further southeast. We continue to monitor for the potential of a closer track which may yield a 2"+ advisory level event (higher probabilities in southeast MA).

KEY MESSAGE 4...Shot of arctic air late Monday into Tuesday bringing below normal temperatures and bitter wind chills, especially Tuesday. Freezing spray/gale force winds possible over the waters.

There is good agreement among ensemble guidance with a deep trough pushing into the eastern CONUS early next week. An arctic cold front will likely move through the region later Monday into early Tuesday morning with anomalously cold air advecting in behind it Monday night through early Wednesday. 850mb temperatures drop into the -20 to -24C range with the core of the coldest temperatures aloft present on Tuesday. This will support below normal temperatures, Tuesday and Wednesday, but especially on Tuesday. Highs range 15-25 degrees on Tuesday with gusty conditions bringing wind chill values into the single digits, close to zero for the high terrain. Overnight lows drop into the single digits with spots dropping into the negative single digits in the higher terrain spots. Chilly again Wednesday with highs in the upper teens to 20s. Cold temperatures and gusty winds will bring the risk for gale force winds and freezing spray across the waters.

AVIATION /19Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

Through Friday...High confidence.

Patchy MVFR cigs in the interior will trend to VFR by late afternoon. Otherwise VFR with isolated brief shower/elevation snow shower possible through this evening. SW wind gusts to 25 kt will shift to W this evening and temporarily diminish, then W gusts to 25-35 kt Friday, diminishing late.

Friday night...Moderate confidence.

VFR, but lowering to MVFR-IFR late Fri night in the interior as light snow develops.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: Mainly MVFR, with local IFR possible. Chance SN, chance RA.

Saturday Night: VFR.

Sunday: Mainly VFR, with local MVFR possible. Slight chance RA, slight chance SN.

Sunday Night: VFR. Slight chance SN.

Martin Luther King Jr Day through Monday Night: VFR. Windy with local gusts up to 30 kt.

Tuesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Tonight through Friday Night...High Confidence.

A strong cold front crosses the waters this evening. Gusty SW winds shift to W behind the front with gusts increasing to 35-40 kt late tonight into Fri, especially outer waters. The cold air and increasing wave action will result in light freezing spray developing. Winds will begin to diminish late Fri and eventually drop to less than 20 kt later Fri night.

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kt. Local rough seas. Chance of rain.

Saturday Night: Winds less than 25 kt. Local rough seas. Slight chance of rain.

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

Sunday Night: Winds less than 25 kt. Local rough seas. Chance of snow.

Martin Luther King Jr Day: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas.

Monday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. Chance of freezing spray, slight chance of snow.

Tuesday: Strong winds with gusts up to 30 kt. Areas of rough seas. Chance of freezing spray, slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Gale Warning from 4 AM to 1 PM EST Friday for ANZ230-236. Gale Warning from 1 AM to 4 PM EST Friday for ANZ231>235-237- 250-251-254>256.