textproduct: Boston / Norton

WHAT HAS CHANGED

1 to 2" of snow is expected very late tonight into Saturday across the lower elevations of interior Massachusetts and northern Connecticut with 2-4" possible across the higher terrain of the Berkshires and northern Worcester Hills. Offshore low pressure may bring a plowable snow to portions of the region Sunday into Sunday night...but the westward extent remains quite uncertain. We continue to have high confidence in a shot of arctic air later Monday into Wednesday with bitterly cold wind chills.

KEY MESSAGES

- Blustery and cold today with windchills in the single digits above and below zero for much of the region.

- 1 to 2" of snow is expected very late tonight into Saturday across the lower elevations of interior Massachusetts and northern Connecticut. 2-4" possible across the higher terrain of the Berkshires and northern Worcester Hills.

- Offshore low pressure may bring a plowable snow to portions of the region Sun-Sun night...but the westward extent remains uncertain.

- Arctic cold front crosses the region later Mon bringing well below normal temps, bitterly cold wind chills, along with Gales/Freezing Spray for our waters into Wed.

- Some moderation in temperatures expected by Wed night-Thu with perhaps a period of snow.

DISCUSSION

KEY MESSAGE 1...Much colder today behind a strong cold front

Much colder today with highs 15-20 degrees colder than Thurs. Values will range from the mid to upper 20s across the interior to the lower to mid 30s closer to the coast. Peak winds expected through 15z as a 45-50kt low level jet moves in behind deepening low pressure to the northeast. Peak gusts will likely be in the 35-45 mph range especially in the higher terrain. Wind chills will bottom out in the single digits for the majority of the region this morning, except zero to 5 below over the Berkshires and Worcester Hills.

Shortwave trough will be quick to move out today with rising heights and shortwave ridging moving into the region. Dry column under high pressure will mean colder but clear conditions today. The core of the coldest air aloft moves out this afternoon 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 2" of snow is expected very late tonight into Saturday across the lower elevations of interior Massachusetts and northern Connecticut. 2-4" possible across the higher terrain of the Berkshires and northern Worcester Hills.

Broad upper trough amplifies over the Great Lakes with a series of embedded shortwaves rotating around it. One shortwave passes to the north and west and brings two rounds of precipitation including elevation snow and rain along the coastal plain Saturday morning and afternoon. Still anticipating a light event with a coating to as much as 2 inches possible, mainly north and west of I-95. Guidance has continued to trend warmer for areas south and east of I-95 bringing boundary layer temps in the upper 30s and even lower 40s along the coastal plain. There still may be a slushy coating in these locations, but impacts will be limited.

A lull in precipitation likely develops as WAA and overrunning wanes Saturday morning. Another period of interior snow and coastal rain develops during the late morning/early afternoon as a mid level front moves into New Eng with area of mid level frontogenesis. Additional measurable snow will be mostly confined northwest of I-95 as a warmer boundary layer will result in mostly rain in the coastal plain. Snow could fall moderately for a period of time Saturday afternoon with omega values falling to -10 to -15 ubar/sec as frontogenesis increases. Additional accum Saturday afternoon of an inch or 2 is possible in the interior Sat afternoon with total accumulation from very late tonight through Sat of 1-2 inches, much of this focused across central/W MA and northern CT with 2-4" possible in the high terrain of the Berkshires and northern Worcester Hills. 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...Offshore low pressure may bring a plowable snow to portions of the region Sun-Sun night...but the westward extent remains quite uncertain.

There still remains considerable uncertainty with the westward extent of a potentially plowable snowfall in association with an offshore low pressure system Sun into Sun night. We normally do not see this much spread in the guidance for a day 3 forecast...but there are so many moving parts. The 00z GFS/NAM/ECMWF operational models keep the low pressure far enough east that taken verbatim would result in very little if any snow ever reaching as far northwest as the Boston to Providence corridor. However...it is more complex than that. We noted that some of the 00z GEFS ensembles have shifted further northwest. In addition...the 00z RGEM/UKMET were considerably further northwest and would bring accumulating snow all the way back across distant interior southern New England. Also of note...is that the GFS/ECMWF AI models continue to be considerably west of the traditional GFS/ECMWF models. These AI models bring a plowable snowfall all the way back across interior southern New England. While we do not have a lot of experience utilizing a lot of the AI guidance...that is certainly concerning.

So in a nutshell...given all the uncertainty there probably is not a ton of value in a deterministic snowfall forecast Sun into Sun night. That being said...appears the greatest risk for a plowable snow might be near and southeast of the Boston to Providence corridor. That possibility could extend back into western MA and CT if the western solutions verify. And it is also possible that there will be little if any snowfall even into the Boston to Providence corridor. We do think we should see the models getting a much better handle on this in the next 12-24 hours.

KEY MESSAGE 4...Arctic cold front crosses the region later Mon bringing well below normal temps, bitterly cold wind chills, along with Gales/Freezing Spray for our waters into Wed.

What ever happens with the western extent of the snow shield Sun into Sun night...the main concern for the first half of next week will be a shot of arctic air. All model guidance and their ensembles remain in excellent agreement. An arctic cold front crosses the region late Mon into Mon night...bringing well below normal temperatures along with bitterly cold wind chills on very gusty west winds. The cold may peak Tue with high temps only between 15 and 25 degrees and low temps mainly in the single digits to the lower teens. Wind chills will drop below zero given the windy conditions. Lastly...moderate freezing spray and gale force wind gusts are a very good bet for our marine zones.

KEY MESSAGE 5...Some moderation in temperatures expected by Wed night-Thu with perhaps a period of snow.

Deep upper trough begins to lift off to the northeast allowing for rising height fields. This will allow for some moderation in temps by Wed night into Thu. We may also see a period of snow in the warm air advection pattern...but plenty of time to sort that out.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today...High confidence.

VFR. W wind gusts of 25 to 35 knot with a few brief gusts up to 40 knots.

Tonight and Saturday...Moderate confidence.

VFR this evening with diminishing winds becoming southwest. Some MVFR conditions will develop across parts of interior southern New England between 6z and 12z in snow with perhaps some brief IFR conditions too. These conditions will continue at times on Sat. Generally looking at 1-2" of snow across parts of western/central MA and perhaps portions of far northern CT with perhaps some 2-4" amounts in the highest terrain. Temps rising above freezing should result in wet runways by Sat afternoon outside the highest terrain of the Berks and northern Worcester Hills.

Meanwhile...across eastern MA and RI just a few brief rain/wet snow showers with mainly VFR conditions persisting.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Breezy.

Sunday through Sunday Night: Breezy. Slight chance SN.

Martin Luther King Jr Day through Tuesday: 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.

Through Saturday...High Confidence. Gusty W winds this morning with gusts as high as 40kts, especially in the outer waters. The cold air and increasing wave action will result in light freezing spray developing. Winds will begin to diminish later this afternoon and eventually drop to less than 20 kt later tonight. Brief lull arrives overnight with winds possibly increasing to SCA levels again Saturday afternoon.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain.

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

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

Martin Luther King Jr Day: Low risk for gale force winds with gusts up to 40 kt. Areas of rough seas.

Monday Night: gale force winds with gusts up to 45 kt. Areas of rough seas. Freezing spray, slight chance of snow showers.

Tuesday: Moderate risk for gale force winds with gusts up to 40 kt. Areas of rough seas. Freezing spray, slight chance of snow showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Gale Warning until 4 PM EST this afternoon for ANZ230>237-250- 251-254>256.