textproduct: Boston / Norton

WHAT HAS CHANGED

Despite marginal boundary layer temperatures, bands of moderate to heavy snow have developed across much of SE Massachusetts and the Cape. Snowfall reports in these areas indicate that around 1-3 inches of snow has already fallen with an additional 2-4 inches possible. Heaviest snow will continue through 06z before gradually tapering tomorrow morning. As a result, have opted to adjust snow totals to 4-6" with localized amounts up to 7 inches possible.

KEY MESSAGES

- Plowable snow tonight with total accumulations of 3-5" with localized 6" amounts possible across eastern MA/RI. Heaviest snow/worst of the travel conditions between 6 pm and 2 am. Snow ends from west to east between 3am and 9am Monday.

- Arctic front delivers cold conditions Tuesday, with some moderation Wednesday and Thursday. Colder again next weekend.

- Quick moving clipper late Wednesday into early Thursday brings the next chance for light precipitation.

DISCUSSION

KEY MESSAGE 1...Plowable snow today-tonight with total accumulations of 2-6" with localized 7" amounts possible across eastern MA/RI. Heaviest snow/worst of the travel conditions between 5 pm and 1 am...but snow showers may linger for the Mon am commute.

An elongated shortwave trough stretching from the northern Great Lakes down into the Gulf Coast will spin up a coastal low offshore, passing just southeast of the benchmark this evening. Guidance held steady, with total QPF across the region ranging from 1/2 to 3/4 inches, south and east of the I-95 corridor, including the Cape and Islands. QPF ticked up a bit north and west of the I-95 corridor all the way to the northern Berkshires with 1/4 to 1/2 inch of QPF. How this translates into snowfall accumulation will depend heavily on temperatures and where mid-level forcing sets up. Temperatures during this event will be marginal, with inland areas staying near freezing and the Cape and Islands staying at or above freezing. With the best mid-level forcing near the Cape and Islands, we will likely see snow at temperatures up to 36F as snow rates will be able to overcome the warm temps. The primary source of uncertainty is the temperature at which snow will accumulate on paved surfaces. Because of the warm temps, we continued to trend total accumulations down to 0.5-1.5 inches for the outer Cape and the Islands; however, if temps cool below freezing, snow accumulations could exceed 2-4.

The highest confidence in snow accumulations remains along the I-95 corridor in eastern MA and into RI, where good mid-level forcing and temperatures near or below freezing will overlap. Snow accumulations in this area will range from 3-5 inches, with localized spots up to 6 inches possible. To the north and west of the I-95 corridor, mid-level forcing begins to drop off; however, cooler temperatures will allow snow ratios to be higher, ranging between 12-1 to 16-1. Snow totals north and west of the I-95 corridor, including all of CT, will range from 2-4 inches, with localized areas of 5 inches.

After the lull in snowfall this afternoon, steady light snow will begin to overspread the region once again through 6 pm. Snow rates will start to tick up after 6 pm, increasing to 0.5 to 1.5 inches per hour. Snow becomes light again after 2-4 am and tapers off from west to east through 6-9 am this morning. Snow showers may linger over eastern MA until noon, but additional accumulations will be light. The most difficult travel period will be between 6 pm and 2 am tonight. However, with lingering snow showers into Monday morning, the AM commute will likely be impacted.

Low temperatures on Monday morning are expected to drop into the low to mid-20s across the region, which could cause any standing water from melting snow to refreeze, especially on untreated surfaces. High temperatures Monday afternoon rebound into the low to mid 30s, allowing some melting. Winds become gusty out of the WSW Monday afternoon at 15-20mph, making it feel like the low to mid 20s.

KEY MESSAGE 2...Arctic front relievers cold conditions Tuesday, with some moderation Wednesday and Thursday. Colder again next weekend.

A potent Arctic cold front crosses the region Monday night into Tuesday, ushering in much colder temperatures and gusty winds. 925mb temperatures fall to around -15C, with steep low-level lapse rates approaching 10C/km, supporting efficient boundary-layer mixing and transport of stronger winds to the surface. Gusts on Tuesday range from 25 to 35 mph, with a few gusts near 40 mph possible across the higher terrain. Combined with high temperatures in the teens and 20s, wind chills fall into the single digits to lower teens Tuesday afternoon, then into the 0F to -10F range Tuesday night. While cold, conditions are expected to fall shy of a Cold Weather Advisory. This Arctic intrusion is short-lived, with moderating temperatures Wednesday afternoon into the upper 20s to lower 30s, followed by mid 30s to lower 40s on Thursday. Global guidance suggests another Arctic air mass may arrives next weekend, with highs returning to the single digits.

KEY MESSAGE 3...Quick moving clipper late Wednesday into early Thursday brings the next chance for light precipitation.

The next chance for precipitation arrives midweek, though confidence remains low. Synoptic pattern features a mid-level low over Hudson Bay with embedded shortwave energy rotating around the cyclonic flow, supporting the potential development of a weak Alberta clipper tracking southeast from the Canadian Rockies. As typical with clippers, moisture limited. While temperatures Wednesday afternoon may be marginal the system could pass during the evening or overnight as temperatures fall below freezing, introducing the potential for light snow accumulations. With little downstream blocking, the system is expected to be progressive. Ensembles guidance remains unimpressive, with probabilities for snowfall exceeding one inch generally in the 10-20% range across interior southern New England. While near the coast, temperatures could be warm enough for light rain showers with southwest flow.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

00z TAF Update...

Moderate confidence.

Previous lull in the snowfall has given way to areas of light to even moderate snow especially SE if I-95. Predominantly IFR conditions in most areas, with areas of 2-4sm in BR and then -SN, especially across southeast areas. The core of the heaviest precipitation will be generally from 00-07z (earliest in the west). Most locations should see a few hours of 1/2SM in moderate to briefly heavy snowfall during this period. Still quite a bit of inconsistency amongst models, so exact timing has an error bar of roughly 2 to 3 hours. Snow will taper off from west to east in the 10-14z period. Expect IFR/MVFR ceilings and visibilities to continue a few hours after that, until drier air starts moving in. Conditions will trend toward VFR Monday afternoon, with west winds gusting 20kt or higher in many areas, especially after 18z.

Monday Night is expected to feature VFR conditions, though with still some west winds gusting 15-20kt or so.

KBOS Terminal...Moderate confidence in TAF. Heaviest snow falls from now through about 07z. Otherwise, additional snow showers develop thru 14-15z Monday morning.

KBDL Terminal...Moderate confidence in TAF. Specific timing of the heaviest of the snow could be off by up to 2 hours.

Outlook /Monday Night through Friday/...

Tuesday through Wednesday: VFR. Breezy.

Wednesday Night: VFR. Breezy. Slight chance SN.

Thursday: VFR. Breezy. Chance RA, chance SN.

Thursday Night: VFR. Breezy. Slight chance SN.

Friday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

* Gale Watch for all waters Monday evening through Tuesday morning.

Tonight through Monday night... High confidence in trends, moderate confidence in Gale Watch.

A quasi-stationary frontal boundary remains offshore this afternoon, with rain and snow continuing over the waters. Low pressure moves along this boundary tonight, exiting east of the Gulf of Maine on Monday. Seas build from 4-6 feet overnight to 5-8 feet on Monday, with up to 10 feet across the southern outer waters. Westerly winds increase Monday afternoon into early evening. A strong Arctic cold front crosses the waters Monday night into early Tuesday, bringing the potential gale-force gusts. Increasing cold air advection may also result in freezing spray across all waters late Monday night into Tuesday morning.

Outlook /Monday Night through Friday/...

Tuesday: Strong winds with gusts up to 35 kt. Areas of rough seas. Freezing spray.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Freezing spray.

Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Wednesday Night through Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain.

Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of freezing spray, slight chance of rain, slight chance of snow.

Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of freezing spray.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory until 7 AM EST Monday for CTZ002>004. MA...Winter Weather Advisory until 7 AM EST Monday for MAZ002>023- 026. RI...Winter Weather Advisory until 7 AM EST Monday for RIZ001>007. MARINE...Gale Watch from Monday afternoon through Tuesday morning for ANZ230>234-236-251. Gale Watch from Monday afternoon through Tuesday morning for ANZ235-237. Gale Watch from Monday morning through Tuesday morning for ANZ250-254>256.