textproduct: Boston / Norton

WHAT HAS CHANGED

A winter weather advisory has been issued for all of CT and into much of Eastern MA and RI. Confidence has increased for 2-5 inches of snow across the advisory areas, with localized spots up to 6 inches possible in SE MA and RI. Confidence remains too low still for NW MA and for Block Island and Nantucket to be included in the advisory at this time.

KEY MESSAGES

- Snow continues across NW MA this afternoon, ending after 5pm this evening.

- Coastal low passes just SE of the Benchmark, bringing 2-5 inches with localized areas of 6 inches, especially in SE MA and RI.

- Arctic cold front pushes through Monday night bringing well below normal temperatures and bitterly cold conditions. The peak of this cold weather will be Tuesday into early Wednesday morning.

- Milder conditions arrive later Wednesday-Thursday, with perhaps a brief period of light snow showers or even light rain showers near the coast.

DISCUSSION

KEY MESSAGE 1...Snow continues across NW MA this afternoon, ending after 5pm this evening.

Snow begins to taper off across western MA this evening after 5pm as the mid level shortwave pulls away from the region and drier air. Additional accumulations across western Mass could reach 1-2 inches this afternoon. Snow transitions to rain as precip approaches the coastal plain as temperatures are currently above freezing through midnight.

KEY MESSAGE 2...Coastal low passes just SE of the Benchmark, bringing 2-5 inches with localized areas of 6 inches, especially in SE MA and RI.

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. Guidance continues to come into Better agreement on track with this low, but remains spread on how far NW the QPF field extends. Guidance generally ticked up total QPF across the region with 1/2 to 3/4 inches across the Cape and Islands, and a 1/4 to 1/2 inch along and south of the I-95 corridor. How this translates into snowfall accumulation will depend heavily on temperatures and mid-level forcing. Temperatures during this event will be marginal, with temperatures staying near freezing inland and at or above freezing for the Cape and Islands. This will impact snow totals as snow will melt and compress as it falls. Even areas inland will see very wet and heavy snow. Even where temperatures are above freezing, still anticipating accumulations for temps between 32-35F as strong mid-level forcing will allow for precipitation to become heavy enough to drag down cooler air and remain as snow rather than turning to rain.

The highest confidence in snow totals across the region is in SE MA and into RI, where strong mid-level forcing and temperatures near or below freezing will overlap. Snow totals in this area will range from 3-5 inches, with localized areas reaching 6 inches. While there will be good forcing over Nantucket and Block Island, temperatures will likely be too warm for significant accumulations, but a slushy inch remains possible. Across CT and into north-eastern MA, the mid- level forcing is not quite as strong, but lower temperatures will allow for 2-4 inches, with localized spots up to 5 inches. Uncertainty is highest across NW MA, where a lack of mid-level forcing and dry air will limit totals. Still could see 1-3 inches, but if the low moves closer to the benchmark, these totals could increase, and the advisory could be expanded NW.

Timing this system out, A band of snow showers associated with the prefrontal wave moves into CT as early as 7 am, but most likely not arriving until after 10 am. Snow spreads north and east into the rest of the region after the lunch hour. Snow will initially start light with rates of under half an inch per hour. Heaviest snow rates arrive later in the afternoon and into the early evening, with rates between 1/2 inch and 1 inch per hour through 10 pm to midnight. Snow rates could exceed 1 to 1.5 inches per hour near the Cape and Far SE MA if temperatures are able to cool below freezing. Snow ends from west to east between 10 pm and 2 am, snow showers may linger into Monday morning. Breezy winds are expected Monday afternoon behind the system, but given the wet and heavy nature of the snow, we do not anticipate blowing snow impacts on Monday.

KEY MESSAGE 3...Arctic cold front pushes through Monday night bringing well below normal temperatures and bitterly cold conditions. The peak of this cold weather will be Tuesday into early Wednesday morning.

High pressure across the southern USA should combine with a low pressure over the Saint Lawrence Valley to funnel significantly colder air into our region towards the middle of next week. Strong signals in the ensemble data, with temperatures some 2-3 standard deviations below normal. While not quite at the minimum of climatology, still looking quite cold, especially at night. Gusty winds Tuesday should lead to minimum wind chill values in the single digits around 0F Tuesday morning.

KEY MESSAGE 4...Milder conditions arrive later Wednesday- Thursday, with perhaps a brief period of light snow showers or even light rain showers near the coast.

Not anticipating quite as much wind into Wednesday as high pressure moves offshore of the Mid Atlantic coast, so some moderation in temperatures starts Wednesday. Winds turn more southerly ahead of a cold front, which is currently expected to move through some time Wednesday night into Thursday. Some light snow or rain showers could accompany this front. Colder air should return for late next week, with another period of below normal temperatures possible.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

TAF Update: 00z

Tonight: High confidence.

VFR. Lull in activity. Light to calm wind.

Sunday & Sunday Night: Moderate confidence.

Between 12z-15z ceiling fall to MVFR/IFR from south to north as snow arrives. Periods of light snow overspreads the region with visby falling to 3SM-5SM. Steadier snowfall overspreads from south to north from 21z-00z, afterwards, snowfall rates could approach near 1" per hour for terminals southeast of I-95. Snow ends west to east between 06z-09z, becoming VFR during this time frame as well. SW winds to begin on Sunday, briefly NNE 20z-02z, then to the NNW there after 02z.

KBOS Terminal...Moderate confidence in TAF.

VFR. Rain/Snow showers possible between 21z-23z, leading to a brief period of MVFR ceilings. VFR tonight, trending back to MVFR Sunday midmorning as snow arrives from south to north. Could arrive as early as 15z, but feel somewhat more confident with arrival around 18z-20z. Conditions lower after 21z tomorrow with widespread snowfall.

KBDL Terminal...Moderate confidence in TAF.

IFR/LIFR with moderate snowfall through 21z, improving conditions between 21z-23z. VFR tonight with light winds. Snow returning early Sunday morning.

Outlook /Sunday Night through Thursday/...

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

Tuesday through Wednesday Night: VFR. Breezy.

Thursday: VFR. Breezy. Slight chance RA, slight chance SN.

MARINE

Today and tonight...High Confidence.

Residual seas of 5-8 feet on the southern waters and 3-6 feet on the eastern waters is the basis for a continued Small Craft Advisory on the outer waters, which is in effect through Sunday morning. Coastal storm passes off shore Sunday, bringing a rain/snow mixture and poor visby conditions. Brief lull in advisory level conditions, seas fall below 5 feet with light winds. Increasing wind speed/gusts with seas coming up late Sunday night into early Monday, likely will need to be handled with a renewed Small Craft Advisory.

Outlook /Sunday Night through Thursday/...

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

Monday Night: Strong winds with gusts up to 35 kt. Areas of rough seas.

Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of freezing spray.

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of freezing spray.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 10 AM to 10 PM EST Sunday for CTZ002>004. MA...Winter Weather Advisory from 10 AM Sunday to 4 AM EST Monday for MAZ005>007-012>023. RI...Winter Weather Advisory from 10 AM Sunday to 4 AM EST Monday for RIZ001>007. MARINE...Small Craft Advisory until 10 AM EST Sunday for ANZ235-237-250. Small Craft Advisory until 7 PM EST Sunday for ANZ254>256.