textproduct: Boston / Norton

WHAT HAS CHANGED

Snow accumulations increased and winter weather advisories expanded to include the rest of southern New England except Nantucket. Total snow accumulations of 2-6" expected with localized 7" amounts possible across eastern MA/RI. Heaviest snow occurs between 5 pm this evening and 1 am Monday...but some snow showers may linger into the Monday morning commute especially across eastern Massachusetts. No other significant changes made to the forecast.

KEY MESSAGES

- 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.

- Arctic cold front sweeps across the region Mon night bringing well below normal temperatures and bitterly cold wind chills & temps. The peak of the cold weather will be Tue into Wed am.

- Milder temps arrive Wed night-Thu with perhaps a brief period of light snow showers or even light rain showers near the coast.

- Another shot of very cold air works into the region behind a cold front Thu night into Fri.

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.

Deepening shortwave near a strong thermal boundary over the south triggers cyclogenesis off the coast of the Carolinas early this morning. Surface low quickly tracks northeast as the parent trough becomes negatively tilted over the Great Lakes. Low pressure will be slow to deepen as upper level energy will be somewhat stretched and elongated along a broad baroclinic zone offshore. Some modest deepening of the low center will occur as the trough goes negative and the right entrance region of a 180kt jet moves over the low center. Models have come into better agreement on the track of the storm, taking the center of the low SE of the 40 70 benchmark from 00z-06z Monday. Despite the weaker low center (~995mb) guidance continues to increase QPF across the region with 0.5-0.7" for SE MA and the Cape/Islands and 0.2-0.5" along and NW of I-95. Periods of mainly light snow develop this morning and continue at times into the afternoon...but a lull in the activity is possible especially north of I-90. The steadier/heavier snowfall will arrive late this afternoon and especially tonight across eastern MA/RI.

Looking at some strong midlevel frontogenesis resulting in ample lift in the DGZ. Guidance shows mid level cooling NW of the 850mb low as it tracks SE of the region. This cooling results in the DGZ growing a depth of 2kft during the afternoon to around 6kft through the evening and overnight. At the same time, 700mb omega of -10 to -15 ubar/sec means vigorous lift will be present resulting in bands of moderate to perhaps heavy snow overnight. Highest chance of more consistent moderate/heavy snowfall rates will be across eastern MA/RI. The highest snow totals are still expected across eastern MA/RI, where strong mid-level forcing and temperatures near or below freezing will overlap for the longest duration. Snow totals in this area will range from 3-6 inches, with localized 7" amounts possible. Would not be surprised if this band extended into parts of central MA/northeast CT where temps are colder and there will be better snow ratios.

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 western MA, the mid-level forcing is not quite as strong, but lower temperatures will allow for 2-5 inches. Have opted to include the rest of western and north central MA in the advisory. While forcing will be less in these areas, higher ratios will contribute to greater snowfall accumulations.

While the steady snow ends before daybreak Mon, a period of lingering snow showers are possible especially across eastern MA through 15z. Guidance shows low level convergence and a bit of instability which could pair to produce areas of snow showers and light snow into MLK day.

In summary...periods of snow overspread the region this morning and continue at times this afternoon but a lull in the activity is possible for a time especially north of I-90. Heaviest snow rates arrive later in the afternoon and especially tonight with the potential of 1"+ per hour snow fall rates.

Otherwise...becomes breezy with partial sunshine returning by Monday afternoon ahead of an arctic cold front. High temps will be in the upper 20s to the middle 30s

KEY MESSAGE 2...Arctic cold front sweeps across the region Monday night bringing well below normal temperatures and bitterly cold wind chills & temps. The peak of the cold weather will be Tue into early Wed morning.

The models are still in very good agreement in an arctic cold front crossing the region Mon night. This will bring a shot of arctic air and well below normal temperatures to the region. 850T will drop to between -18C and -20C with the cold peaking Tue into early Wed. Gusty west winds will result in wind chills below zero to the single digits. High temps Tue will only be in the upper teens to the middle 20s with low temps Tue night mainly in the single digits to the lower teens. This shot of arctic air will be short-lived as high pressure shifts to our east by Wed. We should see high temps recover into the middle 20s and lower 30s by Wed afternoon...which is still below normal.

KEY MESSAGE 3...Milder temps arrive Wed night-Thu with perhaps a brief period of light snow showers or even light rain showers near the coast.

Low pressure tracking across Quebec will result in much milder air working back into the region on southwest flow. We may see a period of light snow showers in the warm air advection pattern and perhaps even light rain showers near the coast as the boundary layer warms. Regardless...these rain/snow showers will likely be short-lived and not expecting a significant precipitation event.

KEY MESSAGE 4...Another shot of very cold air works into the region behind a cold front Thu night into Fri.

Another shot of very cold air works into southern New England behind a strong cold front Thu night into Fri. Highs may be held in the upper 20s and lower 30s on Fri but with mainly dry weather.

AVIATION /08Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

Today and tonight: 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 mid-morning 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 /Monday Night through Thursday/...

Monday Night: VFR. Windy with gusts up to 35 kt.

Tuesday through Wednesday: VFR. Breezy.

Wednesday Night: VFR. Breezy. Chance SHSN.

Thursday: VFR. Breezy. Chance SHSN, slight chance SHRA.

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 today, 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 tonight into early Monday, likely will need to be handled with a renewed Small Craft Advisory.

Outlook /Monday Night through Thursday/...

Monday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 12 ft. Freezing spray.

Tuesday: Low risk for gale force winds with gusts up to 30 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: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft.

Wednesday Night: Strong winds with gusts up to 30 kt. Areas of rough seas. Chance of rain showers.

Thursday: Strong winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 7 AM this morning to 7 AM EST Monday for CTZ002>004. MA...Winter Weather Advisory from 7 AM this morning to 7 AM EST Monday for MAZ002>023-026. RI...Winter Weather Advisory from 7 AM this morning to 7 AM EST Monday for RIZ001>007. MARINE...Small Craft Advisory until 10 AM EST this morning for ANZ235- 237-250. Small Craft Advisory until 7 PM EST Monday for ANZ254>256.