textproduct: Boston / Norton

WHAT HAS CHANGED

Blizzard Warnings were expanded to all of southern New England except northwest MA. Winter Weather Advisory for Nantucket upgraded to a Blizzard Warning.

KEY MESSAGES

- A powerful winter storm brings significant to extreme impacts to much of the region later tonight into Monday, with heavy snowfall, damaging winds and blizzard conditions. Worst of the impacts will be felt south and east of the I-95 corridor.

- Expecting significant coastal impacts with areas of moderate coastal flooding during the late tonight/early Monday morning high tide across eastern MA. The greatest impacts will be from Plymouth county southward to Cape Cod and the Islands.

DISCUSSION

KEY MESSAGE 1...A powerful winter storm brings significant to extreme impacts to much of the region later tonight into Monday, with heavy snowfall, damaging winds and blizzard conditions. Worst of the impacts will be felt south and east of the I-95 corridor.

Overview: Potent upper level trough/shortwave becoming negatively tilted and closing off south of New Eng with significant height falls over 300m will result in explosive cyclogenesis tonight into Monday morning. 24 hr pressure falls exceed 35 mb in 24 hr period which more than satisfies bomb cyclone criteria. This is a classic climatological set up for a blockbuster blizzard for the coastal plain with sub 970 mb low tracking near the benchmark. Very good model agreement on track and intensity so we have high confidence on impacts across the region. Still some differences in QPF for the storm event but we leaned toward hi- res guidance which should have a better handle on mesoscale processes with the banding across the region. Exact placement of mesoscale bands and localized heaviest snowfall may still shift until we can see where banding sets up but overall high confidence on snowfall amounts. We have noted a slight shift in the storm track to a bit south of the benchmark which would increase confidence of max snowfall occurring in SE MA, but lower confidence in amounts across interior northern and western MA/CT.

Timing and snowfall amounts: Some light snow may develop by late afternoon, but main accumulating snow should hold off until 7-11 pm from SW to NE. Then we await the very heavy snow band which will be moving up from the south associated with strong low level warm advection/frontogenesis enhanced by the low level jet which will also be lifting northward. This will initially be a laterally translating band lifting northward across the region between 3 am and 10 am. Snowfall rates during this time will likely reach 2-4"/hr at times, especially across RI and eastern MA given strong and deep forcing for ascent through the DGZ and some instability aloft supporting the heavy snowfall rates and possible thundersnow. Then it will transition to a pivoting band late morning into the afternoon and where this pivots will determine where heaviest snowfall sets up and potential for amounts exceeding 2 feet. Overall looking at 18-24 inches from eastern CT and central MA through RI and eastern MA. While QPF is less across eastern CT and central MA SLRs will be greater. Within this area we think the best chance of seeing locally up to 30 inches will be across SE MA where pivoting band is most likely which fits climatology of a mid level low tracking near or a bit south of the benchmark. For the Cape and Islands we have amounts dropping off a bit over the outer Cape and especially Nantucket where snow may briefly mix or change to rain in the morning before flipping back to snow, but still looking at 8-12 inches for Nantucket and 12-18 inches for the outer Cape. Across western MA/CT there is a bit more uncertainty as this area will be on the western fringe of the heaviest snow. Looking at 8-14 inches in this area. Given the strong wind gusts across the entire region, blizzard conditions are expected for much of the region and so the blizzard warnings were expanded westward. Very poor vsbys with whiteouts at times and blowing and drifting are likely, especially late tonight into Monday afternoon. The intensity of the snow will begin to taper off around midday in western New England and mid to late afternoon across eastern MA, ending by late afternoon in the west and 7-10 pm across SE MA and Cape Cod.

Impacts: We can't stress this enough but this storm will likely bring extreme to destructive impacts south and east of the I-95 corridor, especially SE MA and Cape Cod due to the combination of heavy wet snow and damaging wind gusts. Heavy wet snowfall amounts up to 2 ft with damaging winds presents a serious concern for tree damage and power outages. Temps across SE MA will be around 32F during the brunt of the heavy snow late tonight and Mon morning, then will begin to fall into the 20s during Mon afternoon. Further north and west into central and NE MA and western MA/CT, temps will be in the 20s for the entire event so the snow will be of a drier consistency which will help to mitigate these impacts somewhat. These areas could still see some power outages but on a much lesser scale than what is experienced south and east of I-95. In terms of travel, it will be near impossible during the period of heaviest snow late tonight through the morning with the heavy snowfall rates and whiteouts and as such travel is highly discouraged.

Damaging wind: Wind will also be a serious concern, especially along and south and east of I-95 corridor. 65 to 75 kt low level jet lifts north across the coastal plain late tonight into Mon before pivoting across SE MA and Cape/Islands. We've seen stronger LLJs but what is different with this event is we have a favorable thermodynamic environment to promote efficient mixing. As such, soundings support hurricane force gusts to 70-80 mph for the outer Cape and Nantucket, 55-65 mph southern RI and SE MA and 40-50 mph for the interior north and west of I-95. These winds alone will be strong enough alone for tree damage and power outages south and east of I-95, not even considering the impacts for heavy wet snow.

KEY MESSAGE 2...Expecting significant coastal impacts with areas moderate coastal flooding during the late tonight/early Monday morning high tide across eastern MA. The greatest impacts will be from Plymouth county southward to Cape Cod and the Islands.

A storm surge of 2-3' at the time of high tide early Monday morning (2AM-5AM) will likely yield moderate coastal flooding from Plymouth county southward to Cape Cod and the Islands. Therefore a Coastal Flood Warning continues for this portion of the coastline. Farther northward from Boston to the NH border, there is more uncertainty, with a higher probability of minor flooding. Therefore, a Coastal Flood Advisory posted there.

The greatest storm surge (up to 4 ft) appears to occur during low tide Monday (9-10AM). However, there could be enough storm surge for minor coastal flooding during the late day high tide (3-6PM). Although, given the astro tide is lower and winds will be more northerly, only expecting minor flooding. Nonetheless, a Coastal Flood Watch remains in effect for this Monday afternoon high tide to account for any storm track changes, including timing.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

Tonight through Monday Morning... High Confidence

LIFR overspreads the region between 04-08z. Wind increase out of the NE this evening/tonight with gusts 20-40 kts, increasing to 40-50 kts after midnight. Gusts 50-70 kts possible in eastern MA/RI, strongest for the Cape and Islands. Snow rates pick up overnight, becoming very heavy, likely after 06Z. Heavy blowing snow/blizzard conditions likely, especially further east. Significant LLWS with 70 knots of wind down to 500 feet.

Monday Afternoon and Night... High confidence.

Gradually improving to VFR from W to E Monday afternoon. MVFR by 18z in western New England and perhaps VFR by 00Z Improvements likely take until midnight to reach the east coast of MA. Snow rates decrease in the afternoon with remaining snow exiting the east by midnight. Winds gradually diminish in the afternoon and evening, especially after midnight. However, gusts 40-55 kts are expected through the evening across the more east terminals.

KBOS Terminal...High confidence in TAF (Blizzard conditions)

Snow becomes heavy with hourly rates greater than 1" after 09z.

Blizzard conditions expected with gusts up to 55 kts developing by daybreak Monday along with wind shear. Snow rates decrease after 18z but BLSN continues. Light snow continues Monday evening, exiting by midnight.

KBDL Terminal...High confidence in TAF (Blizzard conditions)

Heavy snow with hourly rates greater than 1" likely after 07/08z. Heavy snow continues Monday morning, with rates decreasing around 15-18z but BLSN continues. Light snow continues before exiting late afternoon/evening.

Winds increase tonight, becoming strong especially after midnight- toward daybreak. Gusts 40-45 kts by 12Z through the afternoon resulting in blizzard conditions. Winds gradually decrease Monday afternoon- evening.

Outlook /Tuesday through Friday/...

Tuesday: VFR. Breezy.

Tuesday Night: VFR. Chance SN.

Wednesday: Mainly IFR, with local MVFR possible. Breezy. SN likely, chance RA.

Wednesday Night: VFR. Breezy.

Thursday: VFR. Chance RA, chance SN.

Thursday Night: Mainly VFR, with areas IFR possible. Breezy. RA likely, chance SN.

Friday: Mainly VFR, with local MVFR possible. Breezy. Chance SN, slight chance RA.

MARINE

Forecaster Confidence Levels...

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

Very dangerous and life threatening storm for mariners late tonight through Monday. Storm warnings are in effect. We expect wind gusts 55-65 kt peaking late tonight into Monday afternoon. A few hurricane force gusts are likely. Winds will begin to diminish from the peak of the storm from late afternoon through Mon evening and drop below gale force late Mon night. Seas peaking at 15-25 ft Mon afternoon and evening with the highest seas over the eastern MA outer waters. Near zero vsby at times in heavy snow late this evening into Monday, possibly briefly changing to rain east of Chatham to Nantucket.

Outlook /Tuesday through Friday/...

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 13 ft.

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

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Rain likely, chance of snow. Areas of visibility 1 to 3 nm.

Wednesday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of rain.

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

Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Rain likely. Local visibility 1 to 3 nm.

Friday: Winds less than 25 kt. Areas of rough seas. Chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Blizzard Warning until 7 AM EST Tuesday for CTZ002>004. MA...Blizzard Warning until 7 AM EST Tuesday for MAZ004>007-009>024- 026. Winter Storm Warning until 7 AM EST Tuesday for MAZ002-003-008. Coastal Flood Advisory from 1 AM to 7 AM EST Monday for MAZ007- 015-016. Coastal Flood Warning from 1 AM to 7 AM EST Monday for MAZ019- 022>024. Coastal Flood Watch from Monday afternoon through Monday evening for MAZ019-022>024. RI...Blizzard Warning until 7 AM EST Tuesday for RIZ001>008. MARINE...Storm Warning from 10 PM this evening to 1 AM EST Tuesday for ANZ230>237-250-251-254>256.