textproduct: Boston / Norton

WHAT HAS CHANGED

Converted the Winter Storm Watch to a Winter Weather Advisory for Nantucket. Increased snow totals across east central Massachusetts and Rhode Island to show a widespread 18-24 inches.

KEY MESSAGES

- A powerful winter storm brings significant impacts to much of the region tonight into Monday, with heavy snowfall, strong winds and blizzard conditions for RI and much of SE and coastal Massachusetts.

- 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.

DISCUSSION

KEY MESSAGE 1...A powerful winter storm brings significant impacts to much of the region tonight into Monday, with heavy snowfall, strong winds and blizzard conditions for RI and much of SE and coastal Massachusetts.

Timing: Felt that the 22/00Z HREF had a good handle on onset timing with snow/flurries developing late this afternoon across CT/RI/MA. The accumulating snow will begin late today into this evening. Then the heaviest snow will occur from late tonight into Monday afternoon.

A deep and negatively tilted trough over the Mid Atlantic provides the necessary forcing for cyclogenesis off the NC coast this morning with rapid deepening expected to commence overnight into Monday morning. System undergoes bombogenesis and strengthens from 1005mb at 15z today to around 965-970mb early Monday. At the mesoscale, high-resolution guidance has resolved quite a deformation zone for the eastern part of southern New England early Monday morning through the afternoon.

The resulting frontogenetic forcing brings in some extreme hourly hourly snowfall rates. The HREF continues to show high probabilities (>80%) of snowfall rates exceeding 2"/hr along the I-95 corridor in Rhode Island and southeast Massachusetts. These extreme rates are the result of exceptionally strong lift within the DGZ. Some guidance such as the NAM even shows -80 ubar of lift for a time early Monday morning! Wouldn't be surprised to see some thundersnow with lift of this magnitude. This period of prolific rates will be somewhat prolonged as the axis of frontogenesis orients itself from W-E early to mid morning Monday. Heaviest snowfall for most in this area will be from 06-15z. A fly in the ointment for this forecast will be pinning down areas of subsidence (sinking air) on the periphery of these bands. The HRRR has been somewhat consistent with showing a zone of subsidence over the CT river valley so we have slightly lower totals there.

Snow amounts: Guidance has continued to show over an inch of QPF for our region, with perhaps 2 inches for the Cape and Islands. Thus, very high confidence in at least a foot of snow across RI and southeast MA, with a solid potential for up to 2 feet in this corridor. Farther west into CT and western-central MA, some uncertainty with subsidence and proximity to the best low level frontogentic forcing. Nonetheless, high confidence for 12-18 inches for CT and western-central MA. Opted to go with a Winter Weather Advisory for Nantucket as the more amplified storm track likely brings a period of heavy rain behind 4-6 inches of snow. Snow loading remains a concern, especially in the corridor where we expect upwards of 2 feet of snow as this be a heavy/wet snow.

The consistency of the snow will initially be heavy/wet, especially along the coast, then gradually becoming lighter with mid level cooling as the winds turn to the northwest Monday afternoon. Across the interior, snowfall ratios should be around 10 to 13:1 at the onset and closer to a pasty 6 to 8:1 at the coast. Ratios may increase to as high as 12 to even 14:1 across the interior Monday afternoon. The lighter, fluffier snow will have a much easier time blowing around and drifting so, while rates may decrease, visibility will remain extremely low through the day.

Strong to Damaging Winds: Big issue with this system will be strong to damaging winds that will likely extend to at least the I-84 and I-395 corridors. Will see a powerful 60-80 knot low level jet in response to surface low pressure rapidly deepening near or just inside the benchmark early Monday morning. Boundary layer winds will average around 40 knots in the interior, and 60 knots closer to the coast. Model soundings also show a fairly deep boundary layer on the order of 2-3k feet. This should allow for some of those stronger winds aloft to mix down with widespread gusts of 40-50 mph spreading even well inland. Closer to the coast, gusts will approach 60 and even 70+ miles per hour on the Cape and Islands. These wind speeds combined with the heavy snowfall rates will create white out conditions at times, hence a Blizzard Warning in those areas. Occasional blizzard and whiteout conditions are possible across the elevated terrain as hills will be closer to the low level jet. Confidence in sustaining blizzard conditions for the necessary three hours in these locations is 50-60% so, just below warning criteria.

Storm will likely be damaging to electrical infrastructure with the combination of strong winds and heavy wet snow bringing a chance for power outages to much of Rhode Island and eastern MA.

Summary of impacts: Heaviest snow and strongest winds are expected from roughly late tonight/early Monday morning into Monday afternoon. Travel will be nearly impossible, with snowfall rates 2-4" per hour at times during this period. Also, heavy wet snow will combine with strong winds, yielding the potential for power outages across RI and eastern MA. Finally, high astronomical tides Monday into Tuesday brings the potential for minor to moderate coastal flooding along eastern MA coast, depending upon timing of highest surge. Surge guidance and pattern recognition suggests a 3 foot surge around high tide early Monday morning and again early Tuesday morning, which could bring the water level to over 6 feet in Nantucket Harbor and to 13.5 feet in Boston, although wave impacts along the coastline would result in greater impacts (likely Moderate) as well as coastal erosion. Keep in mind large pressure falls would also add to the water level.

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 /18Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

Today...Moderate confidence.

MVFR with ENE winds 6-11 kts. Light snow showers/flurries possible ahead of the approaching winter storm.

Tonight through Monday Morning... High Confidence

MVFR turning IFR/LIFR as light snow overspreads the region between 22-04z. 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. VFR by 00Z across the western half of southern New England, but likely taking 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...Moderate confidence in TAF (Light snow arrival)

Light snow showers possible late this afternoon-evening. Steadier snow arrives 00-03Z. Snow becomes heavy with hourly rates greater than 1" likely after 09z.

Blizzard conditions expected with gusts up to 55 kts developing by daybreak Monday along with wind shear. Snow rates decrease in the afternoon. Light snow continues Monday evening, exiting by midnight.

KBDL Terminal...Moderate confidence in TAF.

MVFR today/tonight. Light snow showers possible this afternoon/early evening. Uniform light snow arrives 00-02Z. Heavy snow with hourly rates greater than 1" likely after 07/08z. Heavy snow continues Monday morning, with rates decreasing mid-late morning. 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.

A very powerful storm develops off the Mid Atlc coast today, rapidly intensifying tonight and Monday into a dangerous storm, as it moves near the 40N/70W benchmark. This storm will be accompanied by very strong winds up to 60 kt and seas 15-25 ft across the RI and MA waters. Heavy snow will reduce vsby to near zero at times tonight and Monday. This storm exits into the Maritimes Monday night, with improving weather conditions Tuesday and beyond.

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 from 4 PM this afternoon to 7 AM EST Tuesday for CTZ002>004. MA...Blizzard Warning from 4 PM this afternoon to 7 AM EST Tuesday for MAZ004>007-009>024-026. Winter Storm Warning from 4 PM this afternoon to 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 from 4 PM this afternoon to 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.