textproduct: Boston / Norton

WHAT HAS CHANGED

Coastal storm for Sunday continues to trend further offshore. Cold Weather Advisory has been issued for western MA and northern CT for wind chills tonight into Sunday morning. Coastal Flood Warning issued for Nantucket Sunday morning into Monday afternoon with Coastal Flood Advisories for the rest of the eastern MA coastline. Wind Advisories issued for the Cape and Islands Sunday and Sunday night.

KEY MESSAGES

- Wind chills are expected to fall below zero again tonight, with the interior near -15F.

- A coastal storm passing offshore of southern New England brings minor accumulating snow, high winds, hazardous marine conditions and minor to pockets of moderate coastal flooding to eastern and southeast New England.

- Mainly minor coastal flooding/beach erosion Sunday morning into Monday afternoon along the eastern MA coast except pockets of moderate coastal flooding for Nantucket. Some of the storm surge may freeze on roadways delaying drainage. Strong Gales to storm warning/s across the waters along with freezing spray advisories.

- Dry weather and not as cold into the middle of next week, turning colder and more active late next week/early weekend.

DISCUSSION

Key Message 1...Wind chills are expected to fall below zero again tonight, with the interior near -15F.

A new Cold Weather Advisory has been issued for western MA and northern CT for tonight into Sunday morning. While clouds are expected to develop tonight as the coastal low approaches, thinking the thickest cloud deck will not quite reach far enough into the interior to minimize cooling. Forecast wind chills tonight in interior Southern New England hover around 15 below zero for several hours... still on the marginal side, but enough to trigger an Advisory for the night.

Key Message 2...A coastal storm passing offshore of southern New England brings minor accumulating snow, high winds, hazardous marine conditions and minor to pockets of moderate coastal flooding to eastern and southeast New England.

The latest guidance has pushed the track of the coastal low even further southeast than yesterday, indicating more of a glancing blow to the Cape and Islands. Elected to reduce snow totals to 1-3" across the region, with the possibility that pockets of Nantucket may see 4". High-resolution guidance has also picked up on ocean effect snow bands forming from the prolonged northeasterly flow this weekend. Thinking these bands will set up over Cape Ann and into Plymouth County, possibly as early as 10PM - 12AM tonight, then begins to taper off around early Sunday evening in Plymouth and Bristol Counties, and closer to late Sunday night over the Cape and Islands. Difficult to narrow down where exactly these bands may form, but snow accumulations may reach up to 2" in coastal Plymouth County.

Expecting snow rates to be relatively manageable as hi-res guidance shows roughly a 40% probability of seeing greater than 0.1" per hour solely over the outer Cape and Nantucket and 20-30% elsewhere. However, with high wind gusts 40-50mph, up to 55mph for Nantucket, expecting large reductions in visibility while snow is falling. While we don't expect the wind to lead to any damage, a High Wind Advisory has been issued for the Cape and Islands.

Key Message 3...Mainly minor coastal flooding/beach erosion Sunday morning into Monday afternoon along the eastern MA coast except pockets of moderate coastal flooding for Nantucket. Some of the storm surge may freeze on roadways delaying drainage. Strong Gales to storm warning/s across the waters along with freezing spray advisories.

The three tides of concern specific to coastal flooding are the Sunday morning and evening high tides, as well as the Monday morning high tide. Despite the storm passing well southeast of the Benchmark...it will become very powerful with SLP dropping near or below 970 mb towards 00z Monday. Pressure falls combined with strong northerly wind gusts of 40 to 50 knots should allow for a 1.5 to 3 foot storm surge...highest being across Nantucket. Given the northerly fetch coupled with the potential for storm tides to reach or even exceed 6 feet over multiple high tide cycles...felt it was worth going with a Coastal flooding warning. The main concern are those vulnerable roads on the sound side of Nantucket. Across the rest of the eastern MA coastline...we have opted to go with coastal flood advisories for minor coastal flooding & splashover.

Another concern will be the sub-freezing temperatures and very cold road surfaces. This may allow some of the storm surge to freeze...which would prolong issues and delay drainage. Lastly...given this is over multiple high tide cycles beach erosion will also be an issue.

As a reminder:

Minor flooding refers to shallow flooding up to 1 foot deep and can result in temporary road closures on more vulnerable coastal roads.

Moderate flooding refers to flooding 1 to 3 feet deep and can cause more widespread road closures, damage to shorefront property, and debris on coastal roads from large waves. Vehicles left in flood prone areas can also be flooded.

Key Message 4...Dry weather and not as cold into the middle of next week, turning colder and more active late next week/early weekend.

Beyond the weekend the upper level pattern becomes more zonal during the week with rising heights as the airmass overhead moderates back toward normal (emphasis on "toward" since we still won't be back to normal which would be highs in the mid to upper 30s). 925 mb temps near -12C over the weekend will rise closer to -5C for the Mon-Tue period before some reinforcing shots of cold air return mid to late week. This brings highs back into the upper 20s and low 30s through mid week before dropping back into the 20s. Mostly dry this week with scant moisture to be had; could see a weak disturbance around Wednesday bringing scattered flurries but ensembles only show a 20- 30% chance of any measurable precip as the better forcing and moisture is elsewhere. The next round looks to be around Friday/Saturday when a deeper shortwave digs into the northeast with a surface low crossing from the Great Lakes and deepening over the Gulf of Maine. This is our next chance of snow, though it doesn't look to be a big/impactful storm at this point with 30-50% chance of at or above 1" of snow in the global ensembles.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18z TAF Update:

Through this afternoon: High confidence.

VFR. Light WNW winds under 10 knots today

Tonight: High confidence thru 06z, then moderate for the eastern terminals.

VFR, though layer of MVFR stratus starts to turn landward from the ocean waters ~00-06z. Periodic bands of ocean effect snow showers develop with MVFR visbys around 09-11z, with steadier light snow starting to break out for the Cape and Islands. Winds turn NE and increase in speed to 10-15 kt with gusts 20-25 kt.

Sunday/Sunday Night: High confidence western terminals, moderate confidence eastern terminals.

MVFR-IFR with steady light snow (1-3 SM visby) across the Cape terminals and ACK. A few inches of accumulating snow likely limited to those terminals. Intermittent ocean effect snow at BOS with 4SM -SN indicated 09-16z but not much if any accumulation expected, perhaps a coating to half inch; Ocean effect precip for BOS starts to taper off around 16z as winds start to turn offshore. VFR inland. NE winds 15-25 kt with gusts 25-40 kt, highest gusts at ACK; winds turn NW with a decrease in speed 21z Sun-00z Mon from west to east. This could cause areas of blowing/drifting snow off existing snow mounds.

KBOS Terminal...Moderate Confidence in TAF.

Biggest uncertainty is when/if -SN makes it far enough north to reach the terminal...thinking best chance is between 09z and 16z with periodic/intermittent ocean effect snow showers. Limited accum (less than an inch) but pavement is cold so it will stick. NE winds increase Sunday morning to around 15 kt with gusts 28-30 kt.

KBDL Terminal...High Confidence in TAF.

VFR. Light WNW winds today under 10 knots, becoming north early Sunday and increasing to around 10-13 kt with gusts 20-23 kt.

Outlook /Monday through Thursday/...

Monday through Monday Night: VFR. Breezy.

Tuesday through Thursday: VFR.

MARINE

Forecaster Confidence Levels...

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

* Strong Gale Warnings to Storm Warnings with seas building to near 15 feet across our eastern outer-waters Sun-Sun night. Minor to moderate areas of freezing spray are also expected.

Low pressure will undergo bombogenesis as it lifts northeast and passes well southeast of the Benchmark Sunday into Sunday night. Despite its offshore track...the pressure gradient will be strong enough to result in N-NE ramping up overnight. N-NE wind gusts of 40 to 50 knots Sun into Sun night with the strongest of those winds across Nantucket sound and points south and east where storm warnings are in effect. This will also allow seas to approach 15 feet across our eastern outer-waters. Lastly cold near shore SST will allow for areas of minor to moderate freezing spray.

So given the above Gale and Storm warnings remain posted along with some Freezing Spray Advisories.

Outlook /Monday through Thursday/...

Monday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 15 ft.

Monday Night: Winds less than 25 kt. Areas of rough seas.

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

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

Wednesday through Thursday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Cold Weather Advisory from 10 PM this evening to 10 AM EST Sunday for CTZ002>004. MA...Cold Weather Advisory from 10 PM this evening to 10 AM EST Sunday for MAZ002-003-008>011. Coastal Flood Advisory from 8 AM Sunday to 2 PM EST Monday for MAZ007-015-016-019-022-023. Winter Weather Advisory from 4 AM Sunday to 4 AM EST Monday for MAZ019-022>024. Wind Advisory from 5 AM to 11 PM EST Sunday for MAZ022>024. Coastal Flood Warning from 8 AM Sunday to 2 PM EST Monday for MAZ024. RI...None. MARINE...Gale Warning from 7 AM Sunday to 1 AM EST Monday for ANZ230. Gale Warning from 4 AM Sunday to 4 AM EST Monday for ANZ231. Storm Warning from 7 AM Sunday to 1 AM EST Monday for ANZ232. Freezing Spray Advisory from 10 PM this evening to 1 PM EST Sunday for ANZ233>237-256. Gale Warning from 6 AM Sunday to 1 AM EST Monday for ANZ233- 234. Gale Warning from 3 AM Sunday to 1 AM EST Monday for ANZ235- 237-256. Gale Warning from 5 AM to 11 PM EST Sunday for ANZ236. Gale Warning from 7 AM Sunday to 4 AM EST Monday for ANZ250. Freezing Spray Advisory from 4 AM to 1 PM EST Sunday for ANZ251. Gale Warning from 6 AM Sunday to 4 AM EST Monday for ANZ251. Storm Warning from 7 AM Sunday to 4 AM EST Monday for ANZ254- 255.