503 FXUS61 KBOX 181044 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 644 AM EDT Thu Mar 18 2021 .SYNOPSIS... Rain develops this afternoon ahead of low pressure moving into the Ohio valley. The rain will change to a period of accumulating snow tonight from north to south, as low pressure tracks south of New England and draws colder air from the north. This storm will bring very strong winds to Cape Cod and the Islands. Blustery and cold weather follows Friday. Dry weather and a warming trend starts this weekend, with unseasonably mild temperatures beginning Sunday and continuing into next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 645 AM Update... Forecast is on track. Expecting rain to overspread SNE starting around noon in CT Valley and closer to 3-4 pm in eastern MA. Highs generally in the mid/upper 40s, but reaching lower 50s in eastern MA where later arrival of rainfall will allow for milder temps. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... * Rain changing to snow tonight with mainly minor accumulations expected * Strong winds developing over Cape/Islands late tonight Potent southern stream upper low opens up as it approaches mid Atlc coast tonight while northern stream trough delivers colder air from the north. Attendant sfc low pres moves off the mid Atlc coast tonight and tracks near or a bit south of the benchmark late tonight. A period of heavy rain will be ongoing this evening, but as colder air moves in from the north and the column cools from top down, rain will flip to snow first over higher terrain this evening, then across rest of SNE from north to south, mainly after midnight. The challenge for snow accums is timing of changeover and how quickly colder air moves in, and duration of heavier QPF after the colder air moves in. Models suggest a decent band developing in response to increasing frontogenesis in the 850-700 mb layer. However, this system is progressive and the window for accums appears to be limited to about a 3-4 hr period with snow tapering off 06-09z in the west and 09-12z in the east. Given that the initial QPF after the changeover will be wasted due to melting and SLR will be less than 10:1, this is looking more like a light accumulation event. We are expecting an area of 1-3 inches snowfall but where the axis of highest snowfall sets up is uncertain. Leaning toward Mass Pike region for the highest amounts around 3 inches but this could still shift further south into portions of CT, northern RI and SE MA. The least amounts should be along the south coast with generally an inch or less. In a worst case scenario, if the colder air comes in quicker and banding duration is longer could see localized snowfall 4-5 inches but this is looking like an unlikely scenario at this time. Expect most of the accum on non paved surfaces with just a light slushy accum on most roads. Temps drop into the 20s after the heavier snow exits so icy spots will develop on roads. Strong wind gusts along the SE New Eng coast will be the other concern late tonight into Fri morning as 50+ kt N/NE jet develops. With colder air moving in and soundings showing a shallow but well mixed boundary layer, expect much of this wind to mix down with potential for 45-55 mph gusts Cape/Islands and Plymouth county coast. Wind advisory will continue for these locations, but upgraded to a high wind warning for Nantucket where gusts to 60 mph possible within the core of the low level jet. The storm exits by Fri morning as it tracks quickly east from near the benchmark. Any lingering snow across SE New Eng will end by mid morning followed by partly to mostly sunny skies developing from NW to SE. The gusty winds will persist into the morning then diminish in the afternoon. Highs will be in the 30s. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Updated at 400 AM: * Mainly dry with a warming trend into next week Upper heights rise through weekend as our region gets caught in between upper trough lifting through western Great Lakes and cutoff low over western Atlantic. This will bring a stretch of dry weather through most of next week along with warmer temperatures. The one caveat is along immediate coast where sea breezes will keep temperatures much cooler this weekend, as a result of weak pressure gradient as high pressure settles over New England. Models also hint at possibility of backdoor front dropping south along coast Sunday, which may also limit extent of warming across eastern MA. Closed low over western Atlantic is expected to track offshore next week as upper flow remains progressive downstream and doesn't seem to favor system staying closer to East Coast. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12z TAF Update: High confidence. MVFR ceilings covering most of SNE hold through morning, but may erode on edges giving brief return to VFR ceilings before rain arrives this afternoon with widespread MVFR/IFR. Widespread IFR/LIFR conditions tonight. Rain, heavy at times, changing to snow over interior high terrain 01-03z, and across rest of the region 04-08z. Brief heavier snow possible with accums generally 1-3", mainly non-paved surfaces. Slushy accums on runways with icy spots developing as temps drop below freezing. Increasing N winds with gusts 35-45 kt developing Cape/Islands after 06z, up to 50 kt ACK, and 25-35 kt in the coastal plain with diminishing wind through the afternoon. Snow ending from NW to SE with improving cigs 06-12z. VFR Friday. KBOS TAF...High confidence. Rain changing to snow by 06z. Minor slushy accumulation possible on runways. KBDL TAF...High confidence. Rain changing to snow by 06z. Minor slushy accumulation possible on runways. Outlook /Friday Night through Monday/... Friday Night: VFR. Windy with local gusts up to 30 kt. Saturday through Sunday Night: VFR. Monday: VFR. Slight chance RA. && .MARINE... We upgraded to storm warnings for the southern outer waters and Nantucket sound for late tonight into Fri morning. Expect a period of 50 kt N/NE wind gusts with diminishing wind during Fri afternoon. Gales elsewhere with max gusts 40-45 kt. Vsbys reduced in rain and fog this afternoon and tonight, with rain changing to snow eastern MA waters and nearshore south coastal waters late tonight, improving Friday. Outlook /Friday Night through Monday/... Friday: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Slight chance of snow. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 13 ft. Saturday: Winds less than 25 kt. Seas up to 5 ft. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Sunday through Sunday Night: Winds less than 25 kt. Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain. && .TIDES/COASTAL FLOODING... Despite NE gales expected late Thu night into Fri morning, low astro tides will preclude any coastal flooding. For example, a storm surge of 1.0-1.5 ft is expected. This will keep total water levels at or below 11 ft at Boston, about 1.5 ft below flood stage. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory from 2 AM to 11 AM EDT Friday for MAZ019-022-023. High Wind Warning from 2 AM to 11 AM EDT Friday for MAZ024. RI...Wind Advisory from 2 AM to 11 AM EDT Friday for RIZ008. MARINE...Storm Warning from 2 AM to 11 AM EDT Friday for ANZ232-254>256. Gale Warning from 2 AM to 6 PM EDT Friday for ANZ231-233>235- 237. Gale Warning from 2 AM to 2 PM EDT Friday for ANZ230-236. Gale Warning from 11 PM this evening to 6 PM EDT Friday for ANZ250-251. && $$ SYNOPSIS...KJC/JWD NEAR TERM...KJC/JWD SHORT TERM...KJC LONG TERM...JWD AVIATION...KJC/JWD MARINE...KJC/JWD TIDES/COASTAL FLOODING...KJC