Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232205 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 505 PM EST Mon Jan 23 2017 .SYNOPSIS... A significant coastal storm will impact the region tonight into Tuesday with a wide variety of weather from snow and/or ice across the interior, a period of strong to damaging wind gusts along the coast including a period of very heavy rain across RI and eastern MA. High pressure builds in behind the departing storm and brings dry mild weather Wednesday and Thursday. A cold front crosses New England Thursday followed by seasonably cool temperatures and blustery winds. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 5 pm update... *** Significant Multi-Facet Winter Storm will bring a wide variety of hazards to the region tonight into Tuesday with greatest uncertainty revolving around ptype across the interior *** Many hazards to consider as moisture laden southern stream system tracks up the Eastern Seaboard. Here are the storm details/attributes for tonight into Tuesday. 1) Timing... Nose of increasing low level onshore jet continues to generate numerous showers of rain and snow across much of the region at 4 pm. Where snow showers are occurring surface temps are above freezing so roadways remain wet. Thus not much impact here other than decorating the landscape with light snowfall amounts. Deep layer moisture and synoptic scale lift arrives onto the south coast around 00z and then spreads northward later this evening. All ready seeing the firehose of Atlantic moisture on the OKX radar coming into NJ/NYC area. This will advect northward into our region this evening. However with steady/heavier precip not arriving until roughly 7 pm along the south coast...about 10 pm up to the MA pike and then to the MA/NH border around midnight. Thus looks like the bulk of the heavy rain will miss the evening commute. The steady/heaviest precip should impact MA/RI and CT from 10 pm to 4 am perhaps lingering until 7 am across northeast MA. 2) Ptypes and Snow/Ice Amounts... This is the most challenging portion of the forecast. For northern and western MA subfreezing temps from 850 mb and to the surface would support lots of snow. However very strong warming aloft occurs in the 850-700 mb layer. This will result in ptype issues. However model guidance differs on the degree of warming in this layer. The 12z NAM/Rgem and 00z NCAR ensembles are the warmest aloft while the 12z EC/GFS are not as warm showing soundings nearly isothermal. This combined with very strong forcing for ascent may cool this portion of the column via diabatic and dynamical cooling processes. This is a very delicate balance as a temp difference of 1C in this layer could be the difference from several hours of heavy wet snow or lots of sleet and freezing rain. In addition snow growth looks favorable 03z to 09z with strong forcing and deep layer moisture in the snow growth region during this time. Thus increasing the risk for a period of heavy snow. Given the model spread/uncertainty we followed a model blend. This yielded 3-6" of snowfall possible across east slopes of the Berkshires, 1-4" along the MA/NH border with an inch or two of snow and sleet across the Worcester Hills. Nevertheless big bust potential here. Therefore do not focus solely on our most likely snowfall forecast as the snow probabilities better communicate the full range of possibilities. Further south across CT a mix of snow/sleet and freezing rain is likely then all rain Tue. For RI and eastern MA, rain/snow showers this evening transitioning to a steady rain later this evening with some sleet mixed in at times...then all rain overnight into Tue. 3) Heavy Rain Across Eastern MA/RI... Global guidance including the 12z EC/GFS have widespread 1-2" rainfall amounts with 2+ across eastern MA where coastal front/inverted trough sets up and enhances low level convergence. Not surprising that the Hi Res guidance such as the ARW/NMM have isolated 3-4" amounts. 00z NCAR ensembles support this as well with probs of 3+ greater than 70% across RI into eastern MA with highest values of 90% over Cape Ann! This seems reasonable given the subtropical connection down into the Bahamas with this southern stream system along with the impressive wind anomalies within this warm conveyor belt of precip. Thus very heavy rainfall expected tonight into early Tue morning, likely impacting the early morning Tue commute especially in northeast MA where heavy rain will linger until 7 am or so. Thus flood watch continues. 4) Strong To Potentially Damaging Wind Gusts... Not much change from previous forecasts. Models locked in on a 70+ kt low level easterly jet moving across coastal RI and MA between 1 am and 7 am Tue. Despite warming aloft strong pressure falls and some enhancement from very heavy rainfall dragging strong winds aloft to the surface will yield wind gusts up to 60 mph or so along the coast. Sustained winds will be very strong as well with speeds up to 45 mph or so. These winds may result in a few downed trees/large limbs and isolated power outages. Less wind away from the coast but still close to wind advisory criteria (sustained 31 mph and higher and/or gusts 46 mph and higher). Thus will keep wind headlines as is. Strongest winds over Cape Cod and islands around 1 am to 5 am. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday... Precip mainly rain except possibly a wintry mix leftover cross northwest MA. Heavy rain will begin exiting northeast MA as low level jet exits. However as dry slot approaches from the south and comma-head begins to crank-up there will be embedded periods of moderate to perhaps briefly heavy rain especially as some dry air aloft on the nose of the dry slot begins to steepen mid level lapse rates. This could be sufficient for isolated thunder. Mild over southeast MA with temps rising to 45-50 but north-northeast winds will keep the interior in the 30s with 40-45 for the Boston to Providence corridor. Remaining windy across northeast MA otherwise winds diminish elsewhere. Tuesday night... Models have trended slower with mid level closed low still not thru the region Tue night. This will result in comma-head rains lingering with possibly some sleet northwest MA as cooler/drier air begins to advect in. Could have some standing water/slush refreeze across the interior with temps falling into the upper 20s and lower 30s. However enough mixing via WNW winds to keep temps above freezing across much of RI and eastern MA. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Overview... Looks like a relatively quiet period from Wednesday through next Monday. The main feature at the surface will be a cold front that sweeps through the area late Thursday/Thursday night. Aloft, there are weak short wave troughs moving through Thursday, Saturday, and late Monday. Temperatures will be quite mild ahead of the cold front Wednesday and Thursday...returning to near normal wintertime readings this weekend into early next week. Details... Wednesday... High pressure off the southeast U.S. coast will ridge northward into southern New England. Clouds will exit the coast early in the morning, followed by abundant sunshine throughout the area. Highs in the mid to upper 40s except lower 40s northwest MA. Wednesday night and Thursday... As a cold front approaches the region, winds turn to the southwest and clouds increase with some warm advection. There is a chance of a few showers late Wednesday night. With some sunshine in eastern MA and RI Thursday, highs should climb to 50 or even the lower 50s. Highs will be in the mid 40s in western MA, where a cold front will move through in the early afternoon. The cold front will move through the rest of the region in the mid to late afternoon and there should be some good mixing as winds shift to the west- northwest. Am forecasting gusts to 25-35 mph for a time late Thu. Cannot rule out a snow shower or flurry in the slopes of the Berkshires late in the afternoon. Thursday night through Monday... Much ado about partly to mostly cloudy skies during the daytime due to broad cyclonic flow with cool air aloft...and clear to partly cloudy skies at night. Can`t rule out a stray flurry especially in northwest MA each day, but probabilities are too low to include in the forecast. Highs Friday mid 30s to lower 40s...and highs in the 30s each day from Saturday through Monday. Lows in the 20s Friday night...then teens and lower 20s Saturday night through Monday night. Previous GEFS ensembles showed development of a coastal storm late Monday, passing to our southeast. New runs are not very bullish on this. For now, have chance PoPs over the coastal waters late Monday to the southeast of our area. && .AVIATION /22Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 22z Update... Tonight...high confidence except low confidence on ptypes Mainly MVFR to start but lowering to IFR across eastern MA later tonight. Rain/snow showers transitions to a steadier/heavier precip 00z -03z except toward 05z for the MA/NH border. Precip mainly rain with possibly some sleet at times but a wintry mix across northern and western MA. Snow and sleet accumulations of 3-6" possible here with an inch or two Worcester Hills. Some freezing rain possible too across the high terrain. Very strong winds overnight from 03z-09z south coast to 06z-12z northeast MA including Boston/Logan. Tuesday...high confidence. MVFR-IFR in periods of light rain and drizzle but improving trend to VFR toward Wed morning. KBOS TAF...Moderate to high confidence in TAF. Strongest winds 08z- 12z Tue. KBDL TAF...Moderate to high confidence in TAF. Outlook /Wednesday through Saturday/...Moderate confidence. Wednesday...VFR. Wednesday night and Thursday... VFR but areas of MVFR cigs/vsbys possible in scattered showers especially southern sections Wed. night and a chance of a snow shower in far northwest MA late Thu. afternoon. Winds behind a cold front turn to the WNW and gust to 25-30 kts Thu. afternoon and evening. Friday and Saturday...VFR. Areas of ceilings 3000-4000 ft. Local MVFR possible with a passing flurry northwest MA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. *** Storm Force Wind Gusts and 20 to 30 foot seas across our outer- eastern MA waters tonight into early Tuesday will result in a dangerous storm for mariners *** 5 PM Update... Tonight... Not much change with strong low pres moving up the Eastern Seaboard. Strongest onshore winds 10 pm to 4 am southern waters of MA/RI... then midnight to 8 am for eastern MA waters. Heavy rain and fog will reduce vsby. By daybreak Tue seas of 20-30 ft likely across eastern MA waters! Tuesday... Low pres moves from DE waters to near Nantucket by dark. Strongest winds exit into ME waters however very rough/dangerous seas will linger. Vsby reduced in rain and fog. Tuesday night... Low pres near Nantucket Tue evening lifts NE to Nova Scotia by Wed morning. Winds become WNW as the night progresses. This results in vsbys improving. Outlook /Wednesday through Saturday/...Moderate confidence. Wednesday through Friday... NW winds gusting to 25 kts early...diminish somewhat during the day then turn to the SW. Rough seas slowly subside to below small craft advisory levels late Wednesday night. A cold front sweeps across the waters late Thursday, followed by colder air and gusty west winds up to 30 knots for a time early Thursday evening. Colder air and gusty winds will continue into Friday and Saturday. Seas will diminish through the period, to lower than WNAWAVE guidance, but still have 5 to 6 foot seas lingering on the outer waters. Small Craft Advisories will likely be needed for some or all of this period. && .TIDES/COASTAL FLOODING... 5 pm update... Timing of strongest winds looks to be 10 pm to 4 am south coast of MA and RI...then midnight to 8 am for the east coast of MA. Psurge performing best when compared to current observations with ETSS and ESTOFS running too low. Thus have followed the more robust Psurge given strength of the low level jet, wind fetch and strong pressure falls from approaching storm. Low to moderate risk for minor coastal flooding this evening for both south and east coasts. Thus have expanded advisory to south coats of RI and MA. More significant high tide is Tue morning. Psurge continues to advertise up to a 3.5 surge. SWANgfs and SWANnam generating seas up to 25 ft across eastern MA waters. Given strength of low level jet and wind fetch would not be surprised to see seas approach 30 ft! Thus the combination of surge and strong wave action ontop of these elevated water levels support widespread minor coastal flooding with pockets of moderate. Thus will issue a Coastal Flood Warning for the entire east coast of MA for Tue morning tide from the NH/MA border to Nantucket. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Advisory until 7 AM EST Tuesday for CTZ002>004. Winter Weather Advisory until 7 AM EST Tuesday for CTZ002-003. MA...High Wind Warning until 7 AM EST Tuesday for MAZ007-019- 022>024. Coastal Flood Warning from 5 AM to 11 AM EST Tuesday for MAZ007-015-016-019. Flood Watch through Tuesday morning for MAZ005>007-013>021. Coastal Flood Advisory until 10 PM EST this evening for MAZ007- 015-016-019. Wind Advisory until 7 AM EST Tuesday for MAZ004>006-009- 011>018-020-021-026. Winter Weather Advisory until 7 AM EST Tuesday for MAZ002>006- 008>012-026. Coastal Flood Warning from 6 AM to noon EST Tuesday for MAZ022- 024. Coastal Flood Advisory until 11 PM EST this evening for MAZ022>024. Coastal Flood Advisory from 6 AM to noon EST Tuesday for MAZ023. RI...High Wind Warning until 7 AM EST Tuesday for RIZ008. Flood Watch through Tuesday morning for RIZ001>007. Wind Advisory until 7 AM EST Tuesday for RIZ001>007. Coastal Flood Advisory from 3 AM to 7 AM EST Tuesday for RIZ006-007. MARINE...Storm Warning until 4 AM EST Tuesday for ANZ231-232-255-256. Storm Warning until 3 AM EST Tuesday for ANZ233-234. Gale Warning until 7 AM EST Tuesday for ANZ230. Gale Warning until 4 AM EST Tuesday for ANZ236. Storm Warning until 2 AM EST Tuesday for ANZ235-237. Storm Warning until 7 AM EST Tuesday for ANZ250-254. Storm Warning until 6 AM EST Tuesday for ANZ251. && $$ SYNOPSIS...Nocera/GAF NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...GAF AVIATION...Nocera/GAF MARINE...Nocera/GAF TIDES/COASTAL FLOODING...Nocera

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