311 FXUS61 KBOX 122046 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 446 PM EDT Sun Mar 12 2017 .SYNOPSIS... A major winter storm will impact the region Tuesday into Tuesday night with heavy snow, strong winds, and possible coastal flooding. Blizzard conditions are possible along portions of eastern MA and Rhode Island.Blustery Thursday, then fair and cold Friday as high pressure builds in. Another round of unsettled weather is possible during the weekend, currently favoring Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Still gusty winds in place, but the wind and CAA are diminishing as high pressure builds east. Clear skies and light wind will allow radiational cooling overnight. With dew points about 5 degrees either side of zero, expect most min temps in the single numbers. A few cold spots may dip a couple of degrees below zero. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... High pressure overhead will keep fair weather through the day and evening. As the high shifts east, mid and high clouds will move overhead. Model high-level moisture fields suggest a period of increased mid-high clouds early afternoon followed by thinning. Temps at the top of the mixed layer will be equivalent to -15C thru -17C at 850 mb, so max temps should be mid 20s to low 30s. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Highlights on Major Winter Storm Tuesday: * High confidence in 12" to 18" of snow for many locations with a low risk for isolated 20" to 24" amounts * Exception is mainly across the Cape/Islands where snowfall amounts are highly uncertain given Ptype uncertainty * Blizzard conditions along with strong to damaging wind gusts possible across eastern MA/RI Details on Major Winter Storm... High confidence in a major winter storm for most of the region with the potential for blizzard conditions and strong to damaging wind gusts possible across eastern MA/RI. Confidence in a heavy snowfall is quite high over much of the county warning area, but given the event is still 48+ hours out in the model world uncertainty remains. The two main issues are where the axis of heaviest snow will setup along with Ptype across extreme southeast new England, focused across the Cape/Islands. Will break it down a bit more below. Timing... Vigorous shortwave diving down from Canada will spawn rapid cyclogenesis along the coast. This will increase the forcing for ascent and snow should rapidly overspread the region between 4 and 8 am Tuesday morning. The rapid intensification of this low will generate an 850 mb easterly low level jet this is 4 to 5 standard deviations above normal. The result will be an area of tremendous omega/frontogenesis developing Tuesday morning and into the afternoon. Both the GFS/NAM indicate a period of 30 to 50 units of omega in the snow growth region for a time. This certainly may yield 2 to 4" per hour snowfall rates for a time and would not be surprised to see a bit of thundersnow. The heaviest snow should taper off to snow showers from southwest to northeast Tuesday night as the intensifying surface low lifts to our northeast. Snowfall Amounts... The models have come into much better agreement over the last 24 to 48 hours, but small nuances are enough to make a significant difference in the overall outcome in some locations. Given the overall synoptics and expected strong omega/frontogenesis...we are quite confident in much of the region receiving 12 to 18 inches of snow. While QPF may be less on some of the guidance in our western zones...many models track the mid level low across eastern CT and up through central MA. Snowfall amounts are often underdone to the northwest of this region in the mid level deformation zone/back. bent mid level warm front. The exact track this storm and its mid level centers remains uncertain...but there certainly is a low risk for isolated 20 to 24" in amounts. Highest risk for this would probably be northwest of the Boston to Providence corridor. Portions of eastern MA/RI may have to contend with a mid level dry slot which may cut down amounts for some of these locations. However, models still show some low level frontogenesis and assistance from a coastal front that may result in another area of enhanced snowfall. Ptype Uncertainty... The most difficult portion of this forecast revolves around ptype across extreme southeast MA and especially across the the Cape/Islands. The NAM model is the most aggressive given its surface track across far southeast MA allowing for a change to rain across the south coast and into much of far southeast MA. We think this is probably too aggressive, while the GFS showing pretty much all snow for the Cape/Islands is likely too cold. Therefore, we blended the guidance and basically have to 4 to 8 inches of snow in this region. A lot of uncertainty in these amounts though and they certainly may need to be adjusted. Headlines and Strong Winds.... Potent northeast low level jet and good mixing should result in 50 to 60 mph wind gusts along the immediate coast, with 40 to 50 mph gusts further back into interior eastern MA and RI. Given the potential for those wind speeds and heavy snow...opted to expand the Blizzard Watch across most of eastern MA and RI. Blizzard conditions are possible with blowing and drifting snow along with near white-out conditions possible at times. Many roads may become impassable on Tuesday. In addition...given our very high confidence in heavy snow across all but the Cape/Islands have converted the Winter Storm Watch to a Warning. Will let the later shifts decide on whether or not confidence is high enough to convert Blizzard Watches to Blizzard Warnings. Across the Cape/Islands too much ptype uncertainty, so continued a Winter Storm Watch and High Wind Watch. Given the wet nature of the snow along the coast...there will be an increased risk for tree damage and power outages. Something will have to watch closely. Thursday and Friday...Moderate confidence. Lingering northwest flow Thursday as high pressure approaches, but less pressure gradient than Wednesday so expect lighter winds. High pressure overhead Friday. Expect dry weather both days. Saturday and Sunday...Low confidence. Shortwave races east from the Canadian Prairies and digs southeast. Associated surface low redevelops along the Mid Atlantic coast. Timing on this feature has been shifting. Previously it looked like a Saturday storm. Now it looks like a Sunday storm. We are indicating chance pops for Sunday, but with low confidence due to timing. && .AVIATION /21Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight and Monday...High confidence. VFR through the period. NW winds gusting to 25 knots in the evening will diminish tonight. KBOS TAF...High confidence. KBDL TAF...High confidence. Outlook /Monday Night through Friday/... Monday Night...High confidence. VFR. Tuesday and Tuesday Night...Moderate Confidence. Conditions rapidly lower to IFR/LIFR in heavy snow, which may change to rain/fog on Cape Cod and Islands for a time during the afternoon. NE winds may gust to 50-60kt along RI and SE MA coast. Wednesday...Moderate Confidence. Conditions improving to VFR in general, but with areas of MVFR in scattered snow showers. Northwest winds will gust to 30 knots. Thursday-Friday...High confidence. VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...High confidence. West northwest winds gusting 25 to 30 kt will diminish early tonight. Areas of light freezing spray will slowly diminish overnight. Lingering 5 to 6 foot seas on the outer waters, but these will subside toward morning. Small Craft Advisory remains in effect through evening, then ends close to shore while an advisory remains in effect overnight over the outer waters. Monday...High confidence. High pressure moves over the waters. Winds will become light, then shift from the south and southwest late afternoon. Some 5 foot seas east of Cape Cod early, otherwise seas below 5 feet through the day. Outlook /Monday Night through Friday/... High confidence. * Dangerous Conditions for Mariners Tuesday and Tuesday Night * Monday Night...Light winds and seas likely below small craft advisory thresholds are expected. Tuesday...Becoming more confident on NE winds reaching Storm Force on all waters Tuesday, possibly as high as 60kt, with strong Gales (40 kt) on Boston Harbor and Narragansett Bay. There is even a low risk of wind gusts to hurricane force over some of the outer southeast coastal waters depending upon the storm's exact track and intensity. Seas should build as high as 20 to perhaps 25 feet late Tuesday into Tuesday night, especially east of Cape Cod. Snow quickly develops Tuesday morning and changes to rain by afternoon on most of waters (especially near Cape Cod), before changing back to snow Tuesday evening. Wednesday...Moderate confidence. Northwest winds 25 to 30 knots. Lingering 8 to 13 foot seas on the outer waters, and 5 to 9 feet on the exposed waters closer to shore such as Mass Bay and RI Sound. Seas further diminishing Wednesday night. Small Craft Advisory will be needed. Low probability for Gales. Thursday and Friday...Moderate confidence. Diminishing northwest wind as high pressure approaches from the west. Lingering 5 foot seas on the outer waters Thursday, then seas below 5 feet Friday. && .TIDES/COASTAL FLOODING... The Tuesday/Tuesday night storm has the potential to wreak havoc on the coastline given a potential for 2 to 3 foot storm surges and waves in excess of 20 feet just offshore. The big question will be timing. Most guidance would point to the surge, waves, and onshore wind peaking out after the Tuesday early afternoon high tide. This could mitigate the impacts some. Even so, there appears to be a risk of at least minor to moderate coastal flooding and significant erosion about the time of the Tuesday early afternoon high tide along the Massachusetts east facing coastlines and possibly again during the Wednesday early AM high tide for north and northeast facing shorelines. A Coastal Flood Watch may eventually be warranted. && .CLIMATE... Record Low Today Boston 7/1900 Providence 10/1984 Hartford 6/1984 Worcester 2/1948 Record Low Max Today Boston 25/1984 Providence 25/1984 Hartford 24/1984 Worcester 22/1998 Record Low for Monday Boston 3/1885 Providence 12/1948 Hartford 5/1941 Worcester 4/1941 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Warning from 5 AM Tuesday to 2 AM EDT Wednesday for CTZ002>004. MA...High Wind Watch from Tuesday morning through Tuesday afternoon for MAZ022>024. Winter Storm Watch from late Monday night through late Tuesday night for MAZ022>024. Winter Storm Warning from 5 AM to 1 PM EDT Tuesday for MAZ005>007-013>021. Coastal Flood Watch from Tuesday morning through Tuesday afternoon for MAZ007-014>016-019-022-024. Blizzard Watch from Tuesday afternoon through late Tuesday night for MAZ005>007-013>021. Winter Storm Warning from 5 AM Tuesday to 2 AM EDT Wednesday for MAZ002>004-008>012-026. RI...High Wind Watch from Tuesday morning through Tuesday afternoon for RIZ008. Winter Storm Watch from late Monday night through late Tuesday night for RIZ008. Winter Storm Warning from 5 AM to 1 PM EDT Tuesday for RIZ002>007. Blizzard Watch from Tuesday afternoon through late Tuesday night for RIZ002>007. Winter Storm Warning from 5 AM Tuesday to 2 AM EDT Wednesday for RIZ001. MARINE...Storm Watch from Tuesday morning through Tuesday afternoon for ANZ232>235-237-254>256. Small Craft Advisory until 4 AM EDT Monday for ANZ232>235-237. Gale Watch from Tuesday morning through Tuesday evening for ANZ230. Small Craft Advisory until 8 PM EDT this evening for ANZ230- 236. Storm Watch from Tuesday morning through Tuesday afternoon for ANZ231-250-251. Small Craft Advisory until 10 AM EDT Monday for ANZ231-250-251- 254>256. Gale Watch from Tuesday morning through Tuesday afternoon for ANZ236. && $$ SYNOPSIS...WTB/Frank/JWD NEAR TERM...WTB SHORT TERM...WTB LONG TERM...WTB/Frank AVIATION...WTB/JWD MARINE...WTB/JWD TIDES/COASTAL FLOODING... CLIMATE...