352 FXUS61 KBOX 060921 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 421 AM EST Fri Mar 6 2020 .SYNOPSIS... High pressure over eastern New England early this morning will drift offshore today. A powerful ocean storm develops off the Mid Atlantic coast this evening, then tracking south of New England tonight into early Saturday. This storm will bring very strong winds to Cape Cod and the Islands with accumulating snow for the south coast of MA and RI, heaviest Cape Cod, Marthas Vineyard and Nantucket before ending early Saturday. Fair and cool weather Saturday night, then fair and much milder weather follows for Sunday and Monday. Another weather system may bring scattered showers and isolated thunderstorms on Tuesday. Cooler air and scattered showers Wednesday and Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 415 AM Update... Today ... Chilly start to the day especially outside the urban areas where light winds, mostly clear conditions (other than thin high clouds) and a dry airmass resulting in many locations in the 20s. This radiational cooling has also lead to patchy ground/shallow fog. This fog will begin to erode shortly after sunrise as onshore winds begin to develop and provide enough mixing for this shallow moist layer to mix out. Otherwise sunshine thru high clouds eventually fades this afternoon as clouds continue to lower and thicken. Cool onshore winds increase this afternoon as low pressure strengthens off the Mid Atlantic coast. Highs will reach into the 40s this afternoon but will feel cooler given increasing onshore winds. Other than low risk of some spotty light rain/sprinkles later this afternoon, dry weather should prevail. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... 415 AM Update... *** Strong to Damaging Winds Likely Cape Cod & Islands *** *** Accumulating Snow Likely Cape Cod & Islands *** *** Minor Coastal Flooding Likely Cape Cod & Islands *** Winds ... high forecast confidence Amplifying mid level northern stream trough phases with southern stream energy off the Mid Atlantic coast tonight yielding bombogenesis with low pressure deepening about 24 mbs in 12 hrs! The low bottoms out around 963 to 967 mb (depending on model of choice) at 09z Sat south of the 40N/70W benchmark. This results in very strong to damaging northeast winds of 30 to 40 mph with gusts up to 65 mph likely across Cape Cod and Islands, strongest over Nantucket where there is a low prob for an hour or two of hurricane force gusts 64 kt/74 mph. As temps cool aloft low level lapse rates steepen and model soundings at ACK and CHH indicate high probability of 60+ mph gusts. Therefore will upgrade the High Wind Watch for Cape Cod, Nantucket and Marthas Vineyard to a High Wind Warning. Wind speeds of this magnitude expecting at least some tree damage along with isolated to scattered power outages with greatest impact across Nantucket. Wind advisories continue for coastal Plymouth to coastal Bristol to Block Island. Model soundings also support gusts greater than 45 mph for coastal Essex county/Cape Ann. Thus will include this area in the Wind Advisory. Accumulating Snow ... low to moderate forecast confidence. 00z Hi Res and Global guidance have all trended stronger with offshore low with models indicating a 963-967 mb low tracking south of the 40N/70W benchmark early Sat morning. Despite the large distance from shore this intense cyclone has a very large wind field with its warm/moist conveyor belt extending well to the northwest and now wrapping cyclonically across southeast MA tonight into early Saturday morning. Thus increasing the risk for accumulating snow. Also wind field in the 700-500 layer supporting potential for mesoscale snow bands via deformation. As a result models have trended up with qpf and much of the guidance offering 0.25-0.50 across Cape Cod and the Islands with potentially greater than 0.50 inches for Nantucket. Snow to liquid ratios could be greater than 10 to 1 as some of the guidance shows max omega occurring the snow growth region. However low level temperatures are marginal for accumulating snow so unless heavier snow bands track northward over Cape Cod and the Islands, snow may be difficult to accumulate on paved surfaces. However if heavier qpf does materialize boundary layer will cool and accumulating snow will commence. HREF hourly snowfall rates support this idea with probs up to 50% for 1 inch hourly snowfall rates. Given recent northwest trends along with anomalous sub-970 mb low and best forcing for ascent potentially in snow growth region along with banding signature, leaned toward this heavier qpf scenario (although not to the extent of the Canadian Rgem model). This translates to a coating to 2 inches of snow across southern RI into south coastal MA including Plymouth county with 2-4 inches of snow for Cape Cod and Marthas Vineyard with 3-5 inches for Nantucket. Therefore have issued a Winter Weather Advisory for this region. However a shift of 50 miles in storm track will alter these snowfall totals so this forecast is subject to change. Regardless of exact snow totals there will be a 6-9 hour of winter weather with strong NE winds yielding blowing snow and poor vsby. Could also be some ocean effect enhancement early Sat morning as 925 mb temps lower to about -10C, steepening low level lapse rates and increasing moisture and heat fluxes for ocean effect snow showers. Saturday ... Morning snow over Cape Cod and Islands should exit by midday but strong north winds will continue. Elsewhere dry weather but windy and cool conditions. Highs in the upper 30s to lower 40s, a few degs cooler than normal but will feel colder given gusty north winds. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Dry and quiet weather expected Saturday night through much of Monday. Temperatures trending upward during this timeframe. * Next system moves in sometime between Monday evening and Wednesday. Drier weather potentially returns on Thursday. Temperatures trend downward Tuesday through Thursday. Details... Saturday night through Monday... A negatively tilted trough will be located offshore Saturday evening, while a ridge axis is just east of the Mississippi River Valley. The trough will continue lifting northeastward offshore. The ridge will build eastward overnight. The surface high will build into the Carolinas and nudge its influence into the Northeast. Winds will gradually back from the north the to the west. Flow aloft at 850 hPa switches from northerly cold air advection to weak warm air advection by Sunday morning. Skies will be clear with gusty winds continuing to diminish as the high relaxes the pressure gradient. Expect this to be the coldest night of the long term with lows generally in the 20s. The ridge axis will continue to build eastward Sunday through Monday. Expect southwesterly warm air advection through this time period with 0 to +10 degree Celsius 850 hPa air moving in. Expect the highest temperatures on Monday as the warmest air moves in. Highs on Sunday will generally be in the 50s. On Monday readings range from the mid 50s to the low 60s. It still may be a bit breezy on Sunday and Monday as lapse rates will be nearly adiabatic during the day. So, it shouldn't be hard to tap into the 15-20 kts of wind aloft once the mixed layer grows. Monday night through Thursday... Confidence in the forecast lowers during this timeframe as models spread out in the track, intensity and speed of the next system moving in. A shortwave trough will lift from the Mid Mississippi River Valley into the eastern Great Lakes. A surface low develops and lifts into the region along with this wave. As of right now it looks like this mainly brings precipitation in on Tuesday, but doesn't appear to be a washout at this point in time. Used the NBM guidance due to uncertainty. Another trough digs in from the the Upper Midwest on Wednesday and could bring more precipitation. Still enough spread amongst guidance in mass fields that it is hard to pin point how things will progress. A broad low may develop nearby or over the region bringing precipitation chances. Confidence still low on Thursday as a ridge axis may build in per the GFS/ECMWF, but the GEM guidance keeps the region under cyclonic flow. There are some significant differences in comparison to the previous runs of these models, so have kept the NBM guidance this far out. && .AVIATION /09Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 06z update ... Today through 21z ... High confidence VFR and dry weather. Light ESE winds this morning become ENE this afternoon and then increase to 15-20 kt by 21z except gusting up to 25-30 kt across Cape Cod and the Islands. After 21z and overnight ... Moderate confidence with uncertainty centered around how far northwest accumulating snow tracks. Very strong winds develop over Cape Cod and Islands with northeast winds peaking 06z-09z gusting up to 50-55 kt. Gusts drop off to 35-40 kt at KBOS and KPVD, capped at 20 kt inland. MVFR develops with light snow overspreading the south coast of RI/MA with steady light snow possibly northward to PVD-PYM. Heavier snow Cape Cod and Nantucket with 2-4" snow accumulation possible. Low risk of 6+ at Nantucket. Inland only spotty light snow/flurries with marginal MVFR/VFR. Saturday .... high confidence. Very strong north winds continue over Cape Cod and Nantucket with gusts up to 50 kt during the morning, then becoming NNW in the afernoon and slowly decreasing but still strong with gusts up to 35-40 kt. MVFR with accumulating snow over Cape Cod and Nantucket ending around midday and improving to VFR. Elsewhere dry weather and VFR. KBOS Terminal...High confidence in the TAF. VFR today then MVFR this evening and overnight with accumulating snow likely staying south of Logan but it's a close call and something to watch today. NE winds this evening gusting up to 30 kt, up to 40 kt overnight. Saturday morning north winds gusting up to 30 kt. KBDL Terminal...High confidence in the TAF. Outlook /Saturday Night through Tuesday/... Saturday Night: VFR. Windy with areas of gusts up to 35 kt. Sunday through Monday: VFR. Breezy. Monday Night: Mainly VFR, with local MVFR possible. Slight chance RA. Tuesday: Mainly VFR, with areas MVFR possible. Chance RA. && .MARINE... 415 AM Update ... Today ... high pressure slowly sliding offshore with SE winds this morning becoming NE this afternoon and increasing to 30 kt late in the day. Dry weather and good vsby prevail. Tonight ... powerful low pressure tracking southeast of the Benchmark 40N/70W with NE winds gusting up to 45-55 kt with low risk of hurricane force gusts southeast of Nantucket. Seas will build to 20-25 ft east of Cape Cod and Nantucket. Vsby will lower to less than a mile in periods of heavy snow across southern waters. Saturday ... powerful low exits across Georges Bank with northerly gales all waters. Snow exits offshore by midday. Very large seas of 20-25 ft early east of Cape Cod and Nantucket. Outlook /Saturday Night through Tuesday/... Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 14 ft. Sunday: Winds less than 25 kt. Areas of rough seas. Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Monday through Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain. && .TIDES/COASTAL FLOODING... 415 AM update ... Powerful low passes south of New England late tonight/early Sat with surface pressure 963-967 mb! Fortunately tides are not astronomically high and also strongest wind and storm occurs around low tide (3 am - 4 am). This combined with pressure rises occurring toward high tide Sat will limit coastal flooding to minor category. Storm surge values at high tide will range between 1-2 ft, perhaps slightly higher at Nantucket and Cape Cod. Given minor inundation expected will post a Coastal Flood Advisory from Plymouth county southward thru Cape Cod, Nantucket and Marthas Vineyard. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory from 9 PM this evening to noon EST Saturday for MAZ007-019-021. Coastal Flood Advisory from 6 AM to 11 AM EST Saturday for MAZ019-022>024. High Wind Warning from 9 PM this evening to noon EST Saturday for MAZ022>024. Winter Weather Advisory from 8 PM this evening to 11 AM EST Saturday for MAZ022>024. RI...Wind Advisory from 9 PM this evening to noon EST Saturday for RIZ008. MARINE...Storm Warning from 9 PM this evening to noon EST Saturday for ANZ231>235-250-254>256. Gale Warning from 9 PM this evening to noon EST Saturday for ANZ230. Gale Warning from 11 PM this evening to 10 AM EST Saturday for ANZ236. Gale Warning from 4 PM this afternoon to 1 PM EST Saturday for ANZ237. Gale Warning from 4 PM this afternoon to 4 PM EST Saturday for ANZ251. && $$ SYNOPSIS...Nocera/BL NEAR TERM...WTB/Nocera SHORT TERM...Nocera LONG TERM...BL AVIATION...Nocera/BL MARINE...Nocera/BL TIDES/COASTAL FLOODING...Nocera