Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 220056 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 756 PM EST Sat Jan 21 2017 .SYNOPSIS... Weak high pressure will bring tranquil weather through tonight. Developing onshore winds Sunday will result in an increase in clouds as well as spotty light rain or drizzle as the day progresses. A significant storm will reach the east coast Sunday night and Monday, then slowly push south of New England Monday night and Tuesday. This will bring a mix of rain and/or snow along with a period of icing well inland. The storm may also bring damaging winds to the coast and the coastal waters. High pressure then brings dry and mild weather Wednesday into Thursday. A cold front crosses New England late week followed by seasonably cool temperatures and blustery winds. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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755 PM UPDATE... Quick update to issue a Dense Fog Advisory along south coastal MA/RI including Cape Cod and the Islands. Visibilities have dropped to 1/4 mile or less on Nantucket and parts of Cape Cod, and expect it to become more widespread this evening. Also had recent report of near zero visibility from ferry on Nantucket Sound. Webcams also show dense fog expanding onto south coast over past hour, so also included South Coast in advisory. No other changes. Mostly clear to partly cloudy skies will prevail this evening and along with light winds temps will fall fairly quickly with sunset. However with dew pts in the low 40s (well above normal for late Jan) this will support at least the formation of patchy fog. This becomes tricky later tonight as mid/high clouds overspread the area. This may result in vsbys improving toward morning. Nonetheless temperatures above normal tonight with just about all locations above freezing. Low risk of some patchy freezing fog across the higher terrain but this is a very low prob.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... 4 PM update... Sunday... High pressure over Quebec will combine with weakening low pres east of DE to yield an increasing moist onshore flow into southern New England. Thus any dim sunshine thru mid/high clouds at sunrise will give way to overcast conditions. This increasing onshore flow will result in lowering clouds and spotty light rain/drizzle especially across the east slopes of the Worcester Hills and Berkshires. Given the clouds and onshore winds temps will be much cooler than today with highs in the 40s...still above normal for late Jan. Sunday night... Tricky forecast as northern stream short wave exits southeast Quebec into New Brunswick. This advects a cooler/drier airmass into the region. This erodes the leading edge of the precip shield as it approaches from the south. However at the same time it provides lowering wet bulb temps increasing the risk for wintry precip...that is if precip makes it this far north. Despite the synoptic scale lift and deep layer moisture possibly remaining south of New England thru 12z Monday...should be enough low level moisture given onshore flow and combined with upslope component will likely see spotty light rain. The problem will be across the higher terrain where the hi res guidance lowers temps into the upper 20s and lower 30s. Thus at least a moderate risk (30-60%) of freezing rain/drizzle across the high terrain of the Worcester Hills and east slopes of the Berks...possibly into northern CT as well. Given the shallow nature of the cold air followed the colder 4 km NAM temps for Sunday night. && .LONG TERM /MONDAY THROUGH SATURDAY/... Overview... Deep trough along with H5 cutoff low pressure digs off the Pacific coast with ridging across the eastern slopes of the Rockies. Another downstream digging trough with cutoff H5 low pressure will push across the western Carolinas into Monday morning. Associated surface low pressure slowly moves to the mid Atlantic coast later Monday, then will slowly push E-NE as the cutoff upper system begins to negatively tilt. Excellent dynamics in place ahead of the system for a multi-faceted storm Monday morning through midday Tuesday across the region. Once this system pushes offshore during Tuesday, H5 system shifts to the Maritimes Wed. This will allow drier and mild conditions to return around mid week. Another front will push across during Thursday on the leading edge of another long wave trough out of the Ohio Valley and Great Lakes. H5 cutoff low will spin over northern Quebec late Friday into next weekend, bringing more seasonable temperatures for late January to the region. Details... Monday-Tuesday... Broad but strong low pressure will move off the mid Atlantic Monday morning, then will only slowly move E-NE during the day as strong high pressure will remain over the Canadian Maritimes. Increasing easterly winds arrive during Monday morning, and will increase during the day especially along east coastal areas. A very strong low level easterly jet (on order of 65-70 kt from H95 to H85) along with excellent low level mixing and the strong high over the Maritimes will combine to bring a good portion of these winds down to the ground. Best shot for gusts up to around 50 kt will occur across Cape Cod and the islands, where high wind warnings from Monday morning into the early morning hours of Tuesday have been issued with this afternoon`s forecast package. Could see strong wind gusts across more of the coastal areas, and possibly a bit further inland across SE Mass and RI, but may not be quite as strong so held off on high wind watches for now. May only need wind advisories, but will let the mid shift take a look at new guidance and decide which headlines may be needed. As for rainfall, will see quite a bit especially across eastern areas during Monday afternoon into Tuesday morning. PWATs continue to increase with each model run thanks to tap of tropical moisture off the Atlantic. Will see area of moderate to heavy rainfall move across the region, with the bullseye focused on E Mass and RI. QPF values on order of 2 to 2.5 inches, with 1.5 to 2 inches further W across central areas to the CT valley. Expect the heaviest rain to fall from late Mon afternoon into early morning hours of Tuesday. Feel that there is a chance for urban, poor drainage flooding as well as in low lying areas. Could also see training of heavy rainfall for a time across some areas. Have issued a Flood Watch with this package for eastern Mass (east of Worcester County) except for the Cape and islands, as well as all of Rhode Island except for Block Island. Will need to monitor whether this heavy rain area shifts either W or E, which will determine whether the watch will need to be shifted. Another facet to this system is the PTYPE issues across N central and western areas, especially across the higher inland terrain. With marginal partial thickness values at both H100 to H85 and H85 to H70, this is the toughest part of the forecast. Will see a mixed bag of precip to start across most areas, even down to the coastal plain, which should change over to rain rather quickly across eastern areas during Monday with the increasing E winds bringing milder air inland. The big question will be how far W will the change go, and the PTYPE issues over far inland areas. At this point, looks like there will be a mix of rain/snow/sleet and even some freezing rain at times. This aspect of the forecast changes with each model forecast update, that`s how critical the partial thickness values have been over the last few days. So, rather low confidence on snow/ice amounts at this point. Do expect some heavy, wet snow, possibly close to advisory level across the highest terrain of NW and N Central Mass, with some light icing in more higher locations of the E slopes of the Berkshires and the Worcester Hills. Again, this will likely change with the 00Z model guidance. Do think that travel mainly N of the Mass Pike will be tricky at best and hazardous in some areas. Stay tuned for later updates. Expect the low to push across Cape Cod around midday Tuesday, followed by the dry slot clearing S coastal areas late Tuesday and the remainder of the region Tuesday night. Some timing issues with this aspect, however. Winds should shift to N-NW and may remain gusty along the E coast. Skies should slowly clear from S-N overnight. Wednesday through Friday... Generally dry weather through this period. Weak high pressure builds over the region Wednesday. A weak cold front moves across on Thursday, but starved of enough moisture for precip most places. There remains a chance of patchy light rain or snow in Northwest MA. Dry and cooler weather moves in behind the front for Friday. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... 2350z update... Tonight...Moderate confidence. Mainly dry conditions. IFR-LIFR CIGS/VSBYS along S coastal areas, with MVFR-IFR conditions moving inland across RI/SE Mass and N CT by around 05Z. Should remain mainly VFR across central and N Mass, though may see local MVFR VSBYS in patchy fog late. Sunday...moderate confidence. Mainly VFR to start but lowering to MVFR as the day progresses along with spotty light rain/drizzle and patchy fog. NE winds increasing up to 20 kt along the coast by days end. Sunday night...moderate confidence... MVFR likely in spotty light rain/drizzle. Concerned about freezing rain/drizzle across the high terrain including KORH. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Monday through Thursday/...Moderate confidence. Monday-Tuesday... Expect MVFR-IFR CIGS/VSBYS in RN/SN/IP to start by midday Monday, then will change over to all RN across coastal areas and most of N CT Mon night. Will see RN/+RN move into Mon afternoon/night across E Mass/RI while mixed precip continues inland. LIFR CIGS push into coastal areas Mon night. Expect best chance for SN/IP and FZRA near and N of the Mass Pike, although N CT has a low chance. Expect mainly rain and fog farther south and east, with +RA late Mon afternoon into Tue morning. Increasing E winds above the surface Monday, with speeds reaching their maximum Monday night and then diminishing Tuesday. Winds at 1000-2000 feet should reach 50 to 60 knots Monday afternoon and continue Monday night. East winds of 70 knots possible over Cape Cod and Islands. Expect strong LLWS during this time along gusts up to around 40-50 kt, highest along the immediate E coast especially Cape Cod and the islands. Winds diminish as they shift to N-NW during Tuesday as the low level jet shifts north into Maine. CIGS/VSBYS gradually improve to VFR from S-N Tuesday night, though MVFR may linger across N Mass into Wed morning. Wednesday and Thursday... VFR most likely. An approaching cold front may bring MVFR cigs/vsbys Thu in isolated rain/snow showers for parts of Northwest Mass during the afternoon/evening. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 7 PM Update... Tonight...Expect Light W-NW winds. Dry conditions. VSBY lowering to at or below 1/2 mile across south coastal waters in areas of fog, some dense near Cape Cod and the islands. Sunday...Increasing NE winds, especially in the afternoon with gusts to 20 kt developing. G25 kt over southern waters by evening. Vsby may lower in patchy rain/drizzle and fog. Sunday night...northeast winds continue to increase to gale force toward daybreak Monday. Seas increase rapidly to 10-15 ft by 12z Mon across the southern RI/MA waters. Vsby may lower in rain and fog esp southern waters. Outlook /Monday through Thursday/...Moderate confidence. Monday through Tuesday... Low pressure will continue to deepen off the mid Atlantic coast Monday, moving slowly NE toward the southern New England coast before crossing Cape Cod around midday Tuesday. E winds will continue to increase Monday, with the strongest winds Mon night/Tue morning. With very strong winds, up to 60-70 knots just above the surface, along with strong pressure gradient between strong high over the Maritimes and strong low pressure that will slowly approach the southern waters. Expect storm force gusts over the open waters, so have issued Storm Warnings for all waters except Gale Warnings for Boston Harbor and Narragansett Bay. Current forecast suggests E wind gusts up to 60 knots on the eastern outer waters. As the low moves across Cape Cod, winds will diminish as they back to N-NW from S-N during the early morning hours Tuesday on the southern waters, then across the eastern waters by late morning. N winds may gust up to 25 kt on the eastern waters for at time Tue, then will shift to NW and increase Tue night. Wednesday and Thursday... Improving conditions as weak high pres builds over the waters Wed. Then increasing WSW winds ahead of an approaching cold front Thu with its passage late Thu/Thu night. && .TIDES/COASTAL FLOODING... Given potent easterly LLJ and period of strong wind gusts, expect seas to probably build to 20-25 feet across the eastern waters late Mon into Tue. Fortunately, astronomical high tides are about as low as they get, and the strongest winds Monday evening will coincide with an astro high tide of only 8.2 feet in Boston. Thus the risk of any significant coastal flooding is low. In fact, it is hard to imagine a scenario of much worse than some splashover for the Monday evening high tide, thanks to the lowness of the astro high tide. The Tuesday morning high tide is higher at 9.4 feet. If winds are still blowing strong out of the northeast along with 20+ foot seas just offshore, minor coastal flooding could occur along the eastern MA coast. It would take a storm surge of nearly 3.5 feet and waves greater than 20 feet to even approach a moderate level of coastal flooding. Unless this system progresses much more slowly than the current consensus of models indicate, it is unlikely that we will experience anything worse than minor coastal flooding and some beach erosion for the Tuesday morning high tide in spite of such dangerous marine conditions just offshore. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...High Wind Warning from 9 AM Monday to 4 AM EST Tuesday for MAZ022>024. Dense Fog Advisory until 10 AM EST Sunday for MAZ020>024. Flood Watch from Monday afternoon through Tuesday morning for MAZ005>007-013>021. RI...High Wind Warning from 9 AM Monday to 4 AM EST Tuesday for RIZ008. Dense Fog Advisory until 10 AM EST Sunday for RIZ006>008. Flood Watch from Monday afternoon through Tuesday morning for RIZ001>007. MARINE...Storm Warning from 1 PM Monday to 3 AM EST Tuesday for ANZ231- 232. Storm Warning from 1 PM Monday to 1 AM EST Tuesday for ANZ233- 234. Gale Warning from 7 AM Monday to 5 AM EST Tuesday for ANZ230. Gale Warning from 3 AM Monday to 2 AM EST Tuesday for ANZ236. Storm Warning from 7 AM Monday to 1 AM EST Tuesday for ANZ235- 237. Storm Warning from 11 AM Monday to 6 AM EST Tuesday for ANZ250- 254. Storm Warning from 2 PM Monday to 3 AM EST Tuesday for ANZ251. Storm Warning from 4 AM Monday to 1 AM EST Tuesday for ANZ255- 256.
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&& $$ SYNOPSIS...Nocera/EVT NEAR TERM...JWD SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT TIDES/COASTAL FLOODING...Staff is the U.S. government's official web portal to all federal, state and local government web resources and services.