Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152053 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 453 PM EDT Sun Apr 15 2018 .SYNOPSIS...
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Low pressure and its associated frontal system will move into New Eng Monday. Rain and freezing rain tonight will transition to a period of heavy rain on Monday along with isolated thunderstorms as the frontal system moves through. The mid level system will keep unsettled conditions across the region through mid week. As this system exits, another fast moving low will cross the region Thursday into Friday, bringing another round of showers. Dry conditions look to finally return across the region next weekend. Temperatures will remain below normal through most of this week, with some moderation possible next weekend.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Light ocean effect precip continues to push inland from the ocean across much of eastern and central MA into RI. Rather cold 925 mb temps around -7C enhancing low level instability and combined with abundant moisture in the low levels contributing to the ocean effect precip. Ptype mostly drizzle/freezing drizzle, but also some light snow as ice production occurring in the low levels where temps just cold enough for ice crystal growth. Traditional snow growth region (-12 to -18C) remains quite dry. These conditions will persist through the afternoon. Steadier precip will overspread the region tonight, between 8pm and midnight as deepening moisture plume moves in from the west and SW. Area of rain expanding along the mid Atlc coast and this will move into SNE tonight. Ptype tonight will be mostly a cold rain south of the Pike, but pockets of freezing rain likely over higher elevations in northern CT and NW RI. North of the Pike, low level cold layer lingers which will result in a mix of sleet/freezing rain and rain with ice accretion confined mostly to higher elevations. Low levels will gradually warm from south to north so any wintry precip will transition to all rain after midnight but pockets of freezing rain will persist through daybreak over higher elevations in the Berkshires and Worcester hills. Ice accum of 0.10 to 0.25 inch possible over higher elevations. Low level wind field increases tonight with gusty E/NE winds expected. Given the cold air around 925/950 mb, shallow mixing will support gusts 20-30 mph and up to 40 mph possible over the Worcester hills. Temps slowly rising overnight, reaching lower 40s along the south coast toward daybreak. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... * Heavy rainfall, isolated thunder and areas of urban and poor drainage flooding Monday * Heavy rainfall... A few pockets of freezing rain may linger into early Mon across higher elevation in northern MA. Otherwise, the main concern is heavy rainfall as potent upper low approaches from the west with right entrance region of upper jet moving over SNE. Very anomalous low level jet 4-5SD above normal approaches from the south with PWATs 1-2SD above normal. This will certainly result in a period of heavy rainfall moving west to east across SNE with timing around 12-18z west and 15-21z east. Soundings show some elevated instability above the frontal inversion with lapse rates approaching moist adiabatic and SWI near or slightly below zero so an isolated t-storm can`t be ruled out. Fortunately, the system is progressive but still expect widespread 1 to 2 inches rainfall with locally up to 3 inches. In fact some of the hi-res guidance members indicate rainfall exceeding 3 inches. Heaviest rainfall will likely be confined to favored upslope areas of the Worcester hills into NW RI and the Berkshires to western Hartford county. Rivers should be able to handle this rainfall, but areas of urban and poor drainage flooding appears likely. Strong wind potential... Strong low level jet 60-70 kts moves across the south coast during the afternoon but it is uncertain how much of this will mix down. Wind direction will remain mostly SE which will be difficult to get very mild air into SNE. Temps should reach the 50s south of the Pike with mid/upper 50s possible across RI and SE MA, but remaining in the 40s north of the Pike. Soundings suggest we probably need to reach upper 50s to lower 60s to enhance low level mixing and bring strong winds to the surface in vicinity of the low level jet. Thinking gusts up to 40 mph near the south coast and Cape/Islands with some risk of 40-50 mph gusts over higher elevations in the Worcester Hills where hi-res guidance targets strongest gusts. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
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Overview... 12Z model suite and their ensembles continue to signal H5 cutoff low pressure and negatively tilted trough slowly pushing across New England through late Wednesday or Wednesday night. Once this finally moves into the Maritimes, another fast moving H5 short wave in the northern stream flow will bring its associated surface low pressure across the region, which means another round of showers. Both the 12Z GFS and ECMWF northern hemispheric graphics finally showing some improvement in sensible weather as H5 long wave trough pushes E of the region by early next weekend, which looks to bring dry conditions. Still noting below normal temperatures even into next weekend with NW flow aloft. Details... Monday night through Wednesday... H5 mid level cutoff low will slowly move across the Great Lakes Monday night, reaching the Maritimes sometime Wednesday. With the continued cyclonic mid level synoptic flow, along S-SE winds will keep decent low level moisture across the region especially along the coast, then may see a brief break in the action as winds shift to SW on Tuesday. Some showers may linger across the E slopes of the Berkshires. Still some uncertainty whether another weak short wave moves across Tue, which may bring isolated showers across with marginally unstable conditions. The mid level system moves across northern New England late Tue into Wed. Will see a general W-SW wind flow, with mainly dry but cool conditions. Wednesday night through Friday night... Another fast moving H5 short wave heads E out of the Great Lakes Wed night. Models in pretty good agreement in cutting off the mid level low and spinning it around New England through the remainder of the work week. A band of cool H85 air works in across interior southern New England Wed night into Thu, which will bring another chance for scattered showers moving across Wed night through Fri. Have best chance for precip from Wed night through Thu from about the Mass Pike northward where best lift is noted. The H5 cutoff low pushes SE out of upstate NY Thu night, along with its cold pool of air (H85 temps drop to -3C to -6C mainly after midnight across the interior), so could see scattered snow showers across the E slopes of the Berkshires as well as the Worcester hills possibly into the higher terrain of N central CT. As the H5 low slowly pushes into Maine Friday, the associated moisture will also lift NE. At this point, should see showers end from W-E Friday afternoon and evening. Temps will continue to run 5 to 10 degrees below seasonal normals as the cold pool moves across. Saturday and Sunday... Large high pressure will move E out of Hudson Bay and central Canada, and will expand SE across New England to the mid Atlantic states through the weekend. However, will still see temps remaining several degrees below normal as H5 heights especially on Saturday as NW flow continues aloft.
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&& .AVIATION /21Z SUNDAY THROUGH FRIDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... Through 00z...Moderate confidence. Widespread MVFR with areas of IFR. Areas of DZ/FZDZ/SN across eastern half New Eng. NE gusts 25-30 kt, strongest along the coast. Tonight...Moderate confidence. Widespread MVFR/IFR trending toward mostly IFR. Steadier rain/freezing rain mixed with sleet overspreading the region tonight and trending toward all rain, but pockets of freezing rain persisting over interior northern and western MA. E/NE gusts 20-30 kt, up to 35 kt Cape/Islands and Worcester hills. Monday...Moderate confidence. Widespread IFR with areas of fog developing and spreading inland from the coast as higher dewpoint air moves in over cooler ocean. Period of heavy rain will move across the region with isold thunder possible. E gusts to 20-30 kt. up to 35 kt Cape/Islands and Worcester hills. KBOS Terminal...Moderate confidence in TAF. DZ/SN becoming rain tonight with period of heavy rain Mon afternoon. KBDL Terminal...Moderate confidence in TAF. Rain developing tonight with period of heavy rain Mon morning through early afternoon. Outlook /Monday Night through Friday/...Moderate confidence. Monday Night: Mainly VFR, with local IFR possible. Windy with local gusts to 30 kt. Slight chance RA. Tuesday: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHRA. Tuesday Night: VFR. Breezy. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Wednesday Night: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA. Thursday through Thursday Night: Mainly MVFR, with local IFR possible. Chance SHRA. Friday: Breezy. Chance SHRA.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Gale warnings will continue. E/NE gusts increasing to 35-40 kt across south coastal waters tonight with E gusts to 40-45 kt over eastern MA waters on Monday. Expanded the gale warning to Boston Harbor. Vsbys lowering later tonight and especially Monday as fog becomes more widespread. Steadier rain developing tonight with a period of heavy rainfall and isold thunder Monday. Outlook /Monday Night through Friday/...Moderate Confidence. Monday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 18 ft. Slight chance of rain. Local visibility 1 nm or less. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft. Slight chance of rain showers. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Wednesday Night through Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Thursday Night through Friday: Winds less than 25 kt. Local rough seas. Chance of rain showers.
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&& .TIDES/COASTAL FLOODING... We issued a coastal flood advisory for coastal Essex county for the midday high tide Monday. Gale force easterly winds will result in seas building to 15-20 ft just offshore from NE MA coast. A storm surge up to 1.5 ft combined with these waves may be enough for minor flooding of vulnerable shore roads along the Essex county coast where highest wave action expected. Tides are a bit higher for the Monday night tide just after midnight but wave action and surge a bit less. Still, some minor flooding is possible for the Monday night high tide cycle which will need to be evaluated further for a possible extension of the advisory. Along the south coast, it appears the strongest winds and surge will occur late afternoon between the high tides so this may limit minor flood potential. && .CLIMATE...
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Record lowest max temperatures for Today April 15 BOS 40 in 1881...High so far today 39 PVD 42 in 1972...High so far today 41 BDL 41 in 1943...High so far today 39 ORH 39 in 1943...High so far today 33 Record lowest min temperatures for Sunday April 15 BOS 28 in 1943 PVD 27 in 1923 BDL 24 in 1957 ORH 19 in 1940
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory until 2 AM EDT Monday for CTZ002>004. MA...Winter Weather Advisory until 2 AM EDT Monday for MAZ010-011. Winter Weather Advisory until 2 AM EDT Monday for MAZ005>007- 012-014. Winter Weather Advisory until 8 AM EDT Monday for MAZ002>004- 008-009-026. Coastal Flood Advisory from 10 AM to 2 PM EDT Monday for MAZ007. RI...Winter Weather Advisory until 2 AM EDT Monday for RIZ001-003. MARINE...Gale Warning until 7 PM EDT Monday for ANZ232>235-237-255-256. Gale Warning from 2 AM to 2 PM EDT Monday for ANZ230. Gale Warning from midnight tonight to 7 PM EDT Monday for ANZ231. Small Craft Advisory until 7 PM EDT Monday for ANZ236. Gale Warning from midnight tonight to 10 PM EDT Monday for ANZ250-251. Gale Warning until 10 PM EDT Monday for ANZ254.
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&& $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT TIDES/COASTAL FLOODING...Staff CLIMATE...Staff

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