Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152024 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 424 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. Showers will linger into Tuesday across interior portions of the region. Another low pressure may approach Thursday, bringing another round of showers. Temperatures will continue to run below normal early this week, then moderate to more normal levels later this week.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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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.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
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* 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.
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&& .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Overview... 15/00Z guidance remains in rather good agreement with the synoptic pattern into Wednesday. Thereafter, differences arise in their handling of a potent shortwave over the northern plains, which will have implications on our forecast later this week. Have above normal confidence in the forecast through Tuesday, then a moderate confidence in the forecast for Wednesday and beyond. Details... Monday through Wednesday... Cutoff mid level low will slowly move over the Great Lakes Monday night, reaching the Maritimes sometime Wednesday. This will generally keep our region within a synoptic cyclonic flow aloft. This would support at least a slight risk for periods of wet weather, depending on available moisture, as well as near to slightly below normal temperatures. Main period of wet weather will be Monday into Monday evening. A triple point low pressure is still expected to move across southern New England during this time. This will help to enhance low level convergence, and make rainfall processes more efficient. Lots of moisture expected to be in place, with precipitable water values 2-3 standard deviations above normal. Instability remains marginal, but also persistent, for several model cycles, now. Maintained the mention of isolated thunderstorms for Monday. Significant anomalies with the 925 mb wind, both for advecting moisture north, as well as colder air from the North Atlantic. Besides the potential for strong gusts towards the east coast, thinking temperatures will also struggle to rise much Monday morning. High temperature MOnday may be later in the day than usual. Will need to consider a Wind Advisory with later forecasts. Thinking it is rather marginal at this point. Will let the next shift get another look. Drier air in 850-700 mb layer punches into our region from the SW-NE behind the triple point low and a cold front Monday night. This should at least cause any remaining rain to taper off for a time. As this low lingers across southern Quebec Tuesday-Wednesday, could see some patchy showers redevelop during the daylight hours, especially across northern MA. Mid level moisture will also keep clouds across much of region through part of Tuesday night. Anticipate mainly dry conditions on Wednesday, though some diurnal showers may still develop across the E slopes of the Berkshires. Wednesday night through Saturday... Another fast moving mid level shortwave approaches, along with a surface low pressure. Timing and track are still the big questions. Model solutions are converging, but still some wiggle room in their solutions. At this time, thinking the window for the greatest risk of showers is Wednesday night into Thursday, but this could change. Also a possibility showers linger into Friday, with a mid level trough still to move past our region. Lower confidence forecast for this timeframe. && .AVIATION /20Z 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. Chance RA, slight chance SHRA, patchy FG. Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Tuesday Night through Wednesday: VFR. Breezy. Wednesday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Thursday: Mainly VFR, with local IFR possible. 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: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 17 ft. Rain, isolated thunderstorms, patchy fog. Areas of visibility 1 nm or less. Monday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 16 ft. Chance of rain, patchy fog. 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. Tuesday Night through Wednesday: Winds less than 25 kt. Areas of rough seas. Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Thursday: Winds less than 25 kt. Local rough seas. Chance of rain showers.
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&& .TIDES/COASTAL FLOODING...
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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.
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&& .CLIMATE... Record lowest max temperatures for Sunday April 15 BOS 40 in 1881 PVD 42 in 1972 BDL 41 in 1943 ORH 39 in 1943 Record lowest min temperatures for Sunday April 15 BOS 28 in 1943 PVD 27 in 1923 BDL 24 in 1957 ORH 19 in 1940 && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory from 7 PM this evening to 2 AM EDT Monday for CTZ002>004. MA...Winter Weather Advisory from 7 PM this evening to 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 from 7 PM this evening to 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 NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Belk AVIATION...Belk/KJC MARINE...Belk/KJC TIDES/COASTAL FLOODING...KJC CLIMATE...

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