Area Forecast Discussion
Issued by NWS Boston, MA

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350 FXUS61 KBOX 191925 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 325 PM EDT Sat May 19 2018 .SYNOPSIS...
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A warm front will bring a steady widespread light rainfall today, becoming more showery late this afternoon into tonight. Chilly conditions today transition to warm and humid later tonight into Sunday with hit or miss showers/thunderstorms. Dry on Monday with passing showers during the mid-week. Cooler, drier air follows for Thursday into Saturday.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 230 PM Update... WAA advection rains continue across the region this afternoon. Cool ENE flow ahead of warm front keeping temps in the 50s. Steady light rain becomes more spotty early this evening and overnight as best WAA lifts into northern New England. However areas of drizzle and fog will maintain damp conditions. Not much change from previous forecast. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Tonight...Warm front lifts north across our region this evening. Increasing low level jet will advect higher precipitable water values and some elevated instability. Expecting at least a risk for showers, with a thunderstorm or two not out of the question. Temperatures should slowly rise overnight behind the warm front. As temperatures rise, so will dewpoints. This should lead to patchy fog formation. Sunday...A cold front should cross our region by evening. Given the modest instability and forcing, not expecting severe weather at this time, just scattered showers with a few thunderstorms. More summer-like temperatures and humidity, with highs in the 70s away from the south coast Cape and islands. && .LONG TERM /MONDAY THROUGH SATURDAY/...
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Highlights... * Dry weather Monday * Passing showers Tuesday and Wednesday * Potential for dry weather but near or below avg temps for the end of the week Overview and model preferences... New England is generally wedged between N stream and S stream flow through the period although most guidance shifts to more N stream dominant flow pattern by late in the week. This is a combination of upstream ridge amplification thanks to warm advection from a SW CONUS cutoff low aloft and reinforcement of a longwave trof just downstream with base through the Canadian Maritimes. This reinforcement comes in the form of several energy lobes ejecting equatorward from a Baffin Bay vortex. Am noting enough agreement between both ensembles and operational guidance that a blend can be used as a baseline for the forecast update. Details... Mon... Slight mid and upper lvl height rises Mon allows high pres to build across the region through the day. Dry conditions prevail. H85 temps near seasonable normals suggest highs near normal, but a W component should allow for some downsloping with areas reaching the the mid-upper 70s. Sea breezes possible given weakening sfc/low lvl flow. Tue and Wed... A series of at least two shortwaves sides S out of the near arctic vortex. The first arrives Tue, first spawning convection across the OH valley and Mid Atlantic, the remnants of which could bring a round of SHRA by late day/overnight Tue. The second, on Wed will be associated with a cold frontal passage. At this point, it does not look like the warm sector fully shifts into S New England as even PWATs remain only between normal and 1 std deviation above. Will need to watch this, but at this point, it looks like mainly on and off SHRA through the period. Thu and Fri... Mean trof axis shifts to the E, but S New England remains under cyclonically influenced flow. However, with stacked predominant NW flow, expecting mainly dry wx associated with nearly 1025mb high pres. The cyclonic flow could allow for diurnal cloud cover. H85 temps also drop given the depth of the downstream trof. Agree with previous forecaster that temps are likely to err below normal. Next weekend... Sat initially looks dry as the mean trof axis shifts further E, allowing for rising heights and implied ridging. However, some suggestion of a return to more unsettled wx mainly as a remnant trof from the SW CONUS low ejects E. Something to watch, but will continue to lean dry at least through Sat.
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&& .AVIATION /20Z SATURDAY THROUGH THURSDAY/...
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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 Sunday Night/...High confidence. 18z update... Widespread IFR/LIFR late this afternoon into tonight in rain transitioning to spotty light rain/drizzle and patchy fog. LLWS marginal across RI and eastern MA tonight as low level SW jet increases. Sunday... IFR/LIFR lifts to MVFR by midday, slightly longer south coast. Drizzle and areas of morning fog erode by midday but give way to a broken line of showers along and ahead of approaching front. Very low prob of isolated T-storm but showers will contain locally heavy rain from about 17z-21z from west to east. Breezy southwest winds 15 to 20 kt with a few gusts up to 25 kt possible. Sunday night... Any leftover IFR/MVFR early evening over Cape Cod improves rapidly to VFR. Otherwise VFR and dry weather. KBOS Terminal...a broke line of showers with heavy rain Sunday 17z-20z. Very low risk of brief isolated T-storm. KBDL Terminal...moderate to heavy confidence in the TAF. Outlook /Monday through Thursday/...Moderate confidence. Monday through Monday Night: VFR. Tuesday: Mainly VFR, with local MVFR possible. Slight chance SHRA. Tuesday Night: Mainly VFR, with local IFR possible. Slight chance SHRA, patchy BR. Wednesday through Thursday: VFR.
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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 Sunday Night/...Moderate confidence. E flow gradually becomes SE this evening as a warm front crosses the waters from S to N. Steady rainfall likely. Increasing S winds tonight, shifting SW Sunday. Building seas across the outer coastal waters tonight into Sunday. Vsbys reduced in rain and fog today, with areas of dense fog developing tonight. Risk for scattered showers and isolated thunderstorms Sunday. Outlook /Monday through Thursday/...Moderate confidence. Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Monday Night: Winds less than 25 kt. Tuesday: Winds less than 25 kt. Slight chance of rain showers. Tuesday Night: Winds less than 25 kt. Slight chance of rain showers, patchy fog. Local visibility 1 to 3 nm. Wednesday through Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday: Winds less than 25 kt.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from midnight tonight to 6 PM EDT Sunday for ANZ231>235-237-250-254>256. Small Craft Advisory from 4 AM to noon EDT Sunday for ANZ230. Small Craft Advisory from midnight tonight to 3 PM EDT Sunday for ANZ236. Small Craft Advisory from midnight tonight to noon EDT Sunday for ANZ251.
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&& $$ SYNOPSIS...Nocera/Doody NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Doody AVIATION...Nocera/Doody MARINE...Nocera/Doody

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