Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 160232 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1032 PM EDT Wed Aug 15 2018 .SYNOPSIS... Very warm to hot weather will prevail through Thursday with slightly less humid conditions. Tropical humidity returns Friday into Saturday, with an increasing risk of showers and thunderstorms with locally heavy rain as a cold front approaches. This front moves through later Saturday, followed by mainly dry and cooler weather Sunday into Tuesday as high pressure builds in from the north. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 1010 PM Update... Area of showers/thunderstorms across central and N NH/VT moving just a bit S of due E as seen on latest NE regional 88D radar imagery. Noted a few showers earlier this evening, but they dissipated as they pushed out of E central NY into S VT, as near term model data had signaled. High clouds associated with the convection has spread E-SE across most of the region, as seen on GOES-East RGB Nighttime Microphysics composite imagery through around 02Z, but noting the higher clouds tending to dissipate as drier air works in aloft from southern NY state. Will still see low T/Td spreads especially across the CT valley into the E slopes of the Berkshires overnight, so have kept mention of patchy fog there. Temps bottom out in the 70-75 degree range. Have updated overnight forecast to bring conditions current. Previous Discussion... Convection across upstate NY into Ontario should remain north of the region as subtle height rises and backing mid level flow should steer this activity into central and northern New England overnight. This subtle height rises are also accompanied by mid level dry air via subsidence and is supported by decreasing K index values on the SPC mesoanalysis across mid to lower Hudson Valley of NY into southern New England. Might see some patchy fog late tonight across central and western areas, as well as near Nantucket, as somewhat higher dewpoints will linger. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... A light sea breeze may develop along E coastal areas with the light pressure gradient. This may be a trigger for a few showers that may develop around midday Thursday. Expecting temperatures just a few degrees higher than today. Given the fairly high dewpoints, may need to consider a Heat Advisory if we can use the lower threshold for consecutive days. Looking rather marginal at this time, so will let the next shift take another look. Regardless, it will be hot and humid for most Thursday afternoon. For Thursday night, a weak cold front will try to push south through our region. This would mean at least a low risk for some showers or thunderstorms. Given the timing, thinking showers would be more likely than thunderstorms. Greatest risk will be towards NW MA. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Updated 420 pm... Highlights... * Showers/T-storms likely Friday into Saturday * Locally heavy rainfall possible along with the potential for a few strong to severe storms Fri afternoon/evening * Mild to warm days and cool nights Sun/Mon/Tue Friday into Saturday... Intensifying low pres moves from the eastern Great Lakes Fri into northern New England Sat. Warm sector airmass overspreads southern New England beginning Fri afternoon. Both deterministic and ensembles offer high probs of CAPEs over 1000j/kg Fri afternoon and evening with highest probs across CT into western-central MA. This in response to high temps 85-90 in the warm sector (+22C at 925 mb) along with sfc dew pts 70-75. Thus could be some strong storms based on instability combined with modest synoptic scale forcing per Qvector convergence. As previous forecaster noted vertical wind shear is initially weak Fri but a developing low level jet late in the day and into Fri night increases helocity values. Therefore could have some rotating storms. Given high dew pts/low LCLs and warm ocean not inhibiting convection, will have to watch this closely. For now included storms with potentially very heavy rainfall/flash flooding Fri into early Sat given tropical PWATs up to +2 standard deviations above climo. Dilemma for Saturday is timing of fropa. Latest trends in ensembles and deterministic is for convection to possibly linger across CT/RI and southeast MA, especially south coast and Islands with a possible drying trend northern MA. In addition, some of the guidance is bullish on gusty NNW winds behind the front especially across Cape Cod and Islands where blyr transfer is best. Not as hot Saturday given cloud cover but still warm and humid with highs in the low 80s across CT/RI and southeast MA, 75-80 across northern MA with post frontal airmass arriving here sooner. So turning slightly less humid late in the day from north to south. Saturday night into Wednesday... Post frontal airmass overspreads the region Sat night on gusty NE winds. Gust up to 25-30 mph possibly across Cape Cod and the Islands given the warm ocean providing good blyr mixing. 1020 mb high descending into the maritimes provides a much cooler airmass via NNE winds with ensembles and deterministic guidance lowering 925 mb temps down to +13C Sunday. This shallow cool airmass will only support highs in the 70s, possibly only upper 60s for the outer Cape! Airmass gradual warms but maritime high remains anchored with ridging lingering into Tue. Thus pleasant and dry weather before warming along with increasing humidity middle of next week ahead of the next trough. && .AVIATION /02Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... 1020 PM update... Overnight...High confidence. VFR. Local MVFR in patchy fog around or after 06Z across CT valley into the E slopes of the Berkshires. Otherwise, dry weather prevails. Thursday...High confidence. VFR. Mainly dry conditions. Patchy fog with MVFR-IFR VSBYS across central and W areas through around 13Z. May see isolated showers/thunderstorms develop across E Mass around midday Thursday. Thursday Night...VFR. KBOS Terminal...Sea breeze may impact the terminal at times late this afternoon. Moderate confidence. KBDL Terminal...High confidence in TAF. Outlook /Friday through Monday/... Friday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, chance TSRA especially across CT and western-central MA. Locally heavy rain and strong winds a concern afternoon and evening. Friday Night: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, chance TSRA, patchy BR. Locally heavy rain and strong winds a concern afternoon and evening. Saturday: Mainly MVFR, with areas VFR possible. Breezy with NNW winds up to 25 kt especially Cape Cod and Islands. Chance SHRA, chance TSRA especially across CT/RI and southeast MA. Saturday Night: Mainly VFR, with local MVFR possible. Breezy with NNE winds up to 25 kt especially Cape Cod and Islands. Slight chance SHRA. Sunday: VFR. Breezy with NNE winds up to 25 kt early across Cape Cod and Islands. Sunday Night: Mainly VFR, with local MVFR possible. Breezy. Monday: Mainly VFR, with local MVFR possible. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. A weak front will push to near or just off the S coast tonight before stalling. W winds will prevail, with some leftover gusts up to 20 kt this evening, then will back to during Thu. Seas up to 5 feet through tonight across the southern outermost coastal waters, then should remain less than 5 feet. Good visibility. Outlook /Friday through Monday/... *** Gusty NNW winds late Saturday then NNE winds Sat night into early Sunday with gusts up to 25 kt possible *** Friday: Winds less than 25 kt. Slight chance of rain showers, slight chance of thunderstorms. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, chance of thunderstorms, patchy fog. Saturday: Low risk for Small Craft Advisory winds with NNW gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, chance of thunderstorms. Saturday Night: Low risk for Small Craft Advisory winds with NNE gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain showers. Sunday: Low risk for Small Craft Advisory winds with NNE gusts up to 25 kt. Seas up to 5 ft. Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Monday: Winds less than 25 kt. Seas up to 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Air Quality Alert from 11 AM to 11 PM EDT Thursday for MAZ020- 022>024. RI...Air Quality Alert from 11 AM to 11 PM EDT Thursday for RIZ006>008. MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ255-256. && $$ SYNOPSIS...Belk/Nocera NEAR TERM...Nocera/EVT SHORT TERM...Belk LONG TERM...Nocera AVIATION...Belk/Nocera/EVT MARINE...Belk/Nocera/EVT

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