Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221936 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 336 PM EDT Tue Aug 22 2017 .SYNOPSIS...
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A cold front will bring the potential for scattered showers and thunderstorms late tonight, which may persist into Wednesday morning along the coastal plain. High pressure brings dry weather with very comfortable humidity levels Thursday through early next week.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Mostly sunny across southern New England this afternoon, except for the south coast of RI and MA, including the Cape and islands, where marine stratus lingered. Minor tweaks to the forecast this afternoon to bring it back in line with observed trends. Tricky forecast tonight. The high heat and humidity resulted in very high CAPE values for our region. Most unstable CAPE values were between 2,500-3,000 J/kg, with mixed layer CAPE values between 1,500-2,000 J/kg. Despite this, our region remains fairly capped this afternoon due to poor mid level lapse rates and low shear. The complication in the forecast tonight will be how quickly the CAPE values drop after sunset, and how quickly stronger shear arrives. It will be delicate balance. At this time, thinking strong convection may arrive east of the Berkshires between 8 and 10 PM, then weaken as it moves east from there. Helicity values in the 0-1km AGL layer range between 150-300 m2/s2. This would mean at least a very low risk for a tornado as these storms move across our region overnight. The other risks from any strong thunderstorms tonight will be gusty winds and locally heavy rainfall. Thinking any flooding risk tonight will be very localized as storms should be moving rather quickly. Training storms also appear to be unlikely. Well above normal temperatures continue tonight. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... The threat for scattered showers and thunderstorms may persist into Wednesday morning across the coastal plain, and perhaps even into early afternoon across the Cape and islands. Much drier air will arrive behind a cold front, which should move offshore by late afternoon. Near normal temperatures expected through this period. && .LONG TERM /THURSDAY THROUGH TUESDAY/... A long break from Summer heat. The return of the ridge-trough dipole stretched from Siberia into the Gulf of Alaska, ridge amplification across the N-Central CONUS into Central Canada shears energy equator- ward downstream promoting trough persistence across the NE CONUS. Several waves through the broader trough reinforcing cooler, drier air across the region. With available moisture and diurnal heating, collocated cold pool and steep lapse rates, can`t rule out isolated to scattered shower activity in the vicinity of S New England ahead of favorable mid-level ascent. Higher confidence over Upstate NY and N New England. Lot of dry air and low theta-E behind early week low pressure persisting over SE Canada. The influence of a 1025 surface high throughout the forecast period, the quasi- stationary tropical frontal boundary remains well S/E off-shore. Overall, looking like a prolonged period of dry weather with low humidity, low dewpoints, making for comfortable conditions. Highs around the mid to low 70s with lows down into the 50s. Scattered cloud decks at times. Dewpoints ranging in the upper 40s to low 50s. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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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 Wednesday Night/... Through 00Z...High confidence. Mainly VFR. Areas of MVFR-IFR towards the south coast, including the Cape and islands. Tonight...Moderate confidence. SCT SHRA/TSRA should move east of the Berkshires between 00Z- 02Z. Storms should weaken as they move east. Mainly VFR, with areas of IFR towards south coast. Strong SW low level jet could result in LLWS, especially toward Nantucket. Wednesday...High confidence in trends, but lower confidence in overall timing. SCT SHRA/TSRA may persist near and especially southeast of the I-95 corridor into early Wednesday afternoon. Improvement to VFR conditions expected from west to east, but timing somewhat uncertain. Wednesday Night...High Confidence. VFR. KBOS Terminal...High confidence in TAF trends. Moderate confidence in timing. KBDL Terminal...High confidence in TAF trends. Moderate confidence in timing. Outlook /Thursday through Sunday/... Mainly VFR. Mainly W-NW flow with sea breezes by day.
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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 Wednesday Night/...High confidence. Strong southwest low level jet moves across the waters tonight into Wednesday morning. Gusty southwest winds ahead of an approaching cold front. Some risk for thunderstorms, but showers more likely. Will continue Small Craft Advisory headlines, but did tweak times. Winds and seas diminish Wednesday afternoon, with more tranquil boating conditions starting Wednesday night. Outlook /Thursday through Sunday/... Generally quiet boating weather. NW winds dominate, with gusts peaking only around 15 kt at any given time. Seas/waves remain below 4 ft.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ231>234- 250. Small Craft Advisory until 2 AM EDT Wednesday for ANZ230-236. Small Craft Advisory until 2 PM EDT Wednesday for ANZ235-237. Small Craft Advisory until 8 AM EDT Wednesday for ANZ251. Small Craft Advisory until 4 PM EDT Wednesday for ANZ254>256. && $$ SYNOPSIS...Belk/Doody NEAR TERM...Belk SHORT TERM...Belk LONG TERM...Doody AVIATION...Belk/Doody MARINE...Belk/Doody

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