Area Forecast Discussion
Issued by NWS Boston, MA

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538 FXUS61 KBOX 220714 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 314 AM EDT Thu Sep 22 2016 .SYNOPSIS...
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High pressure remains over the region today resulting in warm and dry weather. A cold front will sweep across southern New England Friday afternoon and evening with scattered showers and isolated thunderstorms. Cool and dry weather will follow this weekend into next week. Another cold front may cross New England Monday night or Tuesday, but will likely come through dry.
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&& .NEAR TERM /THROUGH TODAY/...
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High pressure will remain in control with sunshine, light winds and unseasonably warm temps. 850 mb temps 14-15C will translate to highs in the mid 80s for much of the region, except a bit cooler higher terrain and along the coast where seabreezes are likely. Dewpoints will be in the 50s so comfortable humidity levels.
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&& .SHORT TERM /TONIGHT THROUGH 6 PM FRIDAY/...
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Tonight... High pressure moves offshore but SNE will be in between systems with a wedge of dry air across the region. Mainly clear skies expected with low in the 50s, but lower 60s BOS and outer Cape/ACK. Some localized late night patchy ground fog possible. Friday... Northern stream shortwave tracking across Quebec will push a cold front south through New Eng, with front reaching northern MA around 18z and the south coast near 00z. Deeper moisture is lagging behind the front so expect sunshine through the morning with clouds increasing from north to south during the afternoon. Airmass remains rather warm with 925 mb temps 20-22C ahead of the front which supports highs in the mid 80s again south of the pike and upper 70s and lower 80s to the north before cooling sharply mid to late afternoon in the north. Surface instability ahead of the front will be limited due to lack of moisture, however models indicate favorable post-frontal elevated instability parameters with a sharp increase in KI values into the mid 30s along with subzero SWI within a theta-e ridge axis. This has the look of an anafront and expect sct showers and possibly an isold t-storm after the fropa with main focus north of the pike in the mid to late afternoon.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Scattered showers and isolated thunderstorms Friday afternoon and night. * Cool and dry weather follows for the rest of the weekend into next week. Patchy frost possible some interior locations Sunday night. Overview... Pattern amplifies during this forecast period. Amplifying trof that closes off over the Canadian maritimes will bring a surface cold front through area Friday night followed by markedly chillier air mass. Another short wave trof rotating about Canadian maritime long wave trof may bring reinforcing chilly air into region but is likely to pass through dry. High amplitude ridge ahead of high amplitude trof in west central North America will likely ensure dry weather from Saturday through most if not all of next week. Forecast becomes highly uncertain late next week. If the pattern shifts far enough east as suggested by the latest GFS run, then we might experience wet weather by the following weekend. Details... Late Fri into early Sat... There looks to be enough low level moisture convergence along a cold front to support scattered showers late Fri afternoon and night. There is some hint that this could be an anafront with much of the rainfall on the back or cold side. There is some instability with K indices near or a little above 30 and showalters near 0. Surface based CAPE is modest, generally under 700 j/kg for a time late Fri afternoon. Models are depicting mid level lapse rates in 6 to 6.5 C/km range. Convection Fri evening/night will most likely be elevated. There is some vertical shear with 500 mb winds 40 to 45 kt, but questionable if deep convection will be able to tap into that shear. Precipitable water values crest about 1.5+ inches. Have not gone higher than chance POPs, however, since most systems have under produced in way of rainfall over the past few months. Sat through Mon... Surface high pressure sprawls into New England during this time frame with 850 mph temperatures probably dropping to between 2C and 4C ,the coolest we have seen in some time. High temperatures will likely remain in the 60s Sat through Mon and provide a fall feel to the air. Clear skies and decoupling winds may cause temperatures to drop into mid/upper 30s Sun night with patchy interior frost a possibility. Went 2 or 3 degrees below a model blend for Sat night and more like 4 to 6 degrees below a model blend for Sun night. Mon night and beyond... Model runs suggest a short wave dropping south about the Canadian maritime long wave trof will bring a secondary dry cold front through the region. This could provide a corresponding reinforcing shot of cool air. Low relative humidities will promote drying of fuels and possible fire weather concerns down the line. Cold frontal approach backed by two weak shortwaves. The first may arrive with little fanfare Fri late afternoon. Although noting increase in moisture and instability, it is later than this initial passage. Only a few widely scattered shra possible. The second, and more robust, on some guidance, arrives late Fri evening into early Sat morning with the actual frontal passage. PWATs reach 1.5 inches, high, but not as high as this last weekends moisture plume along with K-values near 30. These are coincident with modest probabilities of 500-1000j/kg of MUCAPE and modest 0-3 km shear near 30 kt. Therefore, during this late night/early Sat period, a few thundershowers and showers with heavier rainfall are possible. While the PWATs only support modest widespread QPF between 0.1 and 0.3 inches, some areas may be able to overachieve thanks to the convective elements at play. Will linger POPs into Sat morning and need to monitor for localized urban/poor drainage flooding as rivers/streams can still well handle the rainfall. Not expecting too much in the way of severe at this time, but will need to monitor given the modest shear/instability parameters. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today through Tonight...High confidence. VFR, except for some late night and early morning patchy valley fog in the typically prone locations. Friday...High confidence. VFR cigs lowering from north to south in the afternoon. Scattered showers and possibly an isold t-storm developing aft 18z north of the pike where patchy MVFR cigs possible toward evening. KBOS TAF...High confidence in TAF. Seabreeze expected 15-16z. KBDL TAF...high confidence in TAF. Outlook /Friday Night through Monday/... Fri eve/night...Moderate confidence. Sct shra/tsra and associated fog may yield areas of MVFR/IFR conditions. Sat through Mon...High confidence. VFR expected. && .MARINE...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Through tonight...High confidence. Quiet boating weather with high pres in control providing light winds and seas. Friday...High confidence. Increasing pre-frontal SW winds but gusts less than 20 kt. Wind shift to NW then N across NE MA waters mid to late afternoon. Outlook /Friday Night through Monday/... Fri night...High confidence. Winds shift to the N and possibly even NE, but should remain at or below 20 kt. Sat through Mon...Moderate confidence. Swells across the outer coastal waters may approach 5 feet due to combination of NE winds and swells from far away Karl. There could be a pulse of higher northerly winds behind a secondary cold front Mon night into Tue, but confidence on this is fairly low. There is at least some risk of needing Small craft advisories for seas approaching 5 feet over the outer east coast waters.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Doody MARINE...KJC/Doody

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