Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221125 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 725 AM EDT Thu Sep 22 2016 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM update... Nearly SKC across S New england this morning as only a few high clouds continue to clear the Islands. This sunshine will once again allow temps to rise into the low-mid 80s across the area. With mixing, dwpts should remain in the upper 40s to low 50s, and this was the only adjustment made this morning of any consequence. Previous dwpts and dwpts provided by guidance seemed too high. Otherwise, forecast on track. Previous discussion... 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 /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... 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. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Scattered showers and isolated thunderstorms Friday night. * Cool and dry weather follows for the rest of the weekend into next week. Patchy frost possible some interior locations Sunday night. Overview and model preferences... The pattern shift is in full swing by the weekend, as sharp ridging continues to allow cutoff low to shift into Quebec and Labrador. Longwave trof settles to the S, allowing colder, drier air from the northern Canadian source region to spill across New England. Base of the trof sits E, suggesting influence from the upstream ridge, so expect drier and cooler wx to linger well into early next week. Finally, guidance seems to be leaning more on a slow, meridional pattern. Even the most progressive GFS now leans more toward the ECENS and GEFS means. The timing will be dependent on how quickly cutoff low pres is able to suppress ridging across the eastern seaboard. For now, will lean most heavily on ensemble means, which have been the most consistent in suggesting this slower pattern. Details... Fri night into Sat... Lingering tsra continue into the evening hours Fri, thanks to MUCAPE values between 500-1000j/kg and modest low-lvl shear around 30 kt. However, with loss of diabatic heating, it will likely become more difficult to tap the upper lvl instability. Still K-values remain above 30 and PWATs near 1.5 inches as weakening cold front slides through late night into early Sat morning. Therefore, lingering showers are expected, with brief heavier downpours. Drier air and high pres Sat should begin to erode these showers from W-E, allowing for an end everywhere by afternoon. However, strong cyclonic curvature and cold advection aloft may allow for a few destabilized showers to last longer. Mins Fri night slightly warmer than previous nights mainly in the upper 40s and low 50s. Sat temps will be dependent on how quickly clouds dissipate, but could approach the low 70s at some locations given colder air does not really filter in until Sunday. Sun and Mon... Cold CP airmass associated with air drawn out of the Yukon settles across New England, bringing us some of the coolest temps we have experienced in some time, truly a taste of fall. Dry conditions are expected, thanks to subsidence. H5 temps drop to nearly 2 std deviations below normal. H85 temps drop to between 0C and +2C late Sun, warming only to about +4C by Mon. Therefore, expect highs only in the mid 60s in spite of mixed sun and clouds. Lows will be dependent on the final pres gradient, but could drop into the upper 30s if fully realized. May need to watch for isolated frost patches, but this remains a low confidence forecast. Temp probabilities are relatively low at this time. Tue and Wed... Will be trending grids toward a drier solution based on the ensemble mean solutions mentioned above. Precip probs are quite low lending credence to this thinking. We remain on the W side of the mean trof, with continued anticyclonic flow and subsidence. H85 temps reach +6C to +9C, each day, allowing temperatures to reach beck to late September normals, and possibility even above normal by late Wed. Thu and beyond... Very uncertain given the widespread amongst both ensembles and deterministic models. Highly amplified flow to blame. && .AVIATION /12Z THURSDAY THROUGH MONDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. No major adjustments with the 12Z TAF update... 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 night into early Sat...Moderate confidence. Mix of MVFR/VFR in showers and fog which should improve Sat morning. Wind shift from W-SW to mainly NW expected. Later Sat through Mon...High confidence. Mainly VFR.
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&& .MARINE... 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 into Sun...Moderate confidence. SW winds shift to the N-NW late Fri night with gusts 20-25 kt continuing through the day Sat. This persistent wind, combined with swell from Karl well offshore will yield 5-7ft seas. So small craft advisories may well be needed. Sat night into Sun, reinforcing gusts 25-30 kt possible, along with slowly subsiding swell. Small craft advisories likely linger. Mon...High confidence. High pres moves over the waters, mainly quiet boating weather expected. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC/Doody SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Doody MARINE...KJC/Doody

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