Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211729 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 130 PM EDT Wed Sep 21 2016 .SYNOPSIS... High pressure remains over the region through Thu resulting in dry weather along with warm afternoons and cool nights. A cold front sweeps south from Canada late Friday into early Sat with a risk of showers, followed by cool weather with a taste of fall this weekend. Another cold front may cross New England either Monday or Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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130 pm update... Mostly sunny skies at mid afternoon with just some thin high cloudiness across the region. High temperatures will likely climb a few degrees from their current readings, topping out mainly in the lower to middle 80s. Dewpoints have dropped into the 40s, to the lower 50s so very comfortable humidity making for a beautiful afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/... Update 400 am ... Tonight ... High pressure and associated dry airmass will remain anchored over the region tonight. Mostly clear skies...light winds and dew pts in the 40s and 50s will set the stage for an ideal night of radiational cooling. Given this setup leaned toward the cooler MAV guid for temps. Low temps will bottom out in the upper 40s to the lower 50s in the normally cooler outlying locations, to the upper 50s and lower 60s in the urban heat islands of downtown Boston/Providence. Thursday ... More of the same...high pressure overhead along with a relatively dry airmass in place with dew pts in the 50s. This will yield mostly sunny conditions with temps warming into the low and mid 80s once again. However humidity will remain in check as dew pts settle in the 50s. Weak pgrad will result in another day of afternoon seabreezes...cooling the shoreline back into the 70s. Nonetheless more fantastic weather. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Highlights... * Showers possible late Friday into early Saturday. * Cool drier weather likely late weekend into early next week. * Uncertainty in unsettled weather mid next week. Overview and model preferences... Strong ridge building (nearly a rex-block formation) continues across the central portions of the US and Canada thanks to continued warm advection out of a cutoff across the middle Pacific region. The result, is a conduit for Hudson`s Bay vortex energy to begin to spill across the northeast CONUS, culminating in a cutoff across Quebec and Labrador with a longwave trof over New England. The initial transition arrives as a slow/weakening frontal boundary. This is followed by a possible pattern shift, shunting the subtropical ridge well the south. Because the final flow pattern is very amplified by early next week across the whole of the CONUS, there are several differences in model outcomes/timing by mid next week, so forecast confidence does drop rapidly after Monday in particular. There is enough agreement early to use a consensus blend of models as a baseline, however, will transition toward a heavily weighted ECMWF/ECENS mean, as these are generally less progressive and more representative of the meridional pattern in which they reside. Details... Thu night into early Fri... Dry wx prevails even as slow moving cold front approaches from the N. Overnight mins Thu night a bit warmer than previous thanks to slight increase in sfc and column dwpts. Looking for lows in the mid-upper 50s. The dry wx lingers until close approach of the front late Fri. Late Fri into early Sat... 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. Late Sat into Mon... Ridge of high pres associated with CP airmass from N Canada will follow the cold frontal passage overnight. Temperatures will dip along with dwpts as drier air moves in and clears out remaining cloud cover. H85 temps near +8C and cooling on Sat suggest with sunshine by afternoon temps could still reach the upper 60s and low 70s. By Sun H85 temps fall to +1 to +3C suggesting highs mainly in the 60s, followed by a slight increase on Mon allowing temps to once again make a run at the 70s thanks to the dry conditions. Dwpts through the whole period will fall as low as the upper 30s and low 40s. While this might suggest a frost risk for some areas of NW MA normally colder in these situations, noting pres gradient may be tight enough to limit the necessary radiational cooling for frost. Will continue to monitor as we approach. All-in-all a true taste of autumn for late September, courtesy of heights almost 2 std deviations below normal. Tue and beyond... As mentioned above, guidance struggling with meridional flow and exactly how amplified the pattern will be by this time. All ensemble probabilities are very low, suggesting very wide spread amongst solutions. In any case, some return flow expected as a second and fairly robust wave moves out of the cutoff the the N. Will need to watch for yet another unsettled period sometime mid week, but timing remains highly uncertain. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight through Thursday night...High confidence. VFR conditions will persist through Thursday night other than perhaps some localized patchy ground fog late Thu night in the typically prone locations. KBOS TAF...High confidence in TAF. KBDL TAF...high confidence in TAF. Outlook /Friday through Sunday/... Fri...High confidence. Mainly VFR, although patchy fog possible early Fri morning. Late Fri into early Sat...Moderate confidence. Occasional shra/tsra possible. This may yield periods of MVFR/IFR conditions, even some fog possible until winds shift to the N early Sat. Late Sat into Sun...High confidence. Winds may reach 20-25 kt at times. Otherwise, VFR.
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&& .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today through Thursday ... High confidence. Other than early morning fog today burning off between 8 am and 10 am...High pres overhead thru Thu will result in light winds...flat seas and good vsby. Overall fantastic boating weather by late Sep standards. Outlook /Friday through Sunday/... Thu into Fri...High confidence. Mainly quiet boating weather as high pres moves E of the waters. Winds shift from SSW to W through the day on Fri. A low risk of showers/thunderstorms possible during the day on Fri. Fri night into Sat...Moderate confidence. Winds shift to the NW, but should remain at or below 20 kt. However, swells on the ocean waters may reach near 5 ft due to swells from winds and from Karl well offshore. Small craft advisories may be needed for this risk. Sat night into Sun...Moderate confidence. A pulse of higher winds possible late Sat night into Sun out of the NW, gusts to 25 kt possible along with a rebound in seas. Small craft advisories may once again be needed. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Doody NEAR TERM...Frank SHORT TERM...Nocera LONG TERM...Doody AVIATION...Frank/Doody MARINE...Nocera/Doody

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