Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211908 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 308 PM EDT Wed Sep 21 2016 .SYNOPSIS...
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High pressure remains over the region through Thursday 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 for a brief band of rain showers, followed by cool and dry weather this weekend. Another cold front may cross New England either Monday or Tuesday, but will most likely come through dry.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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High pressure in control will result in mainly clear skies other than a bit of high cloudiness along with light winds tonight. This combined with the relatively dry airmass in place will allow temps to quickly drop after sunset. Low temps by daybreak should range from the upper 40s to the lower 50s in the normally coolest outlying locations, and between 60 and 65 in the urban heat islands of downtown Boston/Providence.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
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Thursday... Another beautiful day with high pressure in control. Mostly sunny skies should allow temps to quickly jump after sunrise. 850T between +14C and +15C should yield afternoon highs in the middle 80s for much of the region along with low humidity. It will be a few degrees cooler across the higher terrain, as well as the immediate coast where sea breezes will result in afternoon temps mainly in the 70s. Thursday night... High pressure slowly moves off the coast Thursday night, but low level moisture will be very slow to return. Low temps should mainly be in the 50s, to the lower 60s. Should see an increase in mid/high level cloudiness from the north after midnight, but dry weather should prevail through daybreak Friday. Some localized patchy ground fog possible very late Thu night, but do not expect it to be widespread at this point.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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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... 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.
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&& .AVIATION /19Z 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 Monday/... 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...
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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. High pressure in the vicinity will result in a weak pressure gradient. This should keep winds and seas below small craft advisory thresholds and nice late season boating weather through Thursday night. Outlook /Friday through Monday/... 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank NEAR TERM...Frank SHORT TERM...Frank LONG TERM...Doody AVIATION...Frank/Doody MARINE...Frank/Doody

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