Area Forecast Discussion
Issued by NWS Boston, MA

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688 FXUS61 KBOX 212317 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 717 PM EDT Wed Sep 21 2016 .SYNOPSIS... High pressure remains over the region through Thursday resulting in dry weather along with warm afternoons and cool nights. A cold front will sweep across southern New England late Friday afternoon and night with scattered showers and isolated thunderstorms. Cool and dry weather will follow the cold front for 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 AM THURSDAY MORNING/...
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715 PM Update... Quiet evening is expected to continue int a quiet overnight. High clouds look as though they will persist through the night. Observed temperatures running below forecast and observered dewpoints running a tad above forecast. Have updated near term hourly temperatures/dewpoints, but low temperature forecast still looks to be on target. Any sea breezes should now be transitioning to land breezes in an otherwise light synoptic gradient. Prior Discussion... 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/... 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. && .LONG TERM /FRIDAY 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 Candian 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 annafront 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. 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 /23Z 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. No significant changes needed with 715 PM update. VFR conditions will persist through Thursday night with the exception of 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. Scattered showers and isolated thunderstorms may develop mid to late afternoon across northern MA. 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.
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&& .MARINE... 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. Winds and seas should be below Small Craft Advisory thresholds. Isolated thunderstorms possible northern coastal waters during late Fri afternoon. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/Thompson NEAR TERM...Frank/Thompson SHORT TERM...Frank LONG TERM...Thompson AVIATION...Frank/Thompson MARINE...Frank/Thompson

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