Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 220541 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 141 AM EDT Thu Sep 22 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 THIS MORNING/... 930 PM Update... High pres in control with light to calm winds but high clouds streaming north have overspread SNE. Expect these clouds to continue to move across the region north of the upper low in the Carolinas. This will slow temp drop tonight with low temps ranging mostly through the 50s with some upper 40s possible upper CT valley in northern MA, but lower 60s urban heat island of Boston. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... 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 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 /06Z 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... 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 Night 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...KJC SHORT TERM...Frank LONG TERM...Thompson AVIATION...KJC/Thompson MARINE...Frank/Thompson

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