Area Forecast Discussion
Issued by NWS Boston, MA

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587 FXUS61 KBOX 212321 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 721 PM EDT Sat Oct 21 2017 .SYNOPSIS... High pressure south of New England will provide very warm and dry weather into Monday. The exception will be across northeast Massachusetts Sunday where a backdoor cold front will yield cooler temperatures but still above normal. A cold front with possibly waves of low pressure will bring gusty showers to the area Tuesday afternoon into Wednesday. Drier and seasonably cool weather is expected behind the front for late in the week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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700 pm update... No significant changes with this evenings update. Low dwpts still suggest another cool night thanks to a very weak pres gradient in spite of the upper lvl CI spilling across the region. Temps generally on track, just made adjustments to bring them up to current trends. Previous discussion follows... Deep layer ridge remains just west of New England providing dry weather thru the night. Light pressure gradient with 1028 mb high just south of the region along with clear skies and a dry atmosphere (dew pts in the 30s) will promote temps to fall fairly quickly with sunset. Thus have leaned toward the cooler bias corrected MOS to derive min temps tonight. Could be some patchy fog over southeast MA where dew pts have come up into the lower 40s from SW winds today off the ocean. Overall very tranquil conditions for late Oct.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... *** Near Record Warmth CT/western MA but much cooler northeast MA *** Sunday... Little wrinkle in tomorrow`s forecast, that being a backdoor cold front dropping southward from the NH/ME coastline. This feature will be accompanied by a wind shift to the N-NE and result in cooler temps for northeast MA including Boston with highs in the 60s. However how far south this boundary slips is somewhat uncertain along with how widespread low clouds will become across this area. Given this is shallow/mesoscale boundary we followed the high res guidance for this portion of the forecast. Elsewhere, mostly sunny conditions and warm sector airmass with +17C at 925 mb will support highs tomorrow 75-80, possibly low to mid 80s CT river valley from Hartford to Springfield. Record high tomorrow for BDL is 85 set back in 1979. Could come close. Sunday night... Boundary begins to lift north as a warm front, thus a mild night with lows 45-55. Still comfortable with dew pts in the 40s and 50s. Dry weather but could have some low clouds and patchy fog in the warm sector airmass especially with dew pts higher than the avg nighttime low. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Heavy showers and gusty winds are possible ahead of a cold front Tuesday afternoon into Wednesday Overview and model preferences... Gradually sharpening and building block downstream (ridgeline is near Labrador) allows deepening trof to establish itself across the E CONUS through mid week, taking on a negative tilt and cutting off toward the latter portion of the week. This yields a much wetter and unsettled setup in comparison to the very dry/anomalously warm setup this past week. The only break comes in the form of a secondary, very deep trof near the continental divide which allows the subtropical ridge/Bermuda high to regain control across the SE and force the mean jet and cutoff across New England to the NE late week. A very amplified, complex flow pattern, so individual guidance is still struggling to resolve the details especially after Wednesday, therefore, with this update, will lean most heavily on ensembles to take into account the full envelope of solutions. Details... Mon and Mon night... The final throes of ridging continues to shift E Mon into Mon night. Column remains generally dry enough to support one final mainly dry day, although with moisture loading from the top-down through the column, will likely see increasing cloud cover, and especially overnight as the BL moistens. Will begin to introduce some low-end POPs late Mon night due to increased moisture and weak LLJ development. Otherwise, mainly dry through the daylight hours. Breezy warm flow with increased pres gradient ahead of the deepening low pres near the Great Lakes. Mixing to H92 at least, where temps rest around +14C to +15C. Even with increasing clouds, highs should still reach the low- mid 70s before. Tue and Wed... Deepening upper trof takes on strong negative tilt, with parent sfc low sliding S-N through the great lakes, dragging a cold front into New England. This front slows as the trof tilts thanks to parallel orienting flow. The system begins a gradual occlusion process and slows as it does so, allowing the front remain hung near or over S New England into the day on Thu. Two weak frontal waves associated with strong LLJ cores slide across the region, the first mid day Tue into Tue afternoon, while the second follows early Wed. Given the initial trof is able to tap Gulf of Mexico moisture, noting high ensemble probabilities of PWATs near, but just below 2.00 inches, or nearly 3 std deviations above late October climo through the period, supported by K-indices near the mid 30s. Also, high TT values (near 50) and modest mid lvl lapse rates suggest instability, and even show some MU CAPE values between 300-500j/kg. All this is to suggest that, particularly with each LLJ passage there is a risk for heavy rainfall, with potential localized urban flooding. Even if rainfall rates diminish between each LLJ core, widespread light-mod rain can still be expected with the unstable profile and high moisture content. Given the potential convective nature to the precipitation, widespread QPF is likely to remain generally between 1.00-2.00 inches but with locally higher amounts possible. The H92 LLJ early Tue reaches near 40-45 kt, while the second, early Wed looks to be stronger, potentially above 50 kt, nearly 4 std deviations stronger the climo. With the destabilized sounding, will need to watch for strong winds at the sfc should at least some of this momentum be efficiently transferred. A convective fine line, especially along the frontal boundary is possible, which could lead to localized strong winds. Overall the pattern does suggest the possibility of wind headlines. Thu... Cooler, drier air is finally allowed to move into S New England as the trof continues to wrap up and begin a slide to the N in response to the ridge building across the SE CONUS. This dry slow moves in aloft first, trapping moisture in the low lvls. Given strong cyclonic flow aloft and cold advection, could see continued, but less widespread SHRA through the daytime Thu, so POPs will be decreasing with time. Could even see some small hail or graupel as FZL lowers with the influence of cooler air. H92 temps drop to an average near +7C. Which should keep daytime temps in the low-mid 60s. Fri into Sat... High pres noses in as modest ridging builds across the NE in advance of yet another very deep longwave trof through the W. Mainly dry conditions favored. The cold air influence is short- lived, given the ridge building is associated with warmer air attendant to the subtropical ridge across the SE. Therefore, looking at still seasonably mild temps by day (mainly mid-upper 60s). Airmass suggests seasonal lows, mainly near the mid 40s. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/.... Overnight...High confidence. Mainly VFR. Could be a few spots of ground fog at typically prone airports. Sunday...high confidence except low confidence across northeast MA VFR, dry weather and light winds. However lots of uncertainty across northeast MA where low clouds (MVFR-IFR) along the coast of NH/ME may slide southward into northeast MA. Forecast confidence was not high enough to go with IFR/MVFR conditions so left TAFs VFR but need to watch this closely. Sunday night...moderate confidence. Uncertainty on how quickly MVFR-IFR conditions overspread the area from the northeast and from the southwest. Mainly dry conditions prevail along with light winds. KBOS Terminal...High confidence in TAF through Sunday morning then uncertainty on MVFR-IFR cigs from NH/ME coast sliding southward. KBDL Terminal...High confidence in TAF through Sunday. Outlook /Monday through Thursday/... Monday...High confidence. Mainly VFR, although with a low risk for early AM fog and lower clouds with isolated IFR conditions possible. Mon night into Wed...Moderate confidence. Mainly a mix of IFR/MVFR in low CIGS, fog and rainfall. Occasional strong S wind gusts. LLWS likely, especially during the day on Tue and Wed. Thu...Moderate confidence. Some improvement to VFR, although lingering IFR/MVFR possible.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/...High confidence. Very tranquil boating weather for late Oct continues this period with light winds, dry weather and good vsby. Only wrinkle will be backdoor front that drops southward from NH/ME coastline into MA waters Sunday, resulting in a wind shift to the N-NE. Post frontal winds speeds generally 10 kt or less especially given mild water temps of 55-60. Outlook /Monday through Thursday/...Moderate confidence. Mon... Increasing S winds should remain generally below 25 kt through Mon evening and seas below 5 ft through Mon night. Advisories not likely to begin until after this period. Tue into Wed... Occasional Southerly Gales with wind gusts 35-40 kt possible. Seas, building 8-10 ft mainly S and SE waters. Even in periods where Gales not experienced, high end small craft advisories likely. Rain/fog lead to vsby restrictions. Some thunderstorms possible over the waters. Thu... Gradual improvement with winds shifting around to the W. A few gusts still to 25 kt at least, along with lingering seas/swells. Small craft advisories likely to follow Gales. && .FIRE WEATHER... Dry weather continues through at least Sunday with minimum RH values 25 to 35 percent, lowest values over CT and western MA. However with high pressure nearby winds will be very light. Thus fire weather parameters remaining below headline criteria. && .CLIMATE... Record High Temperature for Sunday, October 22: only BDL is in jeopardy. BDL 85 in 1979 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Doody NEAR TERM...Nocera/Doody SHORT TERM...Nocera LONG TERM...Doody AVIATION...Nocera/Doody MARINE...Nocera/Doody FIRE WEATHER... CLIMATE...

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