Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 231437 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1037 AM EDT Fri Sep 23 2016 .SYNOPSIS... Another warm afternoon is on tap for today, but a cold front will cross the region this evening bringing much cooler temperatures. A band of rain showers is also expected behind the front this evening into tonight, but amounts will be light. Cool and dry weather will follow this weekend into early next week. A slow-moving low pressure and cold front could lead to some showers late next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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***High temps mainly in the 80s at mid afternoon will fall into the 60s to around 70 in most locations between 5 and 9 pm this evening*** 1030 am update... Impressive low level inversions in place across southern New England this morning, strongest towards the Cape and weakening toward Albany and Long Island. Lots of dry air below 700 mb, too. Although the Albany sounding showing moisture is increasing already, albeit slowly. This inversion was impacting KBOX radar data, mainly by producing a lot of ground clutter. We`ve done all we can to mitigate those impacts for now. Tweaked the forecast timing this morning for rainfall chances and clouds based on observed trends. This lead to some temperature adjustments as well. Mixed in with the ground clutter were a few sprinkles along and north of the Mass Pike. This far, not a lot of reports of measurable rainfall reaching the ground, which is in line with expectations. Abundant clouds have moved in ahead of a cold front. This will present some limitations to developing instability for this afternoon. Will either need to get some breaks in these clouds, or very good low level advection, to get enough instability for anything more than an isolated thunderstorm or two this afternoon. Previous frontal timing still looks reasonable. Previous discussion... One more warm afternoon is on tap before a cold front brings much cooler weather to the region by evening. A shortwave passing to our north this morning may result in a few brief sprinkles north of the MA turnpike. Otherwise, expect dry weather with partial sunshine through mid to late afternoon ahead of the approaching cold front. This should allow much of the region to see high temps in the middle 80s by mid afternoon. A big change will take place in our weather during the evening. A strong cold front will move across the region from north to south, bringing a surge of gusty north to northeast winds /especially along the coast/. Temps will quickly fall into the 60s to near 70 between 5 and 9 pm, with the coolest of those readings across northeast MA. The other concern will be for precipitation chances very late this afternoon and evening. While there will be impressive wind fields/decent mid level lapse rates this afternoon, dry mid level air will likely prevent convection ahead of the front. However, band of frontogenesis coupled with increasing mid level moisture will result in some showers behind the front mainly very late this afternoon and especially this evening. Low risk of an isolated thunderstorm or two, but not expecting anything more than that.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight... Band of rain showers will continue to press from north to south with band of mid level frontogenesis/moisture behind the cold front. Much cooler air will continue to work into the region tonight and northeast winds will be a bit gusty along the coast. Low temps will bottom out mainly in the upper 40s and 50s by daybreak Saturday. Saturday... Drier air will move in from the northwest allowing for skies to become mostly sunny during the morning. However, a fall like atmosphere will be in place with 850T between +3C and +5C. High temps should still recover well into the 60s to near 70 given lots of sunshine, but certainly be our first true taste of early fall. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Cool and dry weather follows for this weekend into next week. * Patchy frost possible for some interior locations Sunday night. * Seasonable temperatures late next week Overview and model preferences... Highly amplified flow regime continues development mainly as sharp rex block continues to move across E Canada and the US. This provides a conduit for cooler air from near Hudson`s Bay into New England, delivering a period of below normal temperatures, and very comfortable dwpts. Ensembles and deterministic guidance are in good agreement through Tue in spite of the meridional pattern. Beyond Tue, there begins to be more significant difference regarding how quickly cutoff low pres moving E of the continental divide is able to open and become more progressive. ECENS mean continues to be a good middle ground in timing/impacts. Splitting the very slow GFS and more progressive ECMWF. Therefore, from Wed onward, will continue to add more weight to the ECENS mean, which agrees with WPC thinking. Details... Sat night through Mon... Cooler and fall-like conditions prevail as H85 temps drop as low as 0C to +2C late Sun before warming back up to about +6C Mon. Expect highs mainly in the low-mid 60s to prevail. Very low dwpts expected thanks to good afternoon mixing, as low as the mid 30s in spots. Therefore, should radiational cooling be observed, there is a low risk for frost each early morning Sun and Mon, particularly in the most sheltered NW MA valleys. Not expecting widespread frost as developing inversions look to reach only the upper 30s at their coolest with 40s elsewhere each night. Tue and Wed... Uncertainty begins, but will need to monitor for stacked low pres to approach from the W-NW with a warm frontal, then cold frontal passage. Timing very uncertain, but using the ECENS mean, will focus POPs mainly late Tue into early Wed. PWATS mean is only about 1.0 to 1.5 inches with highest probs closer to 1.0 inches, so while rain showers are possible it will be dependent on timing and still may not amount to much more than a quarter inch. Temperatures rise, but may hold close to seasonal normals. Mainly upper 60s and low 70s for highs while mins drop into the 50s. Thu and Fri... As previous forecaster noted, given the slow movement and involvement of a cutoff low pres featuring heights 2 std deviations below normal, not confident on how quickly the impacts from this feature leave New England. Low pres could redevelop in the Gulf of Maine or Maritimes as what is left of the cutoff shifts to the E leading to a resurgence of moisture back into New England should the new cyclogenesis be close enough. At this time, will continue with NIL or low POPs due to uncertainty until the Meridional pattern is better progged. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...High confidence. Mainly VFR through 00Z. Showers also hold off most locations (except a few sprinkles this morning) until after 20Z. Winds shift to the NW this afternoon, and could be accompanied by a brief increase in wind gusts 20-30 kt before settling back down. Tonight...Moderate to high confidence. A period of MVFR CIGS in a band of showers are likely behind a cold front. Northeast wind gusts of 20 to 25 knots expected along the coast. Saturday...High confidence. Any left over MVFR Cigs and perhaps a touch of patchy early morning fog in the typical locations will improve to VFR by mid morning. Otherwise, VFR conditions on tap for Saturday. KBOS TAF...Moderate to high confidence in TAF. Wind shift to the northeast with brief gusts to 25 possible between 20z and 22z, followed by a period of MVFR CIGS/showers. KBDL TAF...Moderate to high confidence in TAF. Outlook /Saturday Night through Tuesday/... Sat night into Mon...High confidence. VFR. Some 25 kt wind gusts possible near shore Sat night into Sun out of the NW. Tue...Moderate confidence. Timing uncertain, but some showers possible on Tue with localized MVFR or lower conditions.
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&& .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today and Tonight...High confidence. Winds and seas will remain below small craft advisory thresholds through late afternoon. A strong cold front should result in period of northeast wind gusts to 25 knots and 3 to 6 foot seas across our open waters early this evening into the overnight hours. Small craft headlines posted for all waters except Narragansett Bay. Saturday...High confidence. Left over small craft wind gusts diminish by mid morning as pressure gradient weakens. However, small craft headlines will continue across our outer-waters into mid afternoon from left over swell. These seas should finally drop below small craft advisory thresholds by late afternoon. Outlook /Saturday Night through Tuesday/... Sat night into Sun night...High confidence. Small craft advisory conditions prevail. NW winds gust around Sat night into daytime Sun with seas building 5-6 ft on the ocean waters. Although winds recede by Sun night, it will take the remainder of the night for seas to fully drop below the 5 ft threshold. Mon and Tue...Moderate confidence. Mainly quiet boating weather but an approaching front could yield more small craft advisory conditions Tue. However, this is timing dependent, which is uncertain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 8 AM EDT Saturday for ANZ231>235-237. Small Craft Advisory from 5 PM this afternoon to 11 PM EDT this evening for ANZ230. Small Craft Advisory from 5 PM this afternoon to 3 PM EDT Saturday for ANZ250. Small Craft Advisory from 5 PM this afternoon to 8 AM EDT Saturday for ANZ251. Small Craft Advisory from 8 PM this evening to 3 PM EDT Saturday for ANZ254>256. && $$ SYNOPSIS...Frank/Doody NEAR TERM...Belk/Frank/Doody SHORT TERM...Frank LONG TERM...Doody AVIATION...Belk/Frank/Doody MARINE...Frank/Doody

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