Area Forecast Discussion
Issued by NWS Boston, MA

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512 FXUS61 KBOX 212048 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 348 PM EST Tue Nov 21 2017 .SYNOPSIS...
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A cold front approaches the region tonight, and will combine with moisture working up the eastern seaboard to bring a period of rain through tomorrow morning, especially SE New England. Quiet weather Thursday and Friday. Another shot of wet weather and breezy conditions Saturday through Monday. Quiet and milder Tuesday and Wednesday, possibly wet and breezy conditions again Thursday.
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&& .NEAR TERM /UNTIL 3 AM WEDNESDAY MORNING/...
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Strong subsidence inversion in a regime of continually rising heights this afternoon, have led to mainly SKC conditions outside of a few CI wisps. This has allowed BL mixing to push into the inversion a bit further aloft than previous guidance suggested, and led to some upper 50s and even low 60s for afternoon highs especially away from the coastline, and in the Merrimack valley where some downsloping is occurring. The sounding is gradually moisture loading from the top down, but have noticed a recent gradual increase in sfc dwpts. The response is a layer of CI building N from offshore, which combined with the maintenance of a strong pres gradient should limit evening decoupling somewhat especially S of the Mass Pike. Mins will likely occur early in the overnight, before 3AM once the lower column moistens. Strong temp gradient will result, with low 30s still possible across the N/NW where some decoupling is possible. Meanwhile, closer to the SE coastline temps are likely to remain in the upper 40s and low 50s, given an overall lack of setup for radiational cooling. Precip will hold off through 3AM, see below for more information.
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&& .SHORT TERM /3 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
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Early AM through Wed evening... Acute upper lvl shortwave will slide E late tonight from the Great Lakes, deepening somewhat as it does so. Merging upper jet setup will allow for ocean cyclogenesis off the mid Atlantic S of the Carolinas after midnight in the equatorward entrance region of the S jet. Meanwhile N jet is associated with slow moving cold front attendant to low pres sliding through ON/QC early AM. All models except the ECMWF and many of its ensemble members were initially keeping each of these features as fully separate entities, even as coastal low pres develops an inverted trof which will impact S New England. 12Z guidance a bit more favorable for at least some phasing of these features, closer to the slower and more amplified ECMWF, which has been more persistent. This forecast update will lean most heavily on the ECMWF given its persistence with this features. Early morning, moisture continues to increase through the column as a prefrontal inverted trof and convergence zone develops mainly across E MA/RI. The moisture plume peaks with PWATs reaching 1.00-1.20 inches (nearly 2 std deviations above normal after sunrise Wed). The lift provided by the convergence boundary and incoming front will lead to a band of light- moderate rainfall developing early morning then peaking by mid morning, especially across E MA/RI. Lift/moisture drops off rapidly to the NW, so am still somewhat uncertain on exact placement of the cutoff for precipitation. Will focus highest categorical POPs along the axis convergence in the ECMWF, mainly E of a line from central CT through the Worcester hills. By afternoon, punch of dry air following the cold frontal passage should allow for precip to rapidly end and skies to clear from W-E, such that much of S New England may see the sun before it sets. This is a little bit slower, as the more robust ECMWF suggests development of meso-low pres at the merger point, slowing the pattern. Final QPF will be focused across E MA and RI where peak moisture/lift reside. Final totals approaching an inch are possible, dropping off toward the NW. Will continue to suggest a risk for TS, mainly in the E, where conditionally unstable lapse rates occur thanks to anomalously high sfc dwpts enhance the risk. This also suggests brief heavy downpours possible during the AM commute. Highs tomorrow will be mainly in the low- mid 50s. Tomorrow night... Skies clear rapidly behind the front through the evening, such that mainly SKC conditions are expected for most of the overnight hours. Even though winds will briefly increase out of the NW thanks to mixing enhanced by differential CAA, am noting enough slackening in the pres gradient by early morning to imply radiational cooling. Therefore, min temps cooler than previous night, with several locations dropping into the 20s by early Thanksgiving morning.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/...
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*/ Highlights... - Quiet weather for Thursday and Friday - Showers progged for Saturday night into Sunday, potentially breezy - Turning mild Tuesday into Wednesday, could also be breezy - A potential near to above seasonable pattern into early December */ Overview... Potential indications of a near to above seasonal pattern into early December. Noting a +WPO trend with EC/UKmet preference, signals of a deamplifying N Pacific trof-ridge-trof pattern, becoming more zonal, a longer, fast fetch of mild Pacific air aimed into Canada. If model stratospheric forecasts are any indication, the main low shifting into NW Europe with slightly higher heights over the CONUS, we lose the Arctic connection via NW fetch, flow more W/SW, the ebb and flow pattern as of late and likely to continue into next weekend relaxing indicating possible milder intrusions with any disturbances sweeping through the continued zonal pattern, lacking an Arctic draw. Neither Atlantic nor Pacific amplification in the long-range to indicate the zonal pattern will end anytime soon as all other teleconnections are near-neutral, subdued. An overall quiet pattern, any disturbances progressive, separated to a degree by the split-flow regime governed and slowly sliding E in concert with a trof disturbance over the SE CONUS extending into the Caribbean. Warm intrusions ahead, cold air following behind. As it was mentioned above, ebb and flow pattern continues. N/S streams mostly split, however closely watching a Saturday night into Sunday disturbance. Touch upon any threats and/or impacts in the discussion below. */ Discussion... Thursday and Friday... Quiet. Cooler Thursday, NW winds, colder aloft. Highs below average. Upper 30s to low 40s. High pressure, light winds, clear conditions, opportune radiational cooling into Friday morning with lows in the 20s. Mild Friday, rebound with S winds, highs in the 40s, thickening clouds late. Saturday through Monday... Watching offshore storm development and parent winds. Progressive flow through the split-flow regime, taking a page from the oncoming disturbance within the next 24 hours. Wet weather late Saturday into Saturday night, chance PoPs, could see decent rains. Breezy SW winds in advance, not anomalously strong. More concerning is NW winds rear- ward, rapid cyclogenesis occurring outside of the 40N/70W benchmark. Undercutting colder air, steeper lapse rates into Sunday, possible that with H85 winds in excess of 50 mph that we could potentially mix-down gusts that suffice WIND ADVISORY criteria, gusts 46-57 mph. An isallobaric component with pressure couplet. Cold air advection. Additional energy acting upon any moisture, perhaps both showers and winds linger into Monday. Tuesday into Wednesday... Warm up. Return S flow as high pressure shifts E, have to closely monitor its interaction with upstream disturbances, a subsequent SW jet with tightening pressure gradient and whether it can effectively mix down to the surface. Greater threat Wednesday. Mild, highs into the 40s and 50s. Thursday onward... Potential sweeping weather disturbance Thursday, however absent an Arctic connection as pointed out in the discussion above. Attendant high pressure to follow of CONUS rather than Canada origin.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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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 Wednesday Night/...High confidence. Through tonight... VFR through at least early morning, with MVFR CIGS and RA arriving mainly after 09Z from the S. Areas W of a line from IJD-ORH-MHT may keep VFR CIGS through sunrise. Winds remain SW, with gusts 25-35 kt through this evening then gradually dissipating overnight. Tomorrow... Mixed MVFR/IFR CIGS through the day tomorrow with RA and VSBYS 1-3SM at times, especially E of the same line (IJD-ORH-MHT). Areas W may remain mixed MVFR/VFR with less RA. The rain clears, and rapid improvement to VFR occurs after 18Z from W-E as winds shift to the NW. Tomorrow night... VFR. NW flow around 10 kt. KBOS Terminal...High confidence in overall TAF trends, timing of lower categories early AM may be off by an hour or two. Clearing occurs mainly after 20Z tomorrow. KBDL Terminal...Moderate confidence in TAF. Uncertainty in MVFR CIGS as CIGS could remain low VFR and bulk of the RA remains E. Outlook /Thursday through Sunday/... Thanksgiving Day through Friday: VFR. Friday Night: VFR. Breezy. Saturday: VFR. Breezy. Chance SHRA. Saturday Night: VFR. Slight chance SHRA. Sunday: VFR. Breezy. Slight chance SHRA.
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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. Short Term /through Wednesday Night/... Overnight into tomorrow...High confidence. Last impulse of Gale wind gusts to 35 kt out of the SW through the evening, Gale warnings continue as issued except Boston Harbor/Narragansett Bay where small craft advisories have been issued. Winds may briefly drop even below small craft thresholds early morning, but then rebound after sunrise. Once winds shift to the NW tomorrow late afternoon/evening gusts increase once more, but should remain generally below Gales. Seas drop overnight but remain at or near 5 ft mainly on the SE waters. In any case, small craft advisories will need to follow Gales. Rainfall/fog over the waters early tomorrow. Tomorrow night...High confidence. NW winds continue to gust to around 25 kt and allow seas to briefly build to about 8 ft on the outer ocean waters. Small craft advisories are likely to continue. Outlook /Thursday through Sunday/... Thanksgiving Day: Winds less than 25 kt. Local rough seas. Thursday Night through Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Gale Warning until midnight EST tonight for ANZ231>235-237-250- 251-254>256. Small Craft Advisory until 10 PM EST this evening for ANZ230. Small Craft Advisory until 1 AM EST Wednesday for ANZ236.
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&& $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Sipprell AVIATION...Doody/Sipprell MARINE...Doody/Sipprell

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