Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 171118 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 718 AM EDT Sat Jun 17 2017 .SYNOPSIS...
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Southerly flow round a Bermuda high will result in increasing humidity levels this weekend. Most of the weekend will be dry, but a few brief showers will be possible especially this morning along the southeast New England coast. An approaching cold front will likely bring a round of showers/thunderstorms with locally heavy rainfall possible sometime later Monday into early Tuesday. Seasonable temperatures with lower humidity will follow mid to late next week but with a continued risk of showers at times.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 am update... Low visibility and cloud decks that are likely to improve through the day except along the S-coast of New England. The warm front presently situated SW to NE along the Cape Cod Canal will gradually lift N through the day behind which winds will veer from NE to SE while at the same time conditions improve. Showers across the Cape and Islands, with some embedded heavier rain, will lift N out of the region. While a spot shower is possible during the day, will remain mostly dry. See the previous discussion below. Previous Discussion... The bulk of the showers across southeast New England should die down by mid to late morning, but the forecast is a bit tricky this afternoon. Somewhat of a dirty upper level ridge with high Pwats and weak warm advection. Despite relatively weak forcing, deep moisture should result in a few spot showers continuing into this afternoon with the low risk of an isolated t-storm. Nonetheless, the majority of the day will be dry in a given location given the limited forcing. As for temperatures, the surface warm front will be very slow to lift north today especially in northeast MA. This may result in some fog lingering through the morning across northeast MA. Otherwise, an abundance of clouds will persist into the afternoon, but a few peeks of sun are possible especially across the interior. High temps will mainly be in the 70s with the warmest readings in the lower CT River Valley where some locations may reach 80 if enough peeks of sun develop. Across northeast MA, temps may be in the 60s much of the day with probably late afternoon/early evening high temps in the lower 70s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
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Tonight... A very moist southerly flow of air will result in a rather mild and humid night. Low temps will only drop into the middle to upper 60s in most locations. Weak warm advection in a high Pwat environment may result in a spot shower/t-storm or two, but the vast majority of the night will end up dry in a given location with limited forcing. Some fog may develop near the south coast, Cape and Islands but enough wind will probably keep it from becoming widespread/dense in nature, but something will have to monitor closely. Sunday... Strong upper level ridge off the mid Atlantic coast will result in a warm and humid day across the region. Despite lots of clouds, will probably see some peeks of sunshine especially in the afternoon. This will probably push highs into the 80 to 85 degree range away from the marine influence of the south coast, Cape and Islands. Developing low level jet and afternoon mixing should yield afternoon southerly wind gusts of 25 to 30 mph with perhaps a few gusts up to 35 mph possible on the coastal plain. As for precipitation chances, upper level ridging and lack of forcing will result in mainly dry weather across our region. However, given continued high Pwat airmass in place certainly can not rule out a brief spot shower/t-storm or two, but dry weather is anticipated for the vast majority of the day.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... */ Highlights... - Heavy rain and possible flood threats Monday night into Tuesday - Thunderstorms possible Wednesday, could be strong to severe - Back and forth pattern the remainder of the week into the weekend */ Overview... Potential threats within a more amplified pattern during the early week period relaxes during the later half of the week into the week- end as the maritime flow becomes more progressive. However overall, ensemble members and parent means emphasis a persistent H5 troughing pattern up against a persistent high over the N/W Atlantic. Simply put, expecting an active weather pattern between airmass exchanges of warm, humid versus cool, dry. */ Details... Sunday Night through Tuesday... Heavy rain, possible flood threat. Expect an increasing sub-tropical connection through the parallel SW flow throughout the atmospheric column. GEFS / SREF both indicating +2 standard deviation (SD) H925- 7 S/SW flow, +4-5 SD of H85 moisture flux, and precipitable waters up to 2-inches into S New England. This as freezing level heights hover around 14 kft. Given any convergent forcing with the H925-7 SW flow would anticipate wet-weather outcomes. This ahead of a broad troughing signature across the Great Lakes Region W of persistent, stout high pressure out across the NW Atlantic. However not all is clear cut. A few struggles: 1.) Not a clean, moist sub-tropical airmass into S New England as broader troughing over S Canada into the Great Lakes neighbors against the persistent Bermuda high. Indications of drier air mixing into the SW moist plume. 2.) Forecast guidance continues to struggle with the occlusion over S Canada and subsequently the timing of the cold front across the region. And 3.) parent with the struggles, understanding the measure of forcing and ascent across the region and possible outcomes / threats. So, Sunday night into Monday it would appear stable, higher heights across the region neighbored against an influential Bermuda high and accompanying ridge, in the absence of stronger ascent, some shower activity is to be expected, possibly heavy given the increasingly moist environment and threat of convergent forcing per low-level winds. Nothing seemingly likely and difficult to nail down precisely where and when, will go with chance PoPs thinking any activity will either be isolated to scattered. Still an eye on the late afternoon period, as to upstream activity and whether we may possibly see a destabilizing environment N/W supportive of thunderstorm activity. Will leave that discussion for later focusing on possible flooding threats noted below. SPC has us in a Day-3 outlook of SLIGHT to ENHANCED, but considering stable heights, drier air mixing in, and marginal lapse rates, feel the threat will be W right on our door- step. As to Monday night into Tuesday, greater likelihood we`ll see heavy rain, possible thunderstorms. Certain we`ll see lift along the cold front slowly sweeping across the region beneath falling heights and the first in a series of mid-level H5-7 vortices. The emphasis of ascent upon an increasing SW fetch of precipitable waters up to 2- inches as freezing level heights hover around 14 kft, would expect an environment supportive of efficient warm rain processes, more so with any instability and/or low-level convergent forcing. Though the forecast guidance struggles with the timing of the cold front, more than likely due to the surface occlusion over S Canada, given the expectation of a slow moving cold front, a rich moist environment, and SW training of outcomes, potential flood threats are possible. Likely PoPs though lesser on thunderstorms, thinking chance, will highlight the possible flood threats in the HAZARDOUS WEATHER OUT- LOOK. Interrogating ensemble probabilistics, N/W portions of New England may be under the greatest threat as the cold front slows over the region per occlusion and loses the emphasis of lift with more parallel SW flow. However considering training aspects over time, the threat is considered for the entire region given how far out we are in the forecast period. Wednesday... Thunderstorms possible, some could be strong to severe. The second of a series of mid-level H5-7 vortices sweeps the region along with the main H5 trough axis. A more continental-origin airmass, drier, with steeper lapse rates aloft, with boundary layer destabilization within an environment of faster flow, stronger 0-6 km bulk shear of around 40 kts, fairly uni-directional and parallel to the 0-6 mean wind, there is the possibility of wind-damage threats. However instability does not look all that great per BUFKIT profiles. Very thin, in some cases tall, the highest values top out around 1k j/kg. Yet given strong forcing ahead of a reinforcing cold front, looking at a convective mode of multi-cell clusters involving into segments and squall-lines. Some early thinking. A low confidence forecast given the uncertainty of synoptics, especially the sweeping cold front, in the earlier period. Into early next week... On and off shower chances. Progressive mid-latitude flow of Pacific- origin disturbances that progress E latching into SW moist flow round the persistent N/W Atlantic high, will see mild, humid periods of wet weather ushered by S flow followed by comfortable, dry conditions with associated high pressure and N flow. A weak amplitude pattern, nothing stands out with respect to threats and impacts. && .AVIATION /11Z SATURDAY THROUGH WEDNESDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...Moderate confidence. IFR/LIFR CIGs and VSBYs improve to MVFR / low-end VFR towards noon except along the S-coast and perhaps NE MA. BKN-OVC CIGs for all terminals. Winds veering out of the S and increasing as is presently observed out on the Cape. Lower conditions may return just prior to evening. Tonight...Moderate confidence. Lowering back down to IFR/LIFR. Lowest conditions will likely be found across the S coast, Cape and Islands with areas of fog forecast. Sunday...Moderate confidence. Widespread low end MVFR-IFR conditions early Sunday morning should improve to mainly MVFR to even VFR for a time Sunday afternoon. SW wind gusts of 20 to 30 knots are expected to develop by afternoon with the strongest of those winds along the coastal plain. KBOS Terminal...Moderate confidence. IFR/LIFR with NE winds. Once those winds begin to veer E/SE will conditions improve and that may not occur until the 16-18z timeframe. Thus low clouds / fog linger. When winds do turn S expect a quick improvement towards possibly low-end VFR and winds to become breezy. KBDL Terminal...Moderate confidence. IFR conditions this morning should improve to MVFR with perhaps even some VFR conditions this afternoon. Outlook /Sunday Night through Wednesday/... Sunday night through Tuesday... MVFR/IFR mix on CIGs and VSBYs. SCT SHRA Sunday night into Monday, likely -RA/RA Monday night into Tuesday. SW winds sustained 15 to 25 mph, gusts up to 30 mph, strongest across S/SE coastal terminals. Possible LLWS impacts. Improving late into Tuesday night. Wednesday... SCT-BKN low-end VFR CIGs with SHRA/TSRA possible. SW flow 10 to 15 mph with gusts up around 20 to 25 mph. Winds turn W late as CIGs improve overnight. Thursday... VFR. Increasing, lowering CIGs late ahead of SHRA overnight. W winds becoming SW 10 to 15 mph, gusts up to 20 mph.
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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 Sunday/... 7 am update... Likely conditions will be slow to improve over the waters this morning, especially over the E waters given the NE wind N of the warm from SW to NE settled over roughly the Cape Cod Canal. No indications of rapid improvement. As the warm front lifts N, expect winds to increase out of the S. Already reports of gusts nearing 25 kts over Nantucket. Today and tonight...Moderate confidence. Warm front will continue to lift very slowly to the N through early this evening. While pressure gradient will be weak enough to keep winds in check, seas across many of our open waters will remain above small craft thresholds as a result of lingering swell. Areas of fog also expected mainly this morning and again later tonight. Sunday...Moderate confidence. Increasing low level jet will result in south to southwest wind gusts of 20 to 30 knots developing by afternoon. Strongest of those winds will be near shore, given better mixing over the land. Have hoisted small craft headlines for all waters, with a special focus on the near shore waters Sunday afternoon for recreational boaters. Outlook /Sunday Night through Wednesday/...Moderate Confidence Sunday night through Tuesday... A slow moving cold front sweeps the waters late into Tuesday night. Likely ahead of which SW winds will be sustained around 15 to 20 kts with gusts up to 30 kts. Increasing humidity, likely lower clouds and reduced visibility prior to the greatest threat of rain ahead of the cold front late Monday night into Tuesday. Waves 6 to 8 feet on the outer waters throughout. Wednesday... Improved boating conditions with persistent S flow 10 to 15 kts that turns W behind a sweeping cold front late. Possible shower activity ahead of the cold front. Waves below 5 feet. Thursday... Good boating weather. Initial W winds becoming SW, sustained around 10 kts with gusts up to 20 kts. Increasing clouds out of the W through the day with the chance of showers late.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 11 AM to 8 PM EDT Sunday for ANZ230>234-236-251. Small Craft Advisory until 8 PM EDT Sunday for ANZ235-237-250- 254>256. && $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Frank/Sipprell SHORT TERM...Frank LONG TERM...Sipprell AVIATION...Frank/Sipprell MARINE...Frank/Sipprell

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