Area Forecast Discussion
Issued by NWS Boston, MA

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905 FXUS61 KBOX 220155 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 955 PM EDT Wed Jun 21 2017 .SYNOPSIS... High pressure building over the region into Thursday. Warmer and more humid air will return Friday. A cold front will cross the region late Friday night and Saturday followed by seasonably warm but less humid conditions for the weekend. Cooler and somewhat unsettled weather returns early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 10 pm update... Shower and thunderstorm activity continues to dissipate over the waters ahead of a cool front. Near-term high-res analysis has MUCape values as high as 500 j/kg lingering ahead of the front while is allowing for some sustaining updrafts. Crux of mid-level vortmax energy lending to ascent ahead of the main H5 trough axis. For any mariners, main threats of frequent lightning and heavy downpours, otherwise a dissipative trend per weakening instability. Behind the front, high pressure building into the region with light westerly flow and drier air. Should see dewpoints continue to drop, and given the light winds and clear conditions, an opportunity for temperatures to radiate out down to lows on average around the mid to upper 50s. The urban centers in the low 60s, while areas notorious for radiating out efficiently get down into the low 50s. Recognize that Nantucket is presently is under low cloud decks. These should erode as winds become westerly and dry air moves in, roughly around midnight. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday and Thursday night... Weak high pressure remains over the region as mid-level ridge begins to build overhead. Overall a dry weather day with westerly winds to start switching to the south through the afternoon. 925 mb winds may be just strong enough to keep any sea breeze at bay, but they do drop off in the late afternoon, so a late sea breeze is possible across the MA east coast. Temperatures will be similar to today with highs in the low to mid 80s. Southerly flow will increase during the overnight hours resulting in more moisture for the region. Dewpoints will be on the rise as clouds increase across the region. Overnight lows will remain mild dropping only to the mid to upper 60s. Passing warm front in the mid- levels combined with increasing theta-e may be enough to trigger showers across the region. Highest confidence is across western zones after 06z. Cannot rule out isolated thunder as both showalters and LIs drop well below 0 and K values increase into the 40s. Perhaps an instability burst to trigger a sunrise surprise Friday morning. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Very warm and humid Fri with sct showers/t-storms interior * Gradually turning less humid this weekend with seasonably warm temps * Cooler early next week with showers or a thundershower possible at times. Overview and model preferences... Last vestiges of the subtropical Bermuda high will give giving way on Friday. This as Hudson bay rotates into Quebec and begins to establish longwave trof across the NE coast. With this initial wave passage, attendant low pres also slides into Quebec, forcing a warm, then cold front through the area Fri- Sat, yielding unsettled conditions. This shifts the mean jet over New England, where it looks to rest for most of the remaining long term. It also allows for a series of weak trofs to potentially impact the region into mid next week. Details... Friday... Warm front will be shifting into N New England through the morning, and noting that with a brief burst of instability with the warm fropa, could be an early round of convection ongoing into the mid morning hours. Noting better forcing remains N, but this early convection could dictate the convection risk during the afternoon given leftover cloud debris. In any case, with some clearing, dwpts rising into the mid-upper 60s, and cooling in the upper lvls in advance of the approaching wave could yield 1500j/kg SB CAPE values during the afternoon. The high dwpts and PWATS between 1.75-2.00 inches suggest another HPE mode for any convection. As previous forecaster noted, best forcing remains W, so convection may struggle to form even given the destabilization. Another somewhat mitigating factor are the meager mid lvl lapse rates which generally rest in the 5.5-6.0C/km range. Should any convection form in-situ or upstream and move into New England, the highest risk for severe would be with a wet downburst, or once again very heavy rainfall and localized flooding. With clearing, temps should easily reach the upper 80s and potentially even low 90s in a few spots. Fri night into Sat... Cold fropa associated with the low pres moving through Quebec. Given mid lvl flow parallels the front it may take most of the day Sat to fully clear S New England. Overnight, elevated instability remains as TTs hover near 50 with elevated K-indices. Therefore, risk for thunder may linger into the overnight especially as vort max slides across the region between the 06-12Z hours. Guidance is highlighting a QPF burst with this as well, raising confidence that some showers/storms may linger through the overnight/early morning hours. Daytime precip chances will be dictated by how efficiently the drier air can filter in behind the front and how much convection/moisture remains with the remnants of Cindy as it slides into PA/NJ. If the front slows, will need to watch this closely as it could be associated with a burst of very high PWATS and more convection. In any case, for now, will feature gradually diminishing POPS from NW-SE through the daylight hours as the front moves. The slow movement of the front should allow highs to once again reach the mid 80s as mid lvl cooling lags upstream. Lows in the 60s Fri night given little change in dwpts during the overnight hours. Sun and Sun night... Mainly dry as the mean jet settles across New England. The dry conditions stem from subsidence between stalled frontal boundary well S of New England and another shortwave passage through Quebec and N New England. Drier dwpts in the 50s make for pleasant conditions as highs remain near normal, in the low 80s. Mon into Tue... Two weak shortwaves slide through the mean jet Mon and Tue. This is accompanied by cold mid lvl temps, noting that H5 temps drop as low as -20C. Although forcing is relatively weak even with each of these waves, the cold mid lvl temps could yield diurnally driven showers and thunderstorms and if the freezing temps are low enough given the cool airmass sliding out of Quebec, can`t rule out some isolated hail in any of the heaver showers/storms. Will feature increasing POPs mainly each afternoon. Otherwise, temps may stay just below normal. Highs in the mid 70s. Mid next week... Ridging out of the W may finally begin to impact the region once again. Allowing forcing the unsettled wx further E. Still some time to see how the pattern breaks down, but will begin to weight the forecast toward this thinking. && .AVIATION /02Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Tonight...High confidence. VFR. SKC expect over ACK where IFR CIGs linger ahead of a wind shift shortly after midnight, SW to W. Thursday...High confidence. VFR. Winds generally light but backing to the south. Low confidence on sea-breezes along the E-coast. Thursday night...Moderate confidence. CIGs lowering and thickening, potentially to MVFR, as South winds increase along with -RA chances. Low chance of iso -TSRA. KBOS Terminal...Sea-breeze possible Thursday. VFR for the entire TAF period. KBDL Terminal...High confidence. VFR. Outlook /Friday through Monday/... Friday into Friday night...Moderate confidence. Some MVFR/IFR cigs/vsbys possible early Fri, otherwise VFR but brief lower conditions possible in any showers/t-storms in the afternoon and evening. MVFR/IFR stratus and patchy fog developing Fri night. Saturday...Moderate confidence. MVFR/IFR may linger into the morning, especially near the coast, otherwise trend should be for improving conditions from north to south. Sunday...High confidence. Mainly VFR. Monday...Moderate confidence. Mainly VFR but a spot shower or thunderstorm is possible during the afternoon/evening hours. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Another cold front will sweep the waters tonight. Seas across the outer waters will remain 5 to 6 feet. High pressure will follow behind the front, allowing for winds and seas to be below SCA. Light westerly winds winds tomorrow will turn to the south by the afternoon. Rain chances return to the region as a warm front lifts N across the area Thursday night. Outlook /Friday through Monday/...High confidence. Fri through Sat night...High confidence. A southerly component to the flow remains through the period. This will allow seas, mainly on the S and SE ocean waters to reach 5-7 ft by Fri night and linger into Sat night. Small craft advisories will likely be needed. During the day Fri, wind gusts to 25 kt are also possible on all waters, so small craft advisories are likely to be needed at least during the day on Fri on all waters. Weaker winds the rest of the period. Some fog/stratus may yield low visibilities at times. Sun and Mon...High confidence. Winds and seas should generally remain below small craft advisory thresholds through the period. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Thursday for ANZ254>256. Small Craft Advisory until 11 PM EDT this evening for ANZ235- 237. && $$ SYNOPSIS...Doody/Dunten NEAR TERM...Doody/Dunten/Sipprell SHORT TERM...Dunten LONG TERM...Doody AVIATION...Doody/Dunten MARINE...Doody/Dunten is the U.S. government's official web portal to all federal, state and local government web resources and services.