Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240217 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1017 PM EDT Fri Jun 23 2017 .SYNOPSIS...
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Mild, humid air continues tonight out ahead of the remnants of Cindy which will bring the threat of showers and thunderstorms through the first part of Saturday. A cold front will sweep the remnants of Cindy out to sea through late Saturday, improving overnight. Seasonably warm and less humid conditions Sunday will be followed by cooler weather and few diurnally driven showers and isolated thunderstorms Monday and Tuesday. A return to summer-like warmth and humidity is expected by the end of the work week.
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&& .NEAR TERM /UNTIL MIDNIGHT TONIGHT/...
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10 pm update... Let`s keep it simple. Scattered showers and thunderstorms are expected tonight given the broad tropical moist plume and the remnants of Cindy out ahead of a mean long-wave mid-level trough axis digging S/E out of the Great Lakes. Particular focus is on any forcing mechanisms, and broadly speaking there are two of focus: 1.) Convergent forcing and subsequent frontogenesis along the nose of H925-85 winds which are starting to amplify with the tightening baroclinic zone between the aforementioned trough axis and NW Atlantic high pressure. Exacerbating theta-E convergence beneath favorable venting per westerly flow aloft. And 2.) Along and ahead of a cold front sweeping S/E parent with the mid-level trough. Expecting overnight clusters of showers and thunderstorms that may pose an isolated, localized threat of nuisance flooding. None of the forecast guidance is quite locking in on one area. Just given the general synoptics and the sultry, tropical air- mass, the upstream activity over the Mid-Atlantic should maintain with the absence of dry air downstream as observed from latest 0z soundings. There aren`t any clear indications as to why scattered wet-weather will not continue overnight. So chance PoPs continue accordingly. The much stronger signal comes towards morning as the remnants of Cindy are flushed N/E with the mean wind out ahead of the sweeping cold front which acts as a lifting mechanism. Ahead of the mid-level long-wave trough axis and parent elongated vortmax, indications of forced ascent beneath falling heights and an accompanying jet streak. It is these sharp dynamics that strongly indicate the likelihood of outcomes, especially across SE New England where there is the possibility of marginal instability that could aid in the invigoration of Cindy remnants, yielding the development of strong thunderstorms, possible severe weather. Then again it may just end up benign. The simple takeaway here is to keep our heads up, maintain an awareness, and be prepared for anything. As the previous forecaster alluded to, this is a tricky forecast. We are dealing with an anomalous setup with very high atmospheric moisture content, and with any squeeze put on the environment, and the stronger that squeeze, there is the threat for some measure of flooding, and incorporating instability and shear as was observed upstream earlier today, there is the possibility of strong to severe weather. We`ll keep analyzing it through the night, especially as 0z guidance becomes available. Please see the discussion below which touches on Saturday interpreted from earlier forecast guidance. Previous discussion... Overnight lows will remain mild, dropping into the upper 60s to low 70s. This may result in another round of fog and stratus with the highest confidence across the south coast once again. May need to watch for a possible dense fog advisory as surface dewpoints will be in the 70s. Muggy to start the day tomorrow but depending on how quickly the front will push through, the later half of the day will begin to improve. Highs will remain in the mid 80s.
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&& .SHORT TERM /MIDNIGHT THROUGH 6 PM SATURDAY/...
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Into Saturday... * Increasing localized flood potential for Southern New England Active weather pattern for the first half of tomorrow as S New England remains in the warm sector for this period. Biggest focus is on precipitation potential. Increasing moisture will begin to pool into the area with PWAT values increasing to above 2.5 inches. This tropical moisture from Cindy will push PWAT near 3 to 4 STD above normal. In fact, moisture flux values at 850 mb during this timeframe is nearing 4 STD above normal. Thus within any heavy shower, the potential for flooding rains is possible. Models are struggling with the placement of the axis of heavy precipitation as there are several features to note. Aside from the approaching front out west, a LLJ will be across the southeast portion of the area. However, the higher instability values will be across the western half of the area. If both of these features could align, then we could see widespread heavy rainfall. But because there is some separation, believe that the threat is more localized. Again the heavy rainfall axis amongst the guidance are in different locations, but there is agreement that the potential for over 1.5 to 2 inches. If this falls in just a few hours, then flash flooding is possible. Best timing for this to occur will be after midnight and into Saturday morning. Cold front sweeping through the region. Latest hi-res guidance is a lot slower than synoptic models so have blended the forecast down the middle. More widespread showers and embedded thunderstorms are anticipated along the frontal passage. Convection along the front continues to be a possibility especially across eastern MA during the late morning hours. CAPE values will be near 1000 J/kg with 40 kts of 0-6 km shear. The caveat is poor lapse rates and the bulk of the moisture will be pushing offshore. One thing to watch for is the potential for a quick spin-up thanks to the Tropical airmass with 70F dewpoints and lower LCLs. Hodographs do show some helicity and good 0-1 km shear. The Frontal system should push through by Saturday afternoon resulting in clearing skies and drier air as westerly flow takes hold.
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&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Highlights... * Seasonably warm/less humid Sun with a spot shower/t-storm possible * A few showers/iso t-storms possible Mon/Tue but not a washout * Below normal temps Mon/Tue return to summerlike readings Thu/Fri Details... Saturday night... Dry and a much less humid airmass will work into the region behind Saturday`s cold front. This should allow low temps to fall into the 50s across many outlying locations, to between 60 and 65 in some of the urban centers. Sunday... A pleasant end to the weekend across southern New England. 850 temps between +11C and +12C should allow highs to reach into the lower to middle 80s in most locations, but dewpoints in the 50s will make it feel much more comfortable outside. Mainly dry weather anticipated, but approaching shortwave may be enough to trigger an isolated shower/t-storm or two by mid-late afternoon across the interior. Monday and Tuesday... Anomalous upper trough/cold pool aloft overhead will result in slightly below normal temps with comfortable humidity levels. Highs mainly in the 70s are expected both days. The cold pool aloft/shortwave energy will result in the potential for a few diurnally driven showers/isolated thunderstorms both days. Not expecting a washout by any means, but the main risk for any activity will be during the afternoon/early evening hours. Wednesday... Axis of upper trough will probably have shifted to the east of the region. This should allow temps to warm a few more degrees than Mon/Tue and result in mainly dry weather. However, if trough ends up slower than currently expected there would be the risk for a few more showers/isolated thunderstorms. Thursday and Friday... Upper trough lifts northeast of the region with rising height fields. This should result in a warming trend for the end of the work week, with a return to summerlike warmth and humidity. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...Moderate confidence. Tonight into Saturday...Moderate confidence. MVFR to IFR conditions expected into Saturday morning. Low risk of LIFR conditions along the south coast in fog, but enough wind in the boundary layer may prevent that from happening. Scattered showers and thunderstorms mainly late tonight into early Sat afternoon. Improving conditions from west to east around midday ahead of a wind shift from the W. Prior to the wind shift will continue to see SW winds with potential gusts 25 to 30 kts LLWS possible for SE coastal terminals as is some embedded TSRA. Improving towards evening to low-end VFR / SKC. KBOS Terminal...Moderate confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Outlook /Saturday Night through Wednesday/... Saturday night through Wednesday...High confidence. Mainly VFR conditions other than a few mainly diurnally driven showers and perhaps isolated t-storms. && .MARINE...
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SW gusts up to 25 to 30 kts through early Saturday prior to a cool frontal passage. Waves building around 5 to 7 feet on the waters as the remnants of Cindy are advected NE around the morning hours. Prior to and during this time, likely to be either a combination of low clouds and fog with visibility down to a quarter mile or less possible. Improving late Saturday as winds become westerly while beginning to diminish along with wave action. Outlook /Saturday Night through Wednesday/... Saturday night through Wednesday...Moderate to high confidence. Quiet boating weather overall during this time period. A few hours of near shore lower 20 knot wind gusts and choppy seas are possible on both Sun and Mon afternoons.
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&& .TIDES/COASTAL FLOODING... Astronomical tides are rather high through this weekend during the night time cycles. Boston has a 12.3 ft tide just before midnight tonight, a 12.4 ft high tide around midnight Sat night and 12.2 ft just after midnight Sun night. While, offshore winds are forecast current conditions suggest a 0.4 surge which will result in minor splashover. Thus will go ahead an issue a coastal flood statement for tonight`s high tide. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 3 PM EDT Saturday for ANZ231>234. Small Craft Advisory until 8 PM EDT Saturday for ANZ235-237- 250-254>256.
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&& $$ SYNOPSIS...Frank/Dunten NEAR TERM...Frank/Sipprell SHORT TERM...Dunten LONG TERM...Frank AVIATION...Frank/Dunten MARINE...Frank/Dunten TIDES/COASTAL FLOODING...WFO BOX Staff

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