Area Forecast Discussion
Issued by NWS Boston, MA

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034 FXUS61 KBOX 231950 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 350 PM EDT Fri Jun 23 2017 .SYNOPSIS... Mild, humid air continues into tonight out ahead of the remnants of Cindy which will bring the threat of heavy rain and possible flooding Saturday morning. 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/isolated thunderstorms Monday and Tuesday. A return to summerlike warmth and humidity is expected by the end of the work week. && .NEAR TERM /UNTIL MIDNIGHT TONIGHT/... 400 PM Update... Clearing skies early this afternoon has resulted in CAPE values increasing to over 2000 J/kg. This combined with shear values of 30- 35 kts and low level lapse rates near 8 C/km has resulted in a few scattered showers. Current Dynamics and SPC meso-analysis indicates that the main risk could be heavy rain and gusty winds. Skinny CAPE and inverted V soundings would also indicate the potential for wet microburst. While most of the region will remain dry this afternoon, the risk for isolated showers will continue through the evening. While risk is low, cannot rule out a quick spin-up as dewpoints are in the 70s. However appears the better helicity and 0-1 km shear will be southwest of the area. Something to watch over the next few hours. While it is difficult to pin where exactly and strong storm will develop, believe the CT valley will be the area to watch over the next few hours, esp with the help of the terrain. This evening... Warm, moist southwest flow will continue across the region this evening into the overnight hours. PWAT values are increasing to 2 inches as tropical airmass continues to pool into southern New England. Biggest question is shower potential for this evening. Increasing LLJ at 925 mb as well as lingering instability should be enough will to trigger scattered showers and thunderstorms across the region. This is support in the hi-res guidance and the latest HRRR. Otherwise a muggy evening is on tap for the region as dewpoints remaining in the 70s. Will have to watch for fog develop along the south coast. && .SHORT TERM /MIDNIGHT THROUGH 6 PM SATURDAY/... After midnight into Saturday... *** Increasing localized flood potential for southern New England *** Active weather pattern for tonight and for the first half of tomorrow as southern New England remains in the warm sector for this period. 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. Biggest focus is on precipitation potential for this time period. Increasing moisture will begin to pool into the area with PWAT values increasing to above 2.5 inches by tonight. 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 will begin to sweep through the region on Saturday. The 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. && .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 /20Z FRIDAY 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. Today... VFR for most of the region with iso MVFR within any shower/TSRA. Stratus and fog will overspread the south coast closer to sunset. Tonight into Saturday... MVFR-LIFR CIGs with MVFR-IFR VSBYs with BR/FG. Scattered SHRA/TSRA tonight approaching from the West, with more widespread potential Saturday morning. 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...High Confidence. VFR today with gusty SW winds up to 25 kts, lower CIGs into this evening with increasing rain/thunder chances. KBDL Terminal...VFR CIGs today with blustery SW winds that will lower into evening prior to onset of RA/+RA. 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... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... SW winds continue to gust up to 25 to 30 kts through early tomorrow prior to a cool frontal passage. Waves building around 5 to 7 feet on the waters as the remnants of Cindy are advected NE across the waters 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. && .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... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 3 PM EDT Saturday for ANZ231>234. Small Craft Advisory until 9 PM EDT this evening for ANZ230- 236. Small Craft Advisory until 8 PM EDT Saturday for ANZ235-237- 250-254>256. && $$ SYNOPSIS...Frank/Dunten NEAR TERM...Dunten SHORT TERM...Dunten LONG TERM...Frank AVIATION...Frank/Dunten MARINE...Frank/Dunten TIDES/COASTAL FLOODING...Staff

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