Area Forecast Discussion
Issued by NWS Boston, MA

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872 FXUS61 KBOX 232334 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 734 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 showers and thunderstorms mainly very late tonight into 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/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/... 725 pm update... Tricky forecast this evening as a cluster of showers and embedded thunderstorms were moving across portions of eastern PA and NJ. Some of the model guidance brings this activity northeastward into southern New England over the next few hours. This a result of increasing low level jet in advance of shortwave energy. However, the higher instability remains to our south which is often where the majority of the showers and thunderstorms tend to gravitate towards. So overall, a low confidence forecast this evening but tend to think the main complex of showers and thunderstorms may pass to our south this evening. Still may see some showers and isolated thunderstorms, especially across our southern zones. A better chance of more showers and thunderstorms with heavy rainfall very late tonight into the first part of Saturday, which is discussed in the next paragraph. Lastly, some patchy fog along the south coast, Cape and Islands tonight. However, enough wind may keep it from becoming widespread/dense overnight but still will need to monitor. && .SHORT TERM /MIDNIGHT THROUGH 6 PM SATURDAY/... After midnight into Saturday... *** Increasing localized flood potential for southern New England *** Active weather pattern for very late 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 /23Z 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. 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... 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. && .MARINE... 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...Dunten/Frank NEAR TERM...Frank SHORT TERM...Dunten LONG TERM...Frank AVIATION...Dunten/Frank MARINE...Dunten/Frank TIDES/COASTAL FLOODING...Staff

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