Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 192018 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 418 PM EDT Mon Jun 19 2017 .SYNOPSIS... A warm and very humid day will bring the potential for scattered severe thunderstorms and localized flash flooding this afternoon and evening across interior southern New England ahead of a cold front. This front will move offshore Tuesday. Another weak front moves across Southern New England Wednesday. High pressure then builds over the region later Wednesday and Thursday with seasonable temperatures and lower humidity. Warmer temperatures return late in the week as the high moves off to the east. Another cold front crosses from the west over the weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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415 pm update... * Severe Weather/Localized Flash Flood Threat continues within southern New England between 2 and 10 pm this evening with the greatest risk across the interior */ Severe Weather Potential: Showers and thunderstorms ahead of an approaching cold front continue to develop and evolve late this afternoon. At 3 pm, the cold front extended from BTV southwest to near BGM, and on into eastern PA. Scattered showers and thunderstorms have traversed thru western MA/CT, with additional convection approaching from the west. The main threat for severe will continue to be across interior southern New England, with the greatest concern Worcester County and west in MA, and across northern CT. Despite weak mid level lapse rates, high temps in the 80s coupled with dewpoints in the upper 60s to the lower 70s are allowing for MLCapes of 1000-2000 J/KG across the interior. The strongest synoptic scale forcing does not look to arrive until early this evening, with a low level jet of 40-50 kts coupled with deep moisture and moderate instability. This should allow for a continuation of scattered showers and thunderstorms within southern New England into the evening hours. The potential exists for severe weather through 10 pm this evening. The main threat will be damaging wind gusts, but hail can not be ruled out with strongest storms. Given high dewpoints/low LCLs along with 0 to 1 km helicity of 100 to 250 units, a tornado is certainly possible. Whether or not this occurs will depend on timing and many mesoscale processes, but will have to be watched. A few strong storms may survive through Central MA, but weak mid level lapse rates should limit severe weather across eastern MA and RI. Main activity may not even reach the I-95 corridor until late evening or even after midnight. */ Flash Flood Threat... The other primary concern will be the potential for localized flash flooding. Precipitable water is around 2 inches ahead of the cold front and extending into southern New England. Surface dew points are in the upper 60s to low 70s. In addition, upper level winds parallel to the front will bring the potential for training showers and thunderstorms, hence enhancing the potential for localized heavy rainfall resulting in localized flash flooding. Rainfall rates could exceed 2 inches in an hour. Will continue to need to monitor the Hartford/Springfield corridor for the potential for urban flash flooding this evening. Severe thunderstorm threat should come to an end mid to late evening, although sufficient instability lingers to warrant continued chance for some thunderstorms past midnight. Cold front will gradually work its way eastward tonight, likely only crossing part-way thru southern New England prior to daybreak. Models indicating front should cross east of I-91. Drier air will be working into the area behind the front. */ Fog along south coast... There has been some lingering fog this afternoon in the vicinity of the Cape and Islands, and south coastal RI. Still some locales with visibility around 3/4 mile. Should see a bit more improvement this afternoon, but then fog should re-materialize this evening. Could have localized visibility down to 1/2 or 1/4 mile at times overnight in that area.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
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Cold front pushes east of southern New England during the day. Much drier air works its way into the region. Low chance for scattered to isolated showers eastern MA/RI, mainly in the morning. However a cap around 10-15 kft accompanied by drier air aloft will limit the vertical extent of any convection. Breezy with W/SW wind gusts 20-25 mph.
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&& .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
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Big Picture... A mainly zonal pattern over the Northern USA through the week. A shortwave within this flow moves across New England during midweek and off to the east Thursday. Closed low ejects from the Gulf of Alaska through Canada by midweek and swings a shortwave axis through New England toward Saturday. Another shortwave approaches from the west on Monday. Contour heights are near normal midweek, and then climb above normal late week and the weekend. Expect temperatures near normal midweek and above normal by the weekend. Model mass fields show agreement through Thursday, with the ECMWF and GFS in general agreement through Friday. Increasing differences, especially regarding timing of weather features, and resulting decreasing confidence after Friday. Details... Tuesday night-Wednesday...Moderate-High confidence. RH fields and cross sections show some lingering shallow moisture Tuesday night. We will be in the right entrance region of the upper jet, which will provide upper venting, but with little to work on we will expect patchy clouds and no pops. Upper trough and its cold pool move overhead Wednesday. The surface reflection will provide some low level convergence, while the cold air aloft will combine with daytime heating to destabilize the airmass. Moisture remains limited but concentrated near the top of the mixed layer. Lifted Index shows negative values while Totals show mid to upper 40s and a couple of patches around 50. We will show low-end chance pops for showers/tstms. Any showers may linger into the evening but then diminish. Mixing to 775 mb with temperatures supporting sfc temps in the lower 80s. Thursday...Moderate-High confidence. High pressure builds over the region. Expect dry weather with temps aloft supporting upper 70s and lower 80s. Friday...Moderate confidence. Diverging mass fields depicted by the various models, but all agree on surface low pressure passing across Northern Quebec. This puts our area in the warm sector with dew points back in the 60s. Upper jet is over or north of the St Lawrence Valley and rather far off for dynamic support. But consensus model sfc pressure and thickness fields show a lee trough along the coastal plain with cold front trailing in the Eastern Great Lakes. Lifted Index is negative and Totals are in the upper 40s. With this pattern we expect showers with scattered thunder, especially Friday afternoon/evening. The actual cold frontal passage looks like either later Friday night or early Saturday. Saturday through Monday...Low confidence. Model mass fields continue to diverge, but consensus suggests the cold front moving offshore early Saturday with an end to precipitation and slight lessening of the dew points to near 60 by Saturday evening and into the 50s Sunday and Monday. Weak high pressure surface and aloft should bring dry weather Saturday night and Sunday. Of concern are signs of shortwaves in the flow that might cross New England Sunday night and Monday. These would support a chance of showers as they pass. We will show chance pops for showers much of the area Sunday night which lift out to the northeast Monday.
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&& .AVIATION /20Z MONDAY THROUGH SATURDAY/...
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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 Tuesday/... This evening and Tonight...Moderate confidence. This evening (thru 02Z), SCT SHRA/TSRA especially across W MA and CT. TSRA likely to be strong to severe with RA/+RA and accompanying MVFR/IFR vsbys. Activity slow to push east, will likely not reach eastern MA/RI until 0-3z. Overall a mix of MVFR and VFR conditions prevailing, with MVFR to IFR conditions anticipated with any +RA/TSRA. Exception is periods of IFR/LIFR conditions along the south coast in fog and in any +RA / TSRA. After 02Z, cigs/vsbys deteriorating to low MVFR/IFR except potential for LIFR along south coastal MA and within RI. Scattered showers and isolated Thunderstorms linger into the overnight. Tuesday...Moderate Confidence. IFR cigs/vsbys in low clouds and fog to start, improving to MVFR/low VFR during the morning and then VFR for the afternoon. Exception is along south coastal MA/RI, where IFR conditions thru the morning should gradually improve to VFR in the afternoon. KBOS Terminal...Moderate confidence. VFR with potential for MVFR/IFR cigs/vsbys in low clouds and scattered TSRA after 23Z and into the evening. Tonight anticipating MVFR/IFR conditions in low clouds and patchy fog, improving to VFR by midday Tue. KBDL Terminal...Moderate confidence. TSRA in vicinity and then crossing thru BDL thru around 02Z. Mainly VFR, with potential for MVFR/IFR in any TSRA/+SHRA. Then conditions deteriorating to IFR after midnight in low clouds and fog. Improving to VFR Tue am. Outlook /Tuesday Night through Saturday/... Tuesday night through Thursday...Moderate-High confidence. VFR. Developing daytime cumulus clouds Wednesday with scattered or widely scattered showers/thunder. One of these showers may briefly lower vsbys to MVFR levels. Southwest wind Tuesday night/Wednesday gusting to 20 knots...turning from the west late Wednesday and becoming variable Thursday. Patches of IFR in fog are possible each night, especially on the South Coast. Friday... Moderate confidence. Cold front to our northwest with warm humid air in place over our area. Scattered showers/tstms possible, especially in the afternoon/evening. Generally VFR, but with brief MVFR/IFR in afternoon/evening showers/tstms. Southwest winds gusting to 20 knots. Saturday... Low confidence. Conditions improving to VFR as the cold front moves offshore.
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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 Tuesday/... Today and tonight...High confidence. Strong low level jet will result in S/SW wind gusts of 20 to 30 knots into tonight. Persistent S/SW flow will build seas to between 5 and 8 feet across our open waters. Small craft headlines remain posted for all waters. The other big concern for mariners will be areas of fog, which will be locally dense at times across our S waters. Scattered thunderstorms are possible across the waters late this evening, diminishing to showers Tuesday morning. Tuesday...Moderate confidence. Overall SW winds around 10 to 15 kts with gusts 20 to 25 kts, with strongest winds during the day. Seas 4-7 ft linger thru the day on the outer coastal waters. SCA headlines posted for outer coastal waters as well as RI/BI Sounds on Tuesday. Outlook /Tuesday Night through Saturday/... Tuesday night and Wednesday...Moderate-High confidence. Winds from the southwest with gusts reaching 25 knots on the Cape and Islands waters Wednesday. Winds shift from the west late Wednesday as a weak cold front moves through. Possible shower or thunderstorm with the front as it moves through. Seas 5 to 6 feet on the outer waters and on RI and Block Island Sounds. Thursday...Moderate-High confidence. Winds diminish and become shifting as high pressure moves overhead. Seas 4 feet or less. Friday-Saturday...Low-moderate confidence. Cold front in the St Lawrence Valley sweeps southeast, crossing the waters Friday night or Saturday morning. Areas of poor visibility in fog Friday, then locally poor in scattered showers/thunderstorms Friday evening/night as the cold front moves through. Non-thunderstorm winds will gust from the southwest 20-25 knots Friday afternoon and night ahead of the front, then less than 20 knots from the west-northwest Saturday. Seas less than 5 feet both days.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Flash Flood Watch until 4 AM EDT Tuesday for CTZ002-003. MA...Flash Flood Watch until 4 AM EDT Tuesday for MAZ002>004- 008>012-026. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ231>234-251. Small Craft Advisory until 2 AM EDT Tuesday for ANZ230-236. Small Craft Advisory until 5 PM EDT Tuesday for ANZ235-237-250- 254>256.
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&& $$ SYNOPSIS...WTB/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...WTB AVIATION...WTB/NMB MARINE...WTB/NMB

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