Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191944 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 344 PM EDT Mon Jun 19 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 1015 am update... Latest radar imagery showed scattered showers and thunderstorms in western MA and vicinity, with some redevelopment. Have upped pops to chance/likely in that area for late this morninng. Otherwise only minor adjustments made to temps and sky cover based on latest trends, and forecast looks on track. Previous discussion follows... First issue this morning is fog along the south coast, Cape and Islands. While a few locations have reported dense fog, more wind in the boundary layer has prevented it from becoming as dense as early yesterday morning. Therefore, will not issue an advisory but handle it with a SPECIAL WEATHER STATEMENT. Fog / misty conditions are likely to persist along the S-coast a majority of today. While many locations will remain dry this morning, widely scattered showers/thunderstorms may develop at any time. MUCapes already over 1000 J/KG early this morning and with a decent low level jet. Therefore, certainly the potential for some widely scattered convection especially into mid morning. Otherwise, low clouds should begin to breakup by late morning away from the south coast. This should allow high temps to warm well into the 80s away from the south coast by mid afternoon under very humid conditions. Gusty south to southwest winds of 25 to 35 mph with perhaps even a few gusts up to 40 mph should develop by mid afternoon. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... */ Highlights... * Severe Weather/Localized Flash Flood Threat continues across interior southern New England between 2 and 10 pm this evening with the greatest risk along and west of I-91 * Main concern is the severe weather/localized flash flood potential across interior southern New England from mid afternoon into this evening. Will break things down in greater detail below. */ Severe Weather Potential: The main threat for severe will be across interior southern New England, with the greatest concern along and west of I-91. Despite weak mid level lapse rates, high temps in the 80s coupled with dewpoints in the upper 60s to the lower 70s should result in MLCapes of at least 1500 J/KG across western MA/northern CT. While the strongest synoptic scale forcing will not arrive until early this evening, impressive 45 knot low level jet coupled with deep moisture/moderate instability should trigger scattered showers and thunderstorms across western MA and northern CT, perhaps by mid afternoon. The issue is that our western zones will initially be on the edge of the deeper moisture/stronger synoptic forcing. That is why our greatest area of concern is west of I-91, while dry weather prevails across eastern MA/RI this afternoon. By early evening, the stronger synoptic forcing will be working into western MA/CT so a large complex of showers and thunderstorms will likely be entering our far western zones. Given expected instability and bulk shear of 30 to 40 knots, the potential exists for severe weather across western MA/northern CT roughly between 2 and 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 LCL`s along with 0 to 1 km helicity likely in excess of 150 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 into Central MA, but weak mid level lapse rates will likely prevent severe weather across eastern MA/RI. In fact, main activity may not even reach the I-95 corridor until late evening or even after midnight. */ Flash Flood Threat... The other concern will be the potential for localized flash flooding. Pwats near 2 inches, strong low level forcing and upper level winds parallel to the front may result in training thunderstorms with torrential rainfall. This threat area is similar to the severe weather potential, focused across interior MA/CT with the greatest concern west of I-91. Given the ingredients, certainly possible to see localized rainfall rates exceed 2 inches in an hour which will bring the potential for localized flash flooding. In addition, will have to watch the Hartford/Springfield metro areas closely with a potential for urban flooding during the evening rush hour depending on when the main activity arrives. && .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 concetrated 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/... Today and Tonight...Moderate confidence. MVFR/LIFR through this morning, eroding across the interior while improving, though hanging low across the S-coast where low CIGs / FG persist. This as SW winds increase, 15 to 20 kts sustained with gusts to 30 kts by afternoon. By afternoon into evening, SCT SHRA / TSRA especially across N/W MA and CT. TSRA likely to be strong to severe with RA/+RA and accompanying IFR VSBYs. Activity slow to push east, will likely not reach eastern MA/RI until 0-3z. Overall, MVFR to IFR conditions anticipated tonight but LIFR conditions along the south coast and in any +RA / TSRA. KBOS Terminal...Conditions improve during the morning hours. Will hint at RA / TSRA moving into the terminal around 1z. Expected to weaken over the terminal during the overnight hours though some concern we will see redevelopment during the Tuesday AM push. KBDL Terminal...Conditions improve during the morning hours. Expecting a solid line of TSRA across the terminal around 21-0z. RA/+RA expected with TEMPO IFR. Gusty winds as well. 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. Strongest of the winds will occur near shore this afternoon where a few gusts near 35 knots are not out of the question. 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. An isolated thunderstorm or two is possible through mid morning, with a better chance of scattered thunderstorms late tonight through early morning 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... CT...Flash Flood Watch through late tonight for CTZ002-003. MA...Flash Flood Watch through late tonight 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. && $$ SYNOPSIS...WTB/Frank NEAR TERM...Frank/Sipprell/NMB SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Frank MARINE...WTB/Frank

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