Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191125 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 725 AM 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. Weak high pressure then builds over the region Wednesday and Thursday with seasonable temperatures and lower humidity. Warmer temperatures return late in the week as the high moves off to the east and another cold front approaches from the west. && .NEAR TERM /UNTIL 2 PM THIS AFTERNOON/...
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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.
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&& .SHORT TERM /2 PM THIS AFTERNOON THROUGH 6 AM TUESDAY/...
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*/ 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.
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/... */ Highlights... - Widely scattered showers Tuesday into Wednesday - Looking dry for Thursday - Periods of wet-weather Friday into the weekend */ Overview... Ensemble means continuing to signal a persistent H5 trough pattern over the Great Lakes Region up against high pressure over the NW Atlantic. The NE CONUS along the dividing line between the two, we`ll likely see an exchange of airmasses over time parent with passage of mid-level disturbances. Warm and mild prior, turning cooler, more comfortable in wake. Chances of wet-weather in-between. Overall conditions holding near-seasonable. */ Details... Tuesday into Wednesday... Generally clearing out though widely scattered light showers linger. The low occluding over S Hudson Bay as the main axis of sub-tropical moisture pushes offshore, drier air wraps into the region. However cyclonic flow prevails through which individual impulses act upon moisture associated with a continental airmass. Thinking we`ll see a seasonably dry day for Tuesday as the last of the moderate to heavy rain pushes offshore during the morning hours. Towards late into the overnight period, increasing clouds and possible showers over the N/W high terrain with W convergence. In addition an enhancement to the S low-level wind profile over the S/E beneath the increasing ascent and left front quadrant of the upper level jet, could see some showers around the Cape and Islands. Possibly lingering into the early morning hours, looking mostly dry for Wednesday, however higher dewpoint air over the S/E with the possibility of boundary layer destabilization ahead of a sweeping cold front, may see the last gasp of showery weather. Highs in the low to mid 80s. Both days expect breezy W winds given boundary layer mixing as high as H8. So to reiterate, a multitude of areas and opportunities that may see light showery weather beneath low-mid level cyclonic flow, dynamics, and cold pool aiding in steeping lapse rates, all of which act upon limited moisture. Certain Wednesday night, high pressure builds into the region as the H5 trough sweeps offshore. Subsidence prevails, clearing conditions, light winds, good bet of radiational cooling, leaning with coolest MOS guidance. Thursday into Thursday night... Going to keep it dry and seasonable. High pressure shifts E and we see return S flow and a moistening environment as the low to mid level warm front lifts N across the region. But beneath progressive flow and absent of ascent, will keep it dry. Indications of some lead shortwave energy to the N collocated with better regions of W low-level convergence of deeper moisture. Perhaps some of this energy reaches S along the Berkshires and Worcester Hills. A low confidence forecast for now, something to watch down the road. Friday onward... Periods of wet-weather expected, however a low confidence forecast. Seeing complications with possible tropical remnants to the S as a H5 trough axis deepens across the Great Lakes Region. Seemingly a pattern similar to that ongoing. Ascent upon high theta-E air as precipitable waters exceed 2 inches initially, there is the threat of heavy rain on Friday per consensus of forecast guidance, the better risk appears to be over S/E New England. May be the case again around Sunday ahead of strong dynamics and ascent emerging with the sweeping mid-level trough axis. && .AVIATION /11Z MONDAY THROUGH FRIDAY/...
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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 Tonight/... 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 through Friday/... Moderate confidence. Tuesday and Wednesday... SCT-BKN low-end VFR CIGs with MVFR/IFR over S/E MA and RI at times. Overall SW winds around 10 to 15 kts with gusts up to 25 kts, with strongest winds during the day, becoming westerly late Wednesday into Wednesday night. Aside from RA early over SE MA, widely SCT SHRA through Wednesday. Thursday and Friday... Thickening CIGs W to E, lowering, beginning Thursday night. MVFR/IFR mix Thursday with -RA/RA. W winds becoming SW Thursday, increasing into Friday 10 to 15 kts with gusts up to 20 kts.
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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 Tonight/... 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 through Friday/... Moderate confidence. Tuesday and Wednesday... Overall SW winds around 10 to 15 kts with gusts up to 25 kts, with strongest winds during the day, becoming westerly late Wednesday into Wednesday night. Aside from rain early over SE MA, widely scattered showers through Wednesday, clearing out overnight. Initial waves 6 to 8 feet on the outer waters slow to diminish, falling below SMALL CRAFT ADVISORY criteria by early morning Thursday. Thursday and Friday... Good boating weather Thursday, however Friday another story. W winds becoming SW Thursday, increasing into Friday 10 to 15 kts with gusts up to 20 kts. Waves building to 5 feet on the outer waters. Rain on the increase across the region Friday.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...Flash Flood Watch from noon EDT today through late tonight for CTZ002-003. MA...Flash Flood Watch from noon EDT today 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...Frank/Sipprell NEAR TERM...Frank/Sipprell SHORT TERM...Frank LONG TERM...Sipprell AVIATION...Frank/Sipprell MARINE...Frank/Sipprell

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