Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180834 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 434 AM EDT Sun Jun 18 2017 .SYNOPSIS...
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Warm and humid weather is on tap for the region today and Monday. A cold front will bring the potential for strong to severe thunderstorms and localized flash flooding Monday afternoon and night to western Massachusetts and northern Connecticut. The showers and thunderstorms will arrive across eastern New England Monday night, but in a weakened fashion. The cold front crosses Southern New England on Tuesday. Seasonable temperatures and lower humidity follow on Wednesday. Warmer temperatures return late in the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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430 am update... Today... Main concern early this morning will be areas of dense fog along the south coast with high dewpoint air advecting over the relatively cool ocean. Will continue the dense fog advisory into mid morning, but should see visibilities improve thereafter with the help of the strong June sun angle. The other issue will be for the potential of a brief band of scattered showers early this morning, mainly across southeast New England with strengthening low level jet. Confidence on whether or not this activity develops is uncertain, but probably will be short-lived if it happens. Otherwise, upper level ridging in control will result in mainly dry weather today other than perhaps a brief spot shower or two. However, it will be humid and it will become breezy by afternoon. Expect south to southwest wind gusts of 25 to 35 mph to develop, with the strongest of those focused on the coastal plain. High temps today are tricky, because model cross sections indicate an abundance of clouds persisting. However, some peeks of sun will probably occur and with 850T of +16C should see highs in the 80 to 85 degree range away from the south coast/Cape and Islands. More importantly though it will feel quite humid with dewpoints in the upper 60s to around 70. Tonight... A very muggy and sticky night is in store for southern New England. The strong upper level ridge of high pressure off the mid Atlantic coast will keep the main forcing for ascent well to our west. Therefore, we expect mainly dry weather to persist through the evening. However, decent low level jet and MUCapes between 500 and 1000 J/KG may allow for some scattered showers and a few embedded thunderstorms to develop after midnight but any activity that develops will be hit and miss. As we mentioned it will remain muggy tonight with high dewpoints in place. Low temps should only bottom out in the upper 60s to the lower 70s. Will have to watch for some more fog across the south coast tonight, but enough wind may prevent it from being as dense/widespread as early this morning.
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&& .SHORT TERM /MONDAY THROUGH MONDAY NIGHT/...
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***Localized Flash Flooding along with the potential for scattered strong to severe thunderstorms exists across western MA and portions of northern CT Monday afternoon and night*** Monday and Monday night... A few elevated showers and perhaps an embedded t-storm or two will be possible Monday morning, but the majority of the time will feature dry weather. Despite an abundance of clouds, some partial sunshine along with gusty south to southwest winds will allow for a warm and humid afternoon. High temps should reach the lower to middle 80s away from the coast, but more importantly it will be quite humid. 1) Severe Weather Potential across western New England: The main severe weather potential roughly between 2 and 11 pm across western MA/northern CT. Given the warm/humid airmass in place, we should be able to generate MLCapes between 1000 and 2000 J/KG. As is normally the case in southern New England, the severe weather potential is not clear cut. Low level wind fields are fairly impressive on the order of 35 to 45 knots. However, limiting factors will be weak mid level lapse rates that will cap updraft strength to some degree. The strongest upper level support will lag behind the strongest low level forcing, but still expect a period of 30 to 40 knots of 0 to 6 km shear. Despite weak mid level lapse rates...dewpoints near 70 and strong low level forcing should be able to compensate and allow for the potential of strong to severe thunderstorms across western New England. The main threat will be damaging winds gusts, with a secondary concern of hail in strong cores. Lastly, given magnitude of low level wind fields/low LCLs and ample 0 to 1 km helicity values there is a low probability for a brief tornado. 2) Heavy Rain/Flash Flood Potential Across Western New England: High Pwat airmass coupled with strong low level forcing and upper level flow parallel to the front will result in the potential for training thunderstorms with torrential rainfall. While this axis remains uncertain, ingredients are in place for localized 2+ inch per hour rainfall rates. This will bring the potential for localized flash flooding and perhaps pockets of significant urban flooding, which may impact the Monday evening commute. Therefore, we have hoisted a Flash Flood Watch across western MA and Hartford county in northern CT Monday afternoon and Monday night. Even when the severe weather risk diminishes, pockets of torrential rainfall and a localized flash flooding risk may continue into Monday night given slow moving cold front. 3) Eastern MA and RI: Mainly dry weather should prevail Monday afternoon and perhaps into early evening along the coastal plain of eastern MA/RI. While low level forcing will eventually arrive Monday night, weak mid level lapse rates should make it difficult to sustain decent instability and best upper support will be northwest of the region. So while showers and embedded t-storms will eventually work into the region Monday night, the threat of severe weather/flash flooding looks to be rather low.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/...
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*/ Highlights... - Heavy rain potentially lingers into Tuesday - Possible showers and thunderstorms Wednesday - On and off shower activity the remainder of the week - Next best chance of wet weather over 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 Tuesday night... Potential for heavy rain and possible embedded thunderstorms exists early on in the forecast period. Not wanting to be too quick pushing the sub-tropical axis offshore. The low having occluded, undergoing a dying phase over the S Hudson Bay, heights are more or less stable as parallel SW flow prevails above the surface cold front to the T- occlusion. Thinking the sub-tropical axis of higher theta-E air lingers longest over SE New England based on consensus timing of mid- level features. Leaning slower of guidance, will hold onto chance PoPs longer, exiting offshore overnight. Possible heavy rain threat, yet at a very minimum expect continued low clouds and potential fog, visibility issues. Can`t rule out partial clearing to the W, perhaps the reinvirogation of shower activity beneath cyclonic flow and mid- level ascent per associated vort-lobe energy. Wednesday into Wednesday night... Possible showers and thunderstorms, the better chance of which may exist over SE New England. Main H5 trough axis sweeping the region with accompanying cyclonic flow, cold pool. Decent low-level lapse rates yet however the column is fairly dry given a more continental airmass. With S flow ahead of the surface cold front reflection to mid to upper level features, higher dewpoint air coming ashore as the boundary layer destabilizes, can`t rule out wet-weather activity over SE New England. Maybe some gusty winds given any cores lofted into the overall dry environment obtaining negative buoyancy, plus noteworthy inverted-V soundings. But the weak Cape, strong shear setup as it would appear, not thinking severe. Otherwise a mix of sun and clouds, mild, seasonable, and given boundary layer mixing, should be breezy. Weak high pressure overnight. Quiet, should dry out overnight. Thursday onward... On and off shower activity through the remainder of the week some- what difficult to nail down on the specifics given weak synoptic features through the overall progressive flow pattern aiding in ascent above return S flow of warmer, humid air. Better chance of wet-weather over the weekend given falling heights ahead of a more amplified mid-level trough axis sweeping across the N CONUS into New England with attendant vortmax energy. Keep with chance PoPs given a forecast timeframe more than 4 days out.
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&& .AVIATION /09Z SUNDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...Moderate confidence. IFR to LIFR conditions in low clouds and fog should improve to mainly MVFR to even VFR later this morning and afternoon. However, low clouds and fog may persist along the south coast. SSW wind gusts of 20 to 30 knots should develop by afternoon. Tonight...Moderate confidence. Low end MVFR to IFR conditions will redevelop, but LIFR conditions will probably be confined to the south coast/Cape and Islands. A few showers and perhaps an isolated t-storm may develop after midnight but activity should be hit and miss. Monday and Monday night...Improvement to MVFR-VFR conditions should occur in most areas by late Monday morning. However...low clouds and fog may persist along the south coast. Another round of SSW wind gusts of 20 to 30 knots are expected Monday afternoon. The main concern will be the potential for numerous showers and thunderstorms Mon afternoon/evening across western New England, some of which may be strong to severe with torrential rainfall. Activity will move east Monday night in a weakened fashion. KBOS Terminal...Moderate confidence in TAF. IFR should improve to MVFR by late morning/afternoon with perhaps even some VFR conditions. KBDL Terminal...Moderate confidence. LIFR conditions in low clouds and fog should improve to MVFR by late morning/afternoon with perhaps even some VFR conditions. Outlook /Tuesday through Thursday/... Moderate confidence. Tuesday... BKN-OVC MVFR/IFR CIGs lingering over SE New England with breezy SW flow around 15 kts, gusts up to 25 kts. Low-end VFR elsewhere. A slow improvement W to E, clearing out overnight. Wednesday... SCT-BKN low-end VFR CIGs with SHRA/TSRA possible, the greater risk over SE New England. SW flow 10 to 15 mph with gusts up around 25 kts. Winds turn W late as CIGs improve overnight. Thursday and Friday... SCT-BKN low-end VFR CIGs with the possibility of MVFR/IFR during the overnight hours. S/SW flow around 10 kts, 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 Monday Night/... Today through Monday night...Moderate to high confidence. Persistent gusty south to southwest flow will continue through Monday night. Wind gusts on the order of 20 to 30 knots are expected with the strongest of those winds near shore this afternoon and again Monday afternoon. Long fetch will also build seas over the open waters and small craft headlines are posted into Monday night in most locations. In addition, areas of fog will impact the waters at times especially this morning across our southern waters where it will be dense at times. Outlook /Tuesday through Thursday/... Moderate confidence. Dominant S/SW flow with gusts up to 20 kts at times, mainly during the daylight hours. Initial waves 6 to 8 feet over the outer waters subsequently is slow to diminish through Wednesday, hovering around 4 feet thereafter. Will see potential heavy rain with embedded thunderstorms, then on and off showers through the remainder of the week as a series of wet-weather disturbances sweep through the waters.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Flash Flood Watch from Monday afternoon through late Monday night for CTZ002. MA...Dense Fog Advisory until 9 AM EDT this morning for MAZ020>024. Flash Flood Watch from Monday afternoon through late Monday night for MAZ002-003-008>011. RI...Dense Fog Advisory until 9 AM EDT this morning for RIZ004>008. MARINE...Small Craft Advisory from 11 AM this morning to 8 AM EDT Tuesday for ANZ231>234-251. Small Craft Advisory from 11 AM this morning to 1 AM EDT Tuesday for ANZ230-236. Small Craft Advisory until 8 AM EDT Tuesday for ANZ235-237-250- 254>256.
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&& $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Frank SHORT TERM...Frank LONG TERM...Sipprell AVIATION...Frank/Sipprell MARINE...Frank/Sipprell

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