Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 161122 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 722 AM EDT Sun Jul 16 2017 .SYNOPSIS... High pressure will bring fair weather along with warmth and humidity today. Warm and humid conditions will continue Monday through Wednesday with the threat for scattered afternoon and evening showers and thunderstorms as a weakening cold front slowly crosses the region. Another cold front may approach Thursday, which looks to slowly push across during Friday. Somewhat drier conditions possible early next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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705 AM Update... Noting area of low clouds and patchy fog along the coast of the outer Cape and Nantucket on latest visible satellite imagery. Several normally prone locations along the immediate coast and across some interior valleys reporting visibilities at or below 1SM at 11Z. Satellite trends showing clouds are starting to dissipate, so should see quick improvement over the next couple of hours. However with surface temps close to ocean temps along the coast, may take a bit longer for those clouds to clear. Weak surface trough appears to be moving off the E Mass coast at 11Z, with bubble high pressure over S VT into W Mass and the Hudson valley of NY state. Noting another band of clouds moving SE out of upstate NY with only a few showers (for now) should remain N of the region this morning. Near term forecast pretty much on track. Only issue will be timing out the patchy fog/low visibilities through about mid morning. The update reflects this thinking. Previous Discussion... High pressure in place over the region today. Mixing should reach to at least 850 mb, possibly to 800 mb. Cross sections show moisture near the top of this mixing, so the daytime heating will lead to a few cumulus clouds. The heating may also generate CAPE of 300-500 J/Kg. But subsidence should keep the instability from being fully realized, so no showers expected. Temperatures at 850 mb will be 15C-16C, and at 800 mb will be 11C-13C. Heating to these levels would support max sfc temps in the mid 80s, possibly near 90 in spots where the deepest mixing takes place. Wind fields will be light, and the daytime heating should generate sea breezes along the coast late this morning and afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Tonight... Dry weather continues tonight. Dew points will be 60-65, similar to water temperatures off eastern Mass but several degrees cooler than water temps south of RI and Mass. Widespread fog moving ashore does not seem likely, but radiation fog inland is possible. With dew points 60-65, expect min temps in the low to mid 60s. Monday... High pressure builds offshore and brings a light south flow across our area. This may bring a slight increase in dew points during the day. Sunshine will bring mixing to at least 850 mb again, tapping temperatures of 14-15C and supporting max temps again in the mid 80s. Shortwave digging over the Great Lakes will turn the current westerly upper flow to come out of the southwest. A cold front being driven by that shortwave will slow to a stall Monday as it aligns with that changing upper flow. A broad zone of 1000-2000 J/Kg CAPEs will extend from Northern New England across New York State. CAPE values diminish rapidly across Western and Central Mass. Precipitable water values will be around 1.5 inches, not excessive but high enough to monitor. Expect thunderstorms to develop in the zone of instability to our west and north. It is possible that some of these storms could move east into our area during the late afternoon/evening. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
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Highlights... * Threat for scattered showers and thunderstorms at times Monday night through Wednesday * Better chance for showers/thunderstorms Thursday into Friday morning along a slow moving front Overview... Pattern starts out with mid level long wave troughing across the eastern U.S., while flattening ridge starts to progress across the Plains states early next week. The steering pattern then tends to flatten out across the northern tier states as a cold front slowly pushes S out of southern Canada. The front will become parallel to the upper flow, becoming a trigger for scattered showers and thunderstorms mainly during the late morning through evening hours through mid week. Noting another mid level short wave in the steering flow moving toward the region around the Thursday timeframe as yet another ridge starts to build over the eastern Rockies/eastern Plains states, with a W-NW flow developing downstream. This will allow another front to shift SE late Thursday into Friday, bringing another threat for convection. Medium range models starting to signal drier and somewhat less humid conditions moving in early next weekend, though timing is uncertain. Continued with an overall blend of available guidance for this forecast. Details... Monday night through Tuesday... Frontal system looks to remain mainly W of the region Monday night, though a few showers/thunderstorms will continue across N central and W Mass into N central CT. May see a few stray showers/isold thunder reach a bit further E overnight, though should remain dry toward the coastal plain. Will see late night/early morning patchy fog develop with continued humid conditions in place. The front over eastern NY state will slowly push E, bringing a better chance for showers/thunderstorms across most of the region during Tuesday. Weak low pressure may develop somewhere on this front which could enhance some convection, though noting marginal instability. Have continued with scattered showers/thunderstorms in the forecast. Could see some locally heavy rainfall in any shower or thunderstorm activity. Overnight lows will mainly be in the 60s, with highs on Tuesday away from the immediate coast in the lower-mid 80s. Wednesday and Thursday... Will see continued high temps and humidity both days. Also continue to see a threat for convection along a weak surface trough, remnants of passing front that is nearly stalled across the region on Wednesday. Carrying chance showers/isolated thunderstorms through Wed, then the trough should slip to the S coast Wed night before dissipating. Next front starts to approach from the N during Thursday. Timing is in question due to developing upstream ridge that may cause a W-NW downstream flow. Looks like best moisture will remain N of the region, but could still see widely sct showers and a few isolated thunderstorms during the day. With fewer clouds, temps could reach to around 90 in some of the normally warmer spots such as the lower CT valley and interior E Mass. Elsewhere, expect temps to top off in the mid-upper 80s except cooler along the immediate S coast. Will need to watch out for excessive heat issues mainly across the lower CT valley both days with heat indices possibly pushing the mid 90s. Thursday night through Saturday... Wide model solution spread during this timeframe lends to continued low confidence forecast. Timing of slow moving cold front is the issue, though looks like it may move a bit more steadily if models are correct in signaling developing western U.S. ridge which could allow for developing W-NW downstream flow which could push the front offshore sometime Friday. Drier and somewhat less humid conditions forecast for Saturday at this point.
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&& .AVIATION /11Z 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. Short Term /through Monday/... 715 AM Update...Forecast on track. Only a few minor updates to bring current. Today... Morning fog burns off quickly. Patchy IFR cigs/vsbys should give way to clear skies by 14Z-15Z. Otherwise, expect VFR conditions through the day. Light winds and daytime heating will allow sea breezes to develop along the coasts starting late morning. Tonight... VFR, but with areas of fog developing especially in the CT Valley. Local IFR cigs/vsbys in fog. Monday... Morning fog again burns off. VFR with developing light south wind. Sea breezes possible along the eastern Mass coastline. Thunderstorms will develop over NY State and Northern New England. It is possible a few of these will move into Western and Central Mass during the afternoon after 18Z, some with strong wind gusts. KBOS Terminal...High confidence in TAF. Seabreezes 14-15Z today, initially NE then turning SE. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/... Monday night and Tuesday...Moderate confidence. Mainly VFR conditions Monday evening, except local MVFR conditions in sct SHRA and isold TSRA across the interior. CIGS/VSBYS lower to MVFR-IFR in areas of low clouds and patchy fog, mainly around or after midnight. Any lower CIGS/VSBYS should improve by mid morning Tuesday. May see local MVFR conditions in scattered late morning and afternoon SHRA/TSRA. Tuesday night through Thursday...Moderate confidence. Mainly VFR. Will see patchy fog develop mainly across E Mass/RI as well as interior valleys around or after midnight each night with areas of MVFR-IFR CIGS/VSBYS. Should see SCT-BKN low-end VFR CIGS with a better shot for scattered afternoon/evening SHRA/TSRA activity.
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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/...High confidence. Winds less than 15 knots and seas less than 5 feet through the period. Sea breezes developing along the coastline. Outlook /Monday Night through Thursday/...Moderate confidence. Expect winds to remain light, mainly 10 kt or less. SW winds may gust up to 20 kt on the outer waters Wednesday and Thursday. Seas mainly 3 ft or less, though may reach 4 ft on the southern outer waters Thursday. Scattered showers or thunderstorms may develop at any time Tuesday through Thursday, with reduced visibilities in locally heavy downpours. Patchy fog during the late night/early morning hours each day with reduced visibilities, locally at or below 2SM across the near shore waters early each morning.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/EVT SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/EVT MARINE...WTB/EVT

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