Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 151144 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 744 AM EDT Sat Jul 15 2017 .SYNOPSIS... Low pressure passes offshore today. High pressure builds over the region tonight and Sunday, bringing drier and more seasonable weather. Warm and humid conditions will continue through mid week with the threat for scattered afternoon and evening showers and thunderstorms Monday through Wednesday. A cold front may approach late Wednesday, then may stall across the region Thursday with the continued threat for scattered showers and thunderstorms. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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715 AM Update... Batch of showers moving across Nantucket and Martha`s Vineyard and the nearby coastal waters continuing to move steadily E, with back edge passing across the Vineyard at 11Z. A few other isolated showers moving across NE CT which have tended to dissipate over the last couple of hours out of the Hartford area. Could still see scattered showers and possibly a few thunderstorms later this morning into the afternoon as strong upper level short wave and associated 90-100 kt jet moves across. With this short wave moving across, could see renewed scattered showers and a few isolated thunderstorms develop mainly across central and western areas by late morning, then shift E this afternoon. Have kept this in the forecast. Still noting area of low clouds and patchy fog across the region, though starting to show signs of lifting. After temps bottomed out in the upper 50s to lower 60s, readings should rebound to the 70s as the clouds break. It will still be rather humid, however. Near term forecast has been updated to reflect this thinking. Previous Discussion... RH cross sections show a moist layer from surface to 700 mb this morning, but with much of it drying out after 18Z. Stability parameters are marginal. Putting it all together, there remains a low chance of showers this morning, then drying as the system moves to our east and winds turn out of the west. Drying out will allow for clearing skies during the afternoon. Temps should climb to the mid 70s to low 80s, warmest in the CT Valley.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... High pressure builds in from the Midwest and provides enough subsidence to bring dry weather. Variable winds tonight will become light south during Sunday. With the light flow, expect sea breezes late Sunday morning and afternoon. Temperatures at 850 mb will be 13-14C, supporting max sfc temps in the low to mid 80s. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Threat for scattered showers and thunderstorms Monday and Tuesday * Better chance for showers/thunderstorms late Wednesday through Friday along slow moving front Overview... H5 short wave trough across the eastern Great Lakes into western NY/PA Sunday night and Monday slows as it digs across the Appalachians while high amplitude ridge builds across the northern Plains into central Canada. With a general southern wind flow across the region bringing warm and humid temperatures, will see the threat of mainly diurnal convection across interior areas Monday, then a better shot for most areas during Tuesday as mid level trough weakens and its reflection and weak surface low pres push into western areas. Expect warmest temps at mid week on Wednesday with some weak surface subsidence builds across coastal areas, but could still see some diurnal convection well inland. Another front approaches out of southern Quebec and Ontario around the late Wed or Thu timeframe, but timing is uncertain. At this point, front will be slow to move across the region in a general flat mid level wind flow across the northern stream. Details... Sunday night... Expect generally dry conditions, though a few showers may sneak into the E slopes of the Berkshires after midnight. Will see redevelopment of areas of late night fog as low level moisture remains in place across the region with dewpts mainly in the 60s. Lows will mainly be in the 60s, but could linger close to 70 along the S coast. Can not rule out some patchy drizzle there as well. Monday through Tuesday... Looks like best instability remains over western Mass/N central CT Mon afternoon/early Mon night as weak surface low/front slowly crosses NY state. May see some isolated activity moves further E, but should weaken as any leftover convection moves into the somewhat more stable air. May also see weak low pres pass SE of Nantucket, with some showers/thunderstorms passing close to Cape Cod and the islands Mon night into Tue. Noting plume of 1.7-1.8 inch PWATs passing across the Cape and islands overnight, so may see some heavier downpours that may push in. As the weak trough and low pres slowly pushes closer to the region, may see better shot for showers/thunderstorms during Tue afternoon/evening across most of the region, possibly approaching NE Mass and maybe as far S as Boston to Worcester areas. PWATs up to 1.4 to 1.6 inches into the best chance areas of precip, so could see a few downpours in those areas. Will continue to see nocturnal areas of low clouds and fog, mainly from around midnight through the mid morning with the sultry airmass in place each night. Daytime highs will run close to seasonal normals. Wednesday and Thursday... Medium range models all signaling the approach of a cold front out of southern Quebec and Ontario during this timeframe. However, with a broad zonal mid level flow across the northern stream, the front will slow as it becomes parallel with the front. Question at this point will be how far S will the front push, along with the chance for showers/thunderstorms as it moves into a nearly zonal steering flow. Rather low confidence due to model solution spread especially in the mid levels. GFS tries to dig another trough, while most of the remaining guidance keeps a broad zonal flow in place during this timeframe. Not much of an airmass change as the front may stall somewhere near or N of the region. However, noting that WPC does bring the front through, but not until Thu night or Fri. Have kept chance of showers/thunderstorms mainly across the interior on Wed, then most areas during Thu. At this point, looks like the warm and humid airmass will hold in place for now. Forecasting warmest temps of the week both Wed and Thu, with readings topping off in the 80s for most locations. A few spots across the interior could touch 90. Friday... Low confidence continues with the possible passage of a cold front across the region. At this point, leaned toward blend of available guidance which keeps chance of precip across the region through most of the day. && .AVIATION /11Z SATURDAY THROUGH WEDNESDAY/...
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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 Sunday/... Today...Moderate confidence. MVFR-IFR and locally LIFR CIGS will improve to VFR from W-E this morning. MVFR CIGS may linger through midday across E coastal areas, but should improve. Patchy fog with local MVFR- LIFR VSBYS will also improve, likely by around 15Z. May see scattered showers and isold TSTMS through this afternoon. Local MVFR conditions possible in any precip. Clearing trend will push into western areas by around 19Z-20Z, then shift E as back edge of precip moves along as well. Tonight and Sunday... High confidence. VFR. Light west wind tonight becomes variable. Light south wind develops Sunday. Light flow will allow coastal sea breezes on Sunday. KBOS Terminal...Moderate confidence in TAF. Ceilings could briefly lower to IFR during the early morning, but more likely will remain at MVFR levels until improving to VFR this afternoon. KBDL Terminal...Moderate confidence. IFR ceilings linger into the morning, then improve to VFR as the winds shift out of the west late morning. Widely scattered showers possible, but vsbys should remain above 5 miles. Outlook /Sunday Night through Wednesday/... Sunday night...Moderate confidence. SCT-BKN low-end VFR CIGS Sun evening. CIGS/VSBYS lower to MVFR- IFR with local LIFR by around 03Z-04Z in areas of fog that will linger through the night. Lowest conditions across coastal plain and higher inland terrain. Monday into Tuesday...Low to moderate confidence. SCT-BKN low-end VFR CIGS across the interior with SCT SHRA/TSRA. Best shot from central-W Mass into N central CT. Will see persistent S-SE wind flow, so MVFR-IFR CIGS move into coastal areas Mon night into early Tue. Will also see areas of MVFR-IFR VSBYS develop as well. Wednesday...Low to moderate confidence. Mainly VFR. Should see SCT-BKN low-end VFR CIGs with a better shot for scattered afternoon/evening SHRA/TSRA activity. MVFR- IFR lingering across the S/SE coast in areas of fog, mainly late night/early morning.
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&& .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...Moderate confidence. Winds remain less than 15 knots. Seas remain less than 5 feet. Patches of light rain and fog this morning, but becoming dry this afternoon. Clearing skies tonight. Outlook /Sunday Night through Wednesday/...Moderate confidence. Sunday night... Light S-SE winds. Seas 2 to 3 feet. Patchy fog with visibility restrictions mainly after midnight. Monday through Wednesday... Persistent S-SE winds at 10 kt or less Monday through Tuesday becoming S-SW Wednesday. Gusts up to 15 kt on the open waters during Wednesday. Best chance for scattered showers and a few thunderstorms Monday night and Tuesday, then again Wednesday night ahead of approaching front. Visibility restrictions in patchy late night and early morning fog each day. && .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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