Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 291741 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 141 PM EDT FRI JUL 29 2016 .SYNOPSIS...
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Low pressure will sweep south of New England into the overnight period. A brief period of drier conditions follow for Saturday with the slight chance of showers over Southwestern New England. Another area of low pressure approaches New England towards Sunday, passing south of New England into Monday bringing with it another round of showers and thunderstorms. High pressure returns with dry and seasonable conditions for the middle and latter portion of next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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130 pm update... Flash flood watch continues for Nantucket only with heavy rain and thunderstorms presently ongoing. Surface low pressure presently situated well south and east of Long Island and the NJ coast ahead of which within the NE quadrant of the H85 low we`re seeing decent low to mid level sub-tropical moisture convergence per SPC mesoanalysis. RAP model solutions are a little off based on latest radar returns, but the trends are there. Strong lift and forcing of sub-tropical moisture yielding widespread heavy rain with embedded thunderstorms, mainly over the ocean waters S/E of Nantucket. Indications per dual-pol that within thunderstorms and accompanying instability that instantaneous rainfall rates are on the order of 1 to 2 inches, with some as a 7 inches per hour. With the main threats mainly over the waters, have trimmed back the flash flood watch to simply include Nantucket through 5 pm. Already roughly upwards of an inch has fallen over the island and presently the heavy rain is falling and will likely continue to do so over the next hour. Otherwise, seeing clearing progress rather quickly across S New England. Destabilizing moist boundary layer lending to low-topped storm development. Slow moving through the rather weak 10 mph mean flow out of the NW, monitoring for possible localized flooding impacts. Limitations on development with likely warmer and sinking air per rising heights around H5. Some measure of suppressed activity. Nevertheless, still enough lift to generate some heavy rain`ers. With sunshine it has easily warmed up into the low to mid 80s, a degree or two below originally thinking. Dewpoints slowly dropping with the weak N flow and as the boundary layer becomes mixed out.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... As the offshore low pressure moves off to the east, weak high pressure over Canada will build with somewhat drier air. Dew points will range from the mid 50s north to mid 60s south, down from the 70s in RI/SE MA today but still noticeable humidity in southern areas. The moisture and light wind will allow fog patches especially in the more humid south. Temps aloft of 14-16C will support max temps in the 80s Saturday. The light wind flow will allow sea breezes to form, keeping coastal areas cooler. Additional concern is potential for convection upstream in NE PA that could move east into Connecticut late in the day Saturday. LI values will be sub-zero across PA and the lower Hudson Valley with SBCAPE values near 1000 J/Kg. We have included chance pops for showers/tstms in CT and Greater Springfield MA, mostly 5-8 pm Saturday. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * High pressure and mainly dry conditions Saturday * Showers and a few thunderstorms possible Saturday night through Monday * Drier and seasonable weather Tuesday through Thursday Overview and model preferences... Fast mid and upper level flow across central and southern Canada has left weaker steering currents across the northern tier of the U.S. into early next week. Noting a slow moving H5 short wave out of the Great Lakes which will cause upper level winds to shift to W-SW across New England. Weak surface low will move slowly along a stalled front south of the region, keeping the chance for showers/ thunderstorms. Models and ensembles continue to signal that the short wave will finally move east Monday night into Tuesday, so should see improving conditions as mid level winds shift to NW. Also noting building heights by late next week, so will see temps run near or slightly above seasonal normals. Used a consensus blend of available model and ensemble guidance for this portion of the forecast. Details... Saturday night through Monday...With relative weak steering currents between the active northern stream flow across southern and central Canada and the normal summer ridging across the southern tier states, expect low pressure to move slowly along a stalled front south of New England keeping low and mid level moisture in place along with marginal instability during this timeframe. PWATs of 1.8 to 2 inches will also linger mainly near and south of the Mass Pike, which could lead to some locally brief, heavy precip mainly Sunday into Sunday night. With the low passing S of the region, winds will generally be E-SE off the cooler ocean, which tend to stabilize any surface based convection but could see some spotty elevated activity. As the H5 short wave slowly pushes across the region on Monday, any showers should push E. However, some may linger across inland areas mainly during the afternoon, though there is individual model solution spread so not a lot of confidence with this. Expect temps to run close to or slightly below seasonal normals especially on Sunday with the steadiest precip. Tuesday through Thursday...H5 high amplitude ridge moving E across Hudson Bay on Tuesday will help push the trough offshore. This will bring generally dry conditions as NW winds aloft take over, albeit on the light side. Noting the 00Z ECMWF is trying to keep some troughing lingering across interior central and southern New England for Wed-Thu, which could mean some isold diurnal convection. With somewhat higher H5 heights, temps should return to near or slightly above normal levels by late next week. && .AVIATION /17Z FRIDAY THROUGH TUESDAY/...
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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 Saturday/...High confidence. 18z update... Rest of Today... Improving W to SE. Light N/NE winds. Lingering MVFR for ACK with -RA/RA but only for a few more hours. SCT +SHRA over N/W interior terminals down through the CT River Valley. TEMPO MVFR-IFR possible. Tonight... All terminals VFR, improving to SKC. Overnight, monitoring for likely MVFR- IFR fog development. Especially focused across the CT River Valley, as well as interior E CT and SE MA. Could bee some issues along the E/SE coast of MA but lesser confidence with respect to the dense fog. Saturday... SCT-BKN cigs especially towards late in the day. Light winds. Sea-breezes developing along the shores late morning into afternoon. SCT SHRA/TSRA possible across S/W New England late but believe it to be a low risk. Saturday Night... BKN-OVC cigs with MVFR-IFR developing along the S-coast of New England towards Sunday morning. Increasing chance of RA from the S/W along with VSBY restrictions. KBOS TAF...High confidence. Clearing line approaching BOS as of 18z. Conditions should remain VFR into this evening. Watching closely overnight for possible E winds to usher MVFR-IFR back across the terminal overnight. Think it a low risk. KBDL TAF...High confidence. Low risk +SHRA towards the late afternoon period with TEMPO MVFR-IFR impacts. Believe it to be a moderate risk. Outlook /Sunday through Tuesday/... Sunday through Monday...Moderate confidence. SCT SHRA with a few TSRA the best chance of which will be Sunday into Sunday night. TEMPO MVFR-IFR but likely VFR dominates. Could see some VSBY issues during the overnight into early morning periods with patchy fog. Monday night and Tuesday...High confidence. Mainly VFR. May see brief IFR CIGS/VSBYS in patchy fog after 06Z through 12Z-13Z in the normally prone areas as well as along the coast.
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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 Saturday/...High confidence. 130 pm update... Strong storms over south coastal waters presently, mainly south of islands and east of Nantucket where brief waterspouts are possible. A Marine Weather Statement continues to highlight this potential, especially for recreational boaters. Warnings have been issued in some cases. Otherwise winds will remain below 25 knots and seas below 5 feet. Rain and fog will reduce visibility Today and this evening. Outlook /Saturday Night through Tuesday/...High confidence. Saturday night through Monday...Expect winds and seas below small craft criteria. Seas 4 ft or less. Locally reduced visibilities in scattered showers through the period, with patchy late night/early morning fog. A few thunderstorms possible. Monday night and Tuesday...NE winds gusting up to 20 kt mainly across the eastern outer waters. Seas may brief build up to 5 ft Tuesday.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Flash Flood Watch until 5 PM EDT this afternoon for MAZ024. RI...None. MARINE...None.
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&& $$ SYNOPSIS...WTB/EVT NEAR TERM...Sipprell SHORT TERM...WTB LONG TERM...EVT AVIATION...Sipprell/EVT MARINE...WTB/Sipprell/EVT

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