Area Forecast Discussion
Issued by NWS Boston, MA

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083 FXUS61 KBOX 291359 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 959 AM EDT FRI JUL 29 2016 .SYNOPSIS... Low pressure along the Mid Atlantic coast will pass offshore late in the day as a weak cold frontal boundary sweeps New England. Dry conditions for Saturday with high pressure. Another low will pass south of New England bringing another round of scattered showers with a few thunderstorms Sunday into Monday. High pressure returns with dry and seasonable conditions for the middle and latter portion of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 am update... Flash flood watch continues for the south-coast of Massachusetts which includes Cape Cod and the Islands. Focus specifically on low to mid level sub-tropical moisture convergence. This beneath mid level forcing ahead of the weak trough axis and accompanying vortex energy, as well as beneath the right-rear-quadrant of the upper level jet. Objective simply is to pinpoint regions of greatest lift / forcing, nailing down areas where precipitation efficiency will be greatest, perhaps with some accompanying instability. Based on latest radar trends and high-res near-term guidance, the area of greatest focus is developing over the S waters progressing N into far SE New England where the flash flood watch is currently in effect. There is a camp of forecast guidance which keeps the bulk of outcomes over the waters encompassing offshore islands, but in monitoring radar trends, including the fine-line mesoscale features, there is certainly some threat to Southeast MA. This along the leading edge of a distinct wave inferred from latest water vapor satellite. A general half to one-inch forecast for far S-coastal RI and MA with the greatest amounts exceeding one-inch over the Cape and Islands. Can not rule out that the batch of heavy rain moving into SE New England is very efficient with respect to rainfall. Rates of 1 to 2 inches per hour of instantaneous rainfall presently being discerned via dual-pol. Otherwise, clearing working in from the W and have this and bulk of rain moving offshore around midday into afternoon. Monitoring upstream thereafter a weak cold frontal boundary sweeping behind the departing offshore system. Boundary layer destabilization but beneath some weak ridging aloft and warmer air around H5. As model trends would concur, any activity that gets organized seems to diminish quickly and/or get suppressed. Will have to watch closely. Storms possible, thinking isolated, maybe a threat of gusty winds and heavy rain given the atmospheric setup. Warming into the low-80s for most locations. Coolest SE with the heavier rain. Soupy air as dewpoints in the upper 60s to low 70s linger into evening. && .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 /14Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short-Term /through Saturday/...High confidence. 15z update... Today... IFR-LIFR for SE terminals with RA/+RA and VCTS. VFR elsewhere with N/NE winds. Conditions clearing from the W presently. There is the moderate risk of additional SHRA/TSRA development N/W. Tonight... Improving VFR over the SE. MVFR-IFR patchy fog may develop over the interior, especially thinking more so in the CT River Valley as winds become light and conditions become SCT to SKC. Saturday... SCT-BKN cigs especially towards late in the day. Light winds. Sea-breezes developing along the shores late morning into afternoon. Risk of SHRA/TSRA into S/W New England late. KBOS TAF...High confidence. Will push the clearing line into the terminal around 16-18z. Prior to which conditions should remain VFR but can not rule out MVFR with any RA. KBDL TAF...High confidence. Improving by 16z. Thereafter monitoring N/W of the terminal for the possibility of additional SHRA/TSRA development. Outlook /Saturday Night through Tuesday/... Saturday night through Monday...Moderate confidence. Scattered showers and a few thunderstorms possible through the period, with the best chance during Sunday into Sunday night. Occasional MVFR conditions, but VFR likely dominates. Patchy late night/early morning fog with IFR CIGS/VSBYS each night. 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. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. 10 am update... Areas of low vsbys in fog along the Southern waters, especially the nearshore bays and sounds. Locally dense fog will have vsbys less than 1/4 mile. Main concern is potential for strong storms over south coastal waters today, mainly south of islands and east of Nantucket where brief waterspouts are possible this afternoon. A Marine Weather Statement continues to highlight this potential, especially for recreational boaters. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Flash Flood Watch until 5 PM EDT this afternoon for MAZ020>024. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/Sipprell/EVT SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/Sipprell/EVT MARINE...WTB/Sipprell/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.