Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 290906 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 506 AM EDT FRI JUL 29 2016 .SYNOPSIS...
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A weak front will slip south of New England by midday. Low pressure along the Mid Atlantic coast will pass offshore late in the day. This will bring showers and isolated thunderstorms today. Dry conditions for Saturday as high pressure builds across. Another low will slowly pass south of New England, bringing another round of scattered showers with a few thunderstorms possible Sunday into Monday. 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/... Low pressure over Eastern Maryland this morning with a primarily zonal flow aloft. There is a faint signal of a shortwave in this flow that could draw the surface low farther north, but most of the push looks out to sea. Also note the 92-knot upper jet crossing New England with our area in the right entrance region through early afternoon. This will support lift and pcpn generation over Southern New England... IF there is enough moisture to tap. Precipitable water values continue to show 2+ inches along the New England South Coast with 1.5 inches north & west. Surface dew points in RI and SE MA are 70-74. On the other hand, convection over the Mid Atlantic is moving more toward the ocean than toward New England. This would intercept the inflow of additional moisture from the south. Broad area of showers over Eastern PA is showing a northeast trend and should bring us wet weather this morning, but its trailing edge is progressing east and should limit the time for precipitation. Model trends early this morning have also shifted the forecast a little farther south. This shifts the heavier pcpn over the South Coast or offshore, so this should be our best chance of higher rainfall amounts. We have shifted rainfall amounts south, with 1 inch or higher south of a Willimantic-Plymouth line. Flash Flood Watch continues in RI and SE MA, but has been discontinued in Northern CT where expected total rainfall has diminished. Even so, remember that extremely dry soils can also lead to rapid runoff, which could result in rapid flooding of some small streams and rivers, again primarily due to urban runoff. The SPC Marginal risk for severe weather remains along the immediate South Coast today. SB CAPE is more limited with this run, with values of 200-500 J/Kg depending on the model of choice. SPC RAP Helicity values are forecast to stay well offshore. The farther-south track of the low will not help, nor will the low lapse rates. But mesoscale features could still generate an isolated damaging wind gust. Rain should fall this morning and early afternoon but taper off from W to E during afternoon, with most of it done toward evening on E MA coast. Highs will top out in 70s to around 80. && .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/...
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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.
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&& .AVIATION /09Z 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. Moderate confidence Today and tonight. High confidence Saturday. Areas of IFR/LIFR along the South Coast and Islands due to a broad area of fog and low clouds along the shore. Showers and isolated thunderstorms moving toward ENE from the Mid Atlantic will bring a period of MVFR vsbys and possibly cigs during the morning and early afternoon. Locally heavy downpours could bring vsbys briefly to IFR. The showers taper off from West to East this afternoon and evening. Conditions will return to VFR tonight, except for local MVFR/IFR in patchy fog. High pressure will maintain VFR on Saturday. The light flow will allow sea breezes late morning through afternoon. There is also be showers developing west of CT that could move into the state during the late afternoon. Possible MVFR vsbys in any showers/tstms. KBOS TAF...Moderate confidence. Heavier rain and lower conditions may stay farther south today and miss the terminal. Higher confidence tonight. KBDL TAF...Moderate confidence. Heavier rain and lower conditions may stay farther south today and miss the terminal. Higher confidence tonight. 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.
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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. 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 or even severe 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Flash Flood Watch through this evening for MAZ017>024. RI...Flash Flood Watch through this evening for RIZ001>008. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/EVT MARINE...WTB/EVT

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