Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 290747 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 347 AM EDT FRI JUL 29 2016 .SYNOPSIS... 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. Weak high pressure brings drier weather Saturday. Another risk for showers follows on Sunday and Monday. && .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/... Highlights... * Showers and storms possible Sunday and Monday * Drier and seasonable weather mid-late next week Overview and model preferences... Interesting weakly split flow regime defines the mid-term period. Weak ridge-over-trof split flow develops and gradually moves across NE CONUS, finally giving way to joined ridging toward the latter half of the forecast period. The trof is the primary wx-maker for late weekend and early next week, providing a focal point for continued shra/tstm activity. Timing disagreements are minimal with the 12Z updates and even show continuity with ECMWF EPS and GEFS means. Therefore, will use a consensus blend for this afternoon`s long-term update. Details... Sun and Mon... Tricky forecast period as diffuse open wave and modest sfc trof rest across much of the mid-Atlantic region and into S New England. This should provide a focal point for continued shra/tstm development especially to the W and S, where highest moisture lies. PWATs linger around 2.0 inches to the SE but then drop into New England. Gradient winds mainly E-NE which, off the cooler Gulf of Maine waters which is likely to limit convective (particularly sfc based). Therefore not confident in how much thunder activity S New England will experience, however leftover shra is possible. CIPS analogs support this thinking, focusing the highest risk for convective activity, SW of New England. Will need to watch a secondary low pres wave development which could provide better low lvl convergence and F-gen. ECMWF is closest to the S coast with this feature Sun night, but if it remains further offshore, less rain will impact S New England. Will need to monitor as we approach as it has similar look to the low pres which could bring heavy rain to the region tomorrow. Tue through Thu... In spite of what occurs earlier in the week, the split flow finally joins in the form of a ridge upstream which gradually shifts to the E. This should bring about a period of mainly quiet wx with sfc high pres in place. However, the orientation of high pres brings in some cooler/drier air with H85 temps really only hovering around +12C, so temps should remain much closer to seasonable levels than they have been for the last week or so. && .AVIATION /07Z FRIDAY THROUGH TUESDAY/... 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/... Sunday and Monday...Moderate confidence. Some showers and a few thunderstorms possible through the period. Occasional MVFR conditions, but VFR likely dominates. Tuesday...High confidence. Mainly VFR. && .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. 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. Mainly quiet boating weather as high pressure slowly builds over the waters. Winds and seas below small craft advisory thresholds. Only caveat could be an isolated shower or thunderstorm mainly Sunday or Monday. && .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/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody

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