Area Forecast Discussion
Issued by NWS Boston, MA

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455 FXUS61 KBOX 281340 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 940 AM EDT THU JUL 28 2016 .SYNOPSIS... Offshore high pressure will bring warmth and humidity to Southern New England today. A cold front over Northern New England drops into our area late in the day and may bring scattered showers and thunderstorms. Low pressure then moves up the front Friday and brings more widespread rain and scattered thunder. Low pressure exits Friday night, with dry and seasonable conditions on Saturday. Another low may bring the chance for showers late Saturday night into Monday. High pressure returns bringing dry but warmer and muggy conditions into the middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 940 AM Update... Early morning fog/stratus has lifted near South Coast. Should have a decent amount of sunshine through afternoon as high clouds stream over region per satellite loop. Concern for this afternoon is potential for showers/storms across interior. Daytime heating will bring modest surface-based CAPE (up to 1000 j/kg) away from coast but 0-6km shear is weak (less than 25kt) as are mid level lapse rates (under 5C/km). High-res models and ensembles are pinpointing western and north central MA for scattered showers/storms this afternoon, which is in line with current forecast. Not expecting to see a lot of activity, but main time frame looks to be 4 pm to 8 pm before loss of daytime heating causes any activity to dissipate. Gradient remains weak enough through most of afternoon to allow for sea breezes to develop near both coasts by midday. However, we do think S winds will begin to increase later this afternoon in response to tightening pressure gradient, so many spots along E MA coast should top out around 90 later in day. Otherwise, forecast highs in 90s for many locations are on track. Dewpoints in 60s will keep heat index no higher than 95 or so. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Some leftover convection possible this evening/early night in Northern/Western MA as the cold front settles through our area. This should diminish with the setting sun and as a weak shortwave ridge races overhead early at night. Attention then turns to another shortwave, this one coming through the Ohio Valley later tonight and across the Northeast on Friday. The previous model suite gravitated to two scenarios. One scenario showed the associated surface low passing close to New England and bringing us a soaking rain, while the second scenario kept the surface low and most of its rain offshore. The overnight suite has converged on the first scenario, which we will follow with this forecast. Low level jet feeds high moisture content air starting around 12Z Friday. This suggests timing the rain for late tonight through Friday afternoon. Precipitable water values are forecast between 2 and 2.5 inches...very juicy air...so the potential will exist for locally heavy downpours embedded in the rain area. Aside from run-to-run consistency, the question remains as to where the axis of heavy rain will set up. The stalled cold front is the most likely feature to focus low level lift. This would be somewhere near the South Coast. The projected tracks of the surface low also favor the South Coast area with heaviest rain along and just north of the low. We used a blend of RFC and WPC rain data for our own forecast. Broad area averages will be an inch to 1.25 inches. Within this, locally heavier amounts up to 2.5 inches is possible. We have considered the possibility of issuing a flood watch but will hold off until the day shift to see if the next model suite remains consistent with the overnight. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Leftover showers will push offshore Friday night as low pressure exits * Seasonable and dry for Saturday * Another weak low will bring the chance of rain Saturday night into Monday Overview and model preferences... Medium range model suite showing better agreement early in this forecast period, but still noting some model spread beyond Sunday mainly with the 00Z Canadian GGEM which appears to be a fast outlier with its development of surface and upper level feature for another low approaching the region late this weekend into early next week. Leaned toward a GFS/ECMWF model blend early in the forecast /into Monday/ then transitioned over to the model ensembles for the remainder of the forecast. Continue to note a rather fast mid level flow in place in the northern stream with developing broad H5 troughing across the Great Lakes Friday night-Saturday, but the trough tends to sit across the eastern Lakes through Saturday night before finally starting to push E Sunday into Monday. With the stalled front S of New England during this timeframe, looks like another wave of low pressure moves along it. This will bring another shot of precip to the region. Once the H5 trough finally clears the region Monday afternoon and evening, ridging builds across central Canada up to Hudson Bay. With increasing H5 heights, will see temperatures rise to near or a bit warmer than seasonal normals during Tuesday and Wednesday with dry conditions. Details... Friday night... As low pressure pushes offshore, will see precip heading E. Conditions will improve from W-E, with skies becoming partly cloudy away from the east coast. Some showers may linger across Cape Cod and the islands through 2 or 3 AM. Patchy fog will likely develop along the coast as well as the normally prone inland areas as dewpts remain in the lower-mid 60s. Expect low temperatures in the lower- mid 60s for most areas, except near 70 along the coast. Saturday... High pressure ridging extending from south central Quebec to off the NJ coast will bring dry conditions to the region. Temps will run close to or slightly above seasonal normals, with readings topping off in the lower-mid 80s for most areas away from the S coast. N-NW winds will start off at 10-15 mph early, but will diminish as the high builds in. With the warm temperatures, sea breezes will likely kick in from mid day through the afternoon. May start to see mid level clouds increase across the CT valley late in the day, with increasing clouds Saturday night. Sunday through Monday... Noting low pressure across PA/NJ region. With the W-SW wind flow aloft, the moisture with this weak low will push slowly NE into the region which is ahead of approaching H5 trough out of central NY/PA. Big question is how quickly this feature will move across the region, as it appears the H5 trough slows into Monday. Still some timing issues in place, so held on to CHC POPs through this timeframe. Looks like some decent instability moving in during Sunday into Sunday night, so have also included a chance for thunderstorms as well. The H5 trough moves across the region during Monday. Will still see some showers lingering, but should end late Monday or Monday night. Tuesday-Wednesday... High pressure builds slowly east during this timeframe, so should see mainly dry conditions. Can not rule out some isolated diurnal showers across far inland areas mainly on Wednesday. Expect temperatures to rise up to around 5 degrees above seasonal normals by mid week. Winds shift to S-SW, so will see dewpts rise as well. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. High confidence through Fri. Light W/SW winds will give way to sea breezes along both coasts between 15z and 17z. Winds along E MA coast expected to shift to S later this afternoon, probably around 21z. Scattered showers/storms possible across western and north central MA after 21z and may bring brief MVFR/IFR vsbys. Tonight and Friday... VFR before midnight. Rain and scattered thunderstorms developing well after midnight. Ceilings/vsbys will lower to MVFR/IFR late at night and Friday in this precipitation. Conditions may start improving from west to east Friday evening. KBOS TAF...High confidence. Sea breeze develops around 15z. Any TS stays well W. KBDL TAF...High confidence. Outlook /Friday Night through Monday/... Friday night...Low to moderate confidence. Patchy MVFR CIGS/VSBYS mainly across central and eastern areas through midnight. Local RA/+RA in any isolated thunderstorms. Patchy fog develops along the coast and normally prone inland areas after midnight with local MVFR- LIFR VSBYS. Light N-NW winds. Saturday...Moderate confidence. Any leftover MVFR-IFR VSBYS in patchy fog through 14Z, otherwise VFR. Light N-NW winds with sea breezes developing along the coast. Saturday night through Monday...Low confidence. Mainly VFR, but local MVFR-IFR conditions in scattered -SHRA. Patchy fog late Saturday and Sunday nights with local IFR-LIFR CIGS/VSBYS. A few thunderstorms possible Sunday into Sunday night. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High confidence. Winds remain below 25 knots and seas below 5 feet through the period. There is a low confidence potential for 25 knot winds on the southern outer waters Friday afternoon. Rain, heavy at times, with possible thunderstorms. This will bring periods of poor visibility. Outlook /Friday Night through Monday/... Friday night...Low to moderate confidence. Light E-SE winds shift to N-NW overnight with gusts up to 20 kt over the open waters. Rain likely with a few thunderstorms possible. Locally heavy downpours. Seas 4 ft or less, though may reach 5 ft on the southern outer waters. Reduced visibilities in areas of fog along with the rain and any thunderstorms. Saturday...Moderate confidence. N-NW winds gusting up to 20 kt early, diminishing during the afternoon. Seas up to 5 ft on the southern outer waters, subsiding during the afternoon. Locally reduced visibilities early in patchy fog mainly near shore, but improving by late morning. A few showers lingering near and E of Cape Cod early. Saturday night through Monday...Low confidence. Light SW winds back to S-SE Sunday into Sunday night. Winds shift to NE late Mon- Mon night with gusts up to 20 kt. Low pressure passes S of the islands, bringing scattered showers. A few thunderstorms possible mainly Sunday into early Sunday night. Seas should remain below 5 ft. && .FIRE WEATHER... Dry conditions will persist today, except possibly in western and north central MA where scattered showers/storms may bring localized rainfall Thu afternoon. Minimum RH values of 30 to 40 percent are expected across much of southern New England, except 40 to 60 percent on the South Coast, Cape Cod, and the Islands. South to southwest winds will stay under 20 mph. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert until 11 PM EDT this evening for CTZ002. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...JWD SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/EVT/JWD MARINE...WTB/EVT FIRE WEATHER...WTB/EVT

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