Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 252320 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 720 PM EDT Tue Jul 25 2017 .SYNOPSIS... Skies will clear tonight. High pressure will bring a return to dry and milder weather for Wednesday and into Thursday. A low pressure wave will move along an approaching cold front late Thursday, passing across the region during Friday. Some leftover showers may linger across southern areas for part of Saturday along with cool northeast winds. Dry, seasonable conditions return late next weekend into early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 705 PM Update... Clearing skies moving southwestward on the NE wind around a bubble of high pressure across VT and upstate NY at 22Z. Visible satellite imagery shows the clearing quite nicely. Expect the clearing to continue to push across the remainder of the region this evening. Some mid and high clouds may linger near the CT valley through around midnight or so. N-NE winds continue, highest across Cape Cod and the islands, which will become mainly N this evening there. Further inland, light/variable or calm winds will be in place. Have updated near term grids to bring conditions current, especially the sky cover, T, Td and winds. Previous Discussion... Through Tonight...Upper trough heads east of New England, and drier air filters into the region. Anticipate gradual clearing this evening. This should allow enough radiational cooling to allow interior temperatures to drop to the lower to mid 50s. There could be isolated pockets in the interior NW valleys that dip into the upper 40s, rather notable for late July. Winds will be light with a drift from the north. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... Surface high pressure and upper ridging will dominate across a dry air mass. Anticipate plenty of sunshine with just some afternoon cumulus and temperatures generally in the upper 70s and perhaps near 80 in the lower portions of the Merrimack and Connecticut Rivers. Temperatures may be capped in the mid 70s along coastal areas due to developing seabreezes. Wednesday night... Expect dry conditions to prevail but will probably experience increasing high clouds late ahead of a shortwave trough dropping southeast from central Canada. A weak SW gradient will become established. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Dry early Thursday * Showers and isolated thunderstorms move into northern and western areas Thursday afternoon/evening * Best chance for showers/scattered thunderstorms Thursday night through Friday night * A few showers linger into Sat, then dry but cool conditions * Dry and seasonable weather likely Sunday into Monday Overview and model preferences... Weak H5 ridge pushes E to the Maritimes early in this period, while digging H5 trough moves E-SE out of the Great Lakes. However, individual model solutions vary on the intensity of the trough as it shifts E. This is giving widening surface solution spread as the surface front and possible wave development during the Thu night-Fri timeframe. 12Z GFS appears to be an outlier as it pushes this system further S late this week and keeps the wave weaker/further S. Also, GFS blasts the H5 short wave offshore by late Thu/Thu night, while most of the remaining suite is slower. A few members even try to develop H5 bubble cutoff in the NW flow over the western Great Lakes Thu night/Fri and move it eastward. While the GFS moves the surface front offshore, the 12Z GGEM and 00Z ECMWF keep the front a bit further N but also have low pres moving along it. The 12Z NAM is a bit more toward the GGEM/EC camp, though a bit further S with the developing low over the lower Appalachians. By Friday night, most models have the low passing S of New England, but northern fringe of the precip may reach into the region though each solution varies on how far N the precip shield extends. Have low to moderate confidence during the late week timeframe. Am leaning toward a non-GFS blend especially for the Thu night to Fri night timeframe. Beyond this, should see the H5 trough move offshore during the upcoming weekend, but noting broad long wave troughing setting up across the eastern half of the U.S. while high amplitude ridge builds from the west coast into British Columbia. Should see generally dry but cooler than seasonal norm temps with continued NE wind flow as surface high builds across the western Great Lakes/northern Plains. Overall, will use blend of models/ensembles for this timeframe. Details... Thursday... High pressure ridge shifts S Thursday with SW winds in place. Leading edge of clouds/scattered showers look to push into northern areas from NE through central-W Mass into N central CT during Thu afternoon. Marginal instability, but could see a few isolated thunderstorms through 00Z or so. Highs will be mainly in the 70s, ranging to around 80 across the lower CT valley. SW winds may gust up to 15-20 kt along S coastal areas, Cape Cod and the islands. Thursday night through Friday night... Models continue to signal low pressure wave shifting E from the Ohio valley along the passing cold front. Some model differences in frontal placement and how quickly the low moves E. At this point, looks like best chance for showers/scattered thunderstorms will occur during this timeframe. Decent instability with this front. Best shot for precip will occur mainly near and S of the Mass Pike where likely POPs are in place. Noting good slug of QPF pushing through, with 1-1.5 inches possible across S coastal RI and SE Mass during this timeframe. Winds shift to N-NE as the low passes during Fri, then will pick up late Fri/Fri night mainly along the S coast as H925 jet up to 40-45 kt passes. Winds may gust up to 20-25 kt along the immediate S coast/Cape Cod and the islands for a time mainly around or after midnight Fri night. Saturday... Showers may linger along the S coast through at least midday Sat, depending upon how fast the low pushes offshore, allowing the front to move further S. Conditions should improve during the afternoon but it will be cool with NE winds in place for most of the day. Highs will only be in the 70s. Sunday-Monday... Looks dry as high pressure to the W holds in place. Light winds will be in place as well. Temps will rebound close to seasonal levels. Tuesday... Low pressure may develop on the front S of New England, while another cold front approaches from the NW with its own wave. Big question will be how quickly the northern front approaches. Kept only slight chance POPs for the entire area for now. Temps continue close to seasonal norms. Rather low confidence. && .AVIATION /23Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Through Tonight...High confidence. VFR with clearing skies moving SW this evening. Patchy late night fog may occur at some interior airports for a few hours. Wednesday...High confidence. VFR. Any patchy interior valley fog should burn off by 12Z-13Z. Wednesday night...High confidence. VFR. KBOS Terminal...High confidence. VFR. Sea breeze expected to develop by mid to late Wednesday morning. KBDL Terminal...High confidence. MVFR cigs lifting to VFR by around 22Z. VFR for rest of the period. Outlook /Thursday through Sunday/... Thursday...Moderate confidence. Mainly VFR. May see local MVFR VSBYS in scattered showers/isolated thunderstorms from about a KBVY-KORH-KIJD line westward Thu afternoon. SW winds may gust to 20 kt along S coastal terminals. Thursday night through Friday night...Moderate confidence. Mainly VFR CIGS, but pockets of MVFR-IFR in heavier SHRA/isolated TSRA. Areas of widespread SHRA mainly from the Mass Pike S late Thu night and Fri, with sct SHRA N of the Pike. Patchy late night/early morning fog with local IFR VSBYS. Winds will be shifting through the period, but cannot rule out some gusts to around 20 kt as these shifts occur. Saturday...Moderate confidence. May see pockets of MVFR conditions along S coast in lingering SHRA and patchy fog. Otherwise mainly VFR. NE winds gusting to around 20 kt from Plymouth county southward through midday then diminish. Sunday...Moderate to high confidence. VFR. Light N-NE winds with local sea breezes. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. Tonight...Seas will gradually subside overnight. Small craft advisories for hazardous seas continues through 11 PM for Mass Bay and Ipswich Bay, as even a light N-NE flow was keeping seas elevated to 5 or 6 feet there. Seas will remain higher through most of the night across the eastern outer waters to E and S of Nantucket, where small crafts remain in place as well. Wednesday...High pres over the waters will result in light winds with near shore seabreezes. Seas will be below SCA thresholds. Wednesday night...Winds shift to S-SE as high moves off the S coast, then will shift to SW and pick up on the eastern outer waters after midnight. Seas 4 ft or less. Outlook /Thursday through Sunday/... Conditions should be mainly below small craft criteria for most of this period. May see N-NE winds gusting up to 25 kt on the southern open waters late Fri night through Sat as front passes. Seas also build up to 5-6 ft on the southern outer waters for a time Sat into Sat evening. Rain and fog will lead to visibility restrictions especially Thu night into Fri. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 4 AM EDT Wednesday for ANZ250-254-255. Small Craft Advisory for hazardous seas until 11 PM EDT this evening for ANZ251. && $$ SYNOPSIS...EVT/Thompson NEAR TERM...EVT/Thompson SHORT TERM...Thompson LONG TERM...EVT AVIATION...EVT/Thompson MARINE...EVT/Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.