Area Forecast Discussion
Issued by NWS Boston, MA

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401 FXUS61 KBOX 272340 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 740 PM EDT Thu Jul 27 2017 .SYNOPSIS... A cold front moves into the region this evening and will be accompanied by scattered showers followed by a drying trend behind the front later tonight. The front weakens or washes out Friday giving way to mainly dry weather along with seasonably warm temperatures and humidity. Low pressure then tracks south of New England Saturday, likely bringing a windswept rain to the Cape and Islands but possibly dry weather for the remainder of the region. A coastal storm will push off the mid Atlantic coast and track south of New England Saturday, which will bring rain and gusty onshore winds with cool temperatures especially close to the south coast. Another low passing off the coast may renew the chance for rain across portions of the region later Monday into Tuesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 720 PM Update... Back edge of precip from passing mid level trough as weak surface reflection pushing off the Mass coast as seen on latest NE regional 88D radar mosaic. Noting a long of showers and scattered thunderstorms moving steadily E-SE out of upstate NY and northern VT, but short range models indicating this should weaken as it moves along. Short range high res models have been signaling a drier solution over the last few hours, which seems to be confirmed by radar. Noting dry conditions into eastern NY state down to NYC where the last of showers are moving through there as well. Agree with previous forecast that it will not really dry out in the low levels as dewpts remain in the mid to upper 60s through most of the night, so will probably see patchy fog develop by around midnight and linger through around 11Z-12Z. Have updated near term forecast to end the showers along the S coast by around midnight, and kept fog in forecast. Also updated remainder of near term to bring conditions current. Previous Discussion... Later this evening and especially overnight the short wave trough moves east of the region with its attending cold front sliding south and reaching the south coast by sunrise. Good drying through the column from top down as K indices fall from the mid 30s this evening to mid teens by 12z Fri. This will result in a drying trend from north to south tonight. However this mid level dry air will not reach the surface with dewpts remaining in the 60s regionwide overnight. So while showers will taper off fog will likely develop as the night progresses. Given the relatively high dewpt air it will be much milder tonight than previous nights with lows in the 60s. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... 4 pm Update... Friday... Behind departing short wave tonight mid level confluent flow develops across the area. This provides mid level subsidence over the region weakens or washes out the frontal boundary near the south coast. This mid level drying and associated subsidence provides a cap tomorrow and should yield a mainly dry day. Given dew pts will remain in the 60s there will be some SB instability present tomorrow afternoon along with some weak low level convergence from seabreezes. Thus can`t rule out an isolated low top shower during the afternoon. Given the mid level cap and limit instability (SB and aloft) decided not to include any thunder. So much of the day looks dry tomorrow along with seasonably warm temps in the upper 70s to low 80s and typical summer humidity with dew pts in the 60s. A combination of some mid level cloudiness along with diurnal clouds should allow for at least a few breaks of sunshine. Friday night... Other than an isolated low top shower the evening hours should be mainly dry as mid level cap and dry air aloft hold til about 03z or so. However thereafter closed mid level low near Pittsburgh moves east and backs the mid level flow along the eastern seaboard, advecting deeper moisture northward into southern New England, especially the south coast. This will result in the risk of overrunning rain /yeah, it really is late July/ to invade the south coast including Cape Cod and the Islands. The remainder of the region likely remains dry especially north of the MA Pike. Seasonably mild with lows in the 60s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Highlights... * Risk continues for showers and a few thunderstorms possible Fri night and Sat, best chance S coast * Long E-NE fetch off the Gulf of Maine leads to continued strong winds especially along the coast from S of Boston to the S coast this weekend into early next week * Another low passes offshore late Monday into Tuesday with more light rain for eastern areas Overview and model preferences... While examining the 12Z operational models, noted changes to a slower solution along the mid Atlantic and northeast U.S. with respect to overall motion of surface systems especially from Sunday through Tuesday. Continued wide solution spread, however, though in the broad sense they keep an overall E-NE wind flow in place with a front stalled to the S and high pressure, albeit weak, to the north. Timing of surface waves along the front remains tough, as some guidance has a couple of waves early in this period, while others have only one from late Sat into early Mon. The biggest question appears to be how each individual model member handles the H5 pattern, with some developing a broad cutoff H5 low (12Z GFS/00Z ECMWF) generally across the mid Atlc states to about the S coast of CT/Long Island, while others keep broad positive tilted troughing. What really makes this forecast particularly difficult is, even if cutoff H5 low forms, there is still quite a bit of dry air to the N which could lead to variable sensible weather into early next week. Where this all sets up, and where the dry vs. somewhat wetter conditions develop, is another big dilemma. At this point, with so much uncertainty in place, will be leaning toward available model and ensemble guidance blend for this forecast package. Depending upon the final blend, may enhance with some more agreeable model blending, such as around the Sunday-Monday timeframe which does overall appear drier for most areas. Then, may see another low pres wave move along the stalled front, but could lift a bit further N which could affect the eastern part of the forecast area Monday night and Tuesday. Beyond Tuesday, looks like broad trough continues across the eastern U.S., though weak surface ridge should keep mainly dry conditions in place for Wednesday and next Thursday. Details... Saturday through Sunday night... As low pres slowly passes S of the region Sat, will see area of rain move in across N CT/RI/SE Mass, with some patchy light rain further N from there. Pretty sharp dry are across N Mass, though, so may remain dry there. Precip pushes offshore Sat night with drier conditions. Will see gusty NE wind especially along the coast with gusts up to 30-40 mph, highest along the S coast, Cape Cod and the islands. May see the northern fringe of heavier rainfall reach portions of Cape Cod and the islands, with some local rainfall of around an inch. Have left mention of thunder out of the forecast with the cooler, stable onshore winds. Expect temps both days to run up to around 10 degrees below seasonal normals. Monday and Tuesday... H5 long wave trough lingers along the eastern seaboard during this time. Another low forms offshore and passes close enough to the 40N/70W benchmark to bring another round of rain across central/E Mass into RI and NE CT Monday afternoon through part of Tuesday. Continued cool NE winds in place, but not quite as strong as over the weekend. As the low passes Tuesday, rain should taper off from west to east. Temps will recover closer to seasonal norms with highs reaching the lower 80s well inland but will remain cooler along the shore. Wednesday and Thursday... Lower confidence due to wide model solution spread. For now, looks dry and warm on Wednesday, then another front may approach during Thu though timing is uncertain. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 720 PM Update... Tonight...High confidence on trends but lower on exact timing. Leftover showers will shift E with drier conditions developing through around midnight. Low level moisture lingers, so will likely see MVFR-IFR conditions developing in low clouds and patchy fog. Friday...Moderate confidence. Any morning IFR-MVFR conditions should lift to VFR by midday. Then marginal MVFR-VFR develops in the afternoon regionwide with isolated shower possible especially near the south coast. Friday night...moderate confidence. Marginal VFR-MVFR in isolated shower threat during the evening will trend toward IFR late especially south coast as risk of rain increases. KBOS Terminal...High confidence on trends but some uncertainty on exact timing of lowering VSBYS overnight then timing improvement Fri morning. KBDL Terminal...High confidence on trends but lower confidence on exact timing of changing categories. Outlook /Saturday through Tuesday/... Saturday and Saturday night...Moderate confidence. Area of rain across N CT/RI/SE Mass with MVFR-IFR CIGS/occasional MVFR VSBYS, with patchy heavier rain along the immediate S coast possible. Patchy fog with local IFR VSBYS late Sat/Sat night. May see patchy -RA/-SHRA reach across most of the remainder of the area but should be mainly VFR. E-NE winds gusting to 25-35 kt along coast, highest along S coast, Cape Cod and the islands. Precip may start to taper off from N-S late Sat night with improving VSBYS. Sunday...Moderate confidence. May see patchy -RA lingering along the S coast through midday Sun with local MVFR-IFR CIGS/VSBYS. Otherwise, should see mainly VFR conditions. NE winds gusting to 20-30 kt along the coast, highest across Cape Cod and the islands, through Sun then should diminish from N-S Sun night. Monday-Tuesday...Moderate confidence. Area of -RA may push back across E Mass/RI/NE CT with local MVFR CIGS/VSBYS mainly along the coast Mon afternoon into Tue, then may improve Tue night from W-E. Isold TSRA possible along the coast Tue. Otherwise mainly VFR. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. 720 PM Update... Tonight... Light S-SW winds this evening becoming west toward daybreak. Showers and patchy fog will reduce visibility late this evening and overnight. Tranquil seas continue. Friday... Light SE winds and mainly dry weather. Vsby may be reduced in morning fog but improving by midday. Tranquil seas continue. Friday night... Winds become NE and increase to 20-25 kt by daybreak Sat. Seas increase rapidly given the long fetch. Chance of rain increases especially southern waters. Outlook /Saturday through Tuesday/... Saturday and Sunday...Moderate confidence. As low pressure passes S of the waters, will likely see NE wind gusts to 30-35 kt during this timeframe mainly from E of Cape Cod southward. Gale watch has been issued for those areas through Sat night, but could continue into Sun. With long onshore fetch, seas will build up to 6-10 ft, possibly higher across the outer waters. Area of rain and patchy fog will likely bring reduced visibilities, which may briefly improve Sun afternoon/night. Monday...Moderate confidence. NE winds will continue to gust up to 25-30 kt, then start to back to N Mon night as the low pushes offshore. Seas remain at or above 5 ft on the outer and southern near shore waters. Another area of rain and patchy fog moves in during Mon with reduced visibilities. Tuesday...Moderate confidence. N winds gusting to around 25 kt Tue morning, then diminishing. Seas at or above 5 ft should subside during Tue night. Areas of rain/fog with reduced visibilities should improve from W-E Tue night. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Watch from Saturday morning through late Saturday night for ANZ231>235-237-254>256. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera/EVT SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT

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