Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 302301 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 701 PM EDT Fri Sep 30 2016 .SYNOPSIS... Low pres moves South of the region through the weekend, yielding cool and damp conditions. Low pressure over the Ohio Valley lifts to the Great Lakes, then moves east across New England Monday and Tuesday. High pressure builds over New England Wednesday and Thursday. Still watching Hurricane Matthew, but uncertain information whether it affects New England or not. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 7 PM Update... Radar shows bands of showers moving north across Southern New England. Observed rainfall was up to one-quarter inch on the mainland and one-quarter to three-quarters of an inch on the islands. No significant changes to the forecast, except to shift the higher pops a little farther north. Previous discussion... Initial band of precip associated with a combination of overrunning from stalled frontal boundary about 100 sm offshore of the S coast and robust 35-40 kt H92 LLJ. In fact, the band is pretty much perfectly colocated with this LLJ. Only meso-scale guidance and 12Z ECMWF have had this timing correctly through the day, as most other OP guidance is far wet/fast, so with this forecast many of the parameters were updated with a blend of the ECMWF/HRRR/RAP. This band will pivot S-N and likely fill in somewhat given the increasing moisture through the column (dry mid-layer from this mornings CHH/ALB soundings continues to erode) through midnight. This is thanks to the inverted-ridging still evidence in the lowest lvls. Even through the LLJ lifts N of the region and dissipated in- situ, the increased moisture and strong overruning signal (relatively deep inversion from sfc-H85, with conditional instability aloft) expect continued on- off shra activity through the overnight, so have POPs peaking during the initial band passage, then very slowly diminishing into tomorrow morning. Overall QPF values have been lowered a bit, as ECENS probabilities are quite low for values greater than 0.5 inches in 24 hours and agree now with it`s operational run. Therefore, given hourly precip values with the banding currently shifting onshore have been 0.05-0.20, totals through the overnight hours generally rest at about a half inch. One other issue tonight and tomorrow is winds. Although near 40 kt LLJ is expected, the very strong inversion will limit mixing to the sfc. However, can`t rule out some gusts approaching 25-35 mph and although this is below the threshold for wind advisories, stressed trees may yield a bit more damage than we would otherwise forecast. Min temps not expected to drop too much from current values even in spite of dwpts in the 40s, so suspect mainly widespread low-mid 50s. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Tomorrow and tomorrow night... Although not a washout, expect continued risk for shra across the region thanks to PWATs increasing to nearly 2.0 inches. This is combined with upper lvl difluence on the periphery of the warm- conveyor/wrapped occlusion attendant to cutoff low pres in the OH valley. The reason a washout is not expected, however, is that overall dynamics driving the rainfall potential is very low and forced only by weak mid lvl overruning from stalled frontal boundary to the S. Latest ECMWF highlights much lower QPF, and will be lowering total QPF values across the region as well. Inverted ridging, although weakening is likely just strong enough to counteract the weak forcing. Final totals (including this evening/overnight) range from about 0.5 inches in NW MA to about 1.5 inches across the Cape/Islands. Although some locations could over-achieve. Temps still below normal thanks to flow off the Gulf of Maine and continued cloud cover. Looking at highs in the mid to upper 50s, and lows mainly in the upper 40s and low 50s. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Big Picture... Longwave pattern continues to show West Atlantic high in place, but shifting south over the weekend. Closed Upper Low over the Ohio Valley shifts north, then moves east as the Atlantic high moves out of the way. This upper low moves over New England Monday and Tuesday. High pressure then moves over the Northeast USA Wednesday and Thursday. Upper low along the British Columbia coast swings across the USA during the week, then lifts northeast across Eastern Canada Friday. And then there is Matthew. There is a great difference in how the models handle the storm, especially north of the Bahamas. The GFS continues to outrun the ECMWF in bringing the storm up the coast, but reacts to the building midweek upper high by dancing and spinning east of our area. The difference continues to be the GFS showing a greater phasing of Matthew with a southern stream shortwave, while the ECMWF shows either no phasing or greatly delayed phasing...and thus a much slower movement north. We favored a model blend for most of the long term. The GFS has been trending slower on Matthew, so a movement away from either extreme seems a good thing. Details... Sunday... Low level east winds are overlaid with 20 knot southwest winds at 4000-5000 feet. Meanwhile the 90-knot upper jet associated with the closed low...this jet approaches Southern New England and brings some meager upper venting. We will maintain clouds and chance pops through the day and through Sunday night. Low level northeast flow will keep us in a small diurnal range, with 50s and 60s during the day and 50s at night. Monday-Tuesday... Upper low and cold pool move overhead Monday and Tuesday. Monday will feature the most instability with GFS-ECMWF-NAM-GGEM showing totals in the upper 40s and lower 50s. This looks mainly to be scattered showers Monday but with enough instability to mention a widely scattered tstm. Less coverage Tuesday, but enough for scattered or widely scattered showers. Surface wind flow will continue from the northeast keeping temps below normal especially in Eastern MA and RI. Mildest temps will be in the CT Valley. Wednesday through Friday... High pressure actually starts building south from Canada on Tuesday, but is most notable starting Tuesday night with temps and dew points cooling into the 40s. The high brings fair and dry weather Wednesday and Thursday with partly sunny skies. Northeast flow continues. Friday currently looks to maintain the dry weather as high pressure moves off through the Maritimes. If Matthew moves close enough, then we could see some showers well out ahead of the system. But very low confidence in such a scenario so we have indicated only slight chance pops in eastern and central sections. Otherwise dry weather. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Tonight into Saturday...Moderate confidence. ENE winds increase overnight just off the deck, could see some minimal LLWS as wind speeds reach 30-40 kt at 2kft. Otherwise, MVFR expands north to the NH border, while ceilings continue to lower to IFR farther south. Low end MVFR or IFR cigs linger through the day Saturday with on and off rain showers lowering vsbys. Saturday night...Moderate confidence. May see some minimal improvement during the overnight hours, however low CIGS remain an issue, at least at MVFR. KBOS TAF...High confidence in trends, moderate confidence in timing. KBDL TAF...High confidence in trends, moderate confidence in timing. Outlook /Sunday through Wednesday/... Sunday...Moderate confidence. MVFR/IFR cigs and vsbys in showers and fog. Diminishing northeast flow. Monday-Tuesday... Low confidence. VFR with areas of MVFR in scattered showers. Wednesday...Moderate confidence. VFR. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... High confidence. No major changes to current marine headlines. Gale warnings continue through the early night for the southern waters, with small craft advisories for all waters now and thereafter. It will likely take until into Sunday for seas to drop below 5 ft as they peak near 11 ft on the SE waters this evening. Outlook /Sunday through Wednesday/... Diminishing Northeast wind Sunday and Monday with gusts 20 knots or less. Northeast winds increase again Tuesday and Wednesday on the Southern and Southeast waters with frequent gusts 20-25 knots. Seas 5-6 feet on the eastern waters Sunday but diminishing. Seas build again Tuesday and Wednesday with 5-7 foot seas on the outer waters. Small Craft Advisory will be needed for much of this time period. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 5 PM EDT Saturday for ANZ232. Small Craft Advisory until noon EDT Saturday for ANZ233-234. Small Craft Advisory from 10 PM this evening to 8 PM EDT Saturday for ANZ230. Small Craft Advisory until 8 PM EDT Saturday for ANZ231. Small Craft Advisory until 11 AM EDT Saturday for ANZ236. Gale Warning until 2 AM EDT Saturday for ANZ235-237. Small Craft Advisory until 8 AM EDT Sunday for ANZ250-251-254. Gale Warning until midnight EDT tonight for ANZ255. Gale Warning until 5 AM EDT Saturday for ANZ256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.