Area Forecast Discussion
Issued by NWS Boston, MA

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988 FXUS61 KBOX 150618 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 218 AM EDT Tue Aug 15 2017 .SYNOPSIS... High pressure moves offshore overnight, allowing some scattered showers to move into the region late, especially along the south coast. The risk for showers lingers tomorrow, with an isolated thunderstorm possible before improvement arrives again tomorrow night. Dry conditions follow Wednesday and Thursday. A warm front approaches Friday, bringing increasing clouds, and scattered showers. Showers and Thunderstorms are possible Saturday as a cold front enters the region. A drying trend likely follows Sunday and Monday along with near seasonable temperatures. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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230 AM Update... A soupy morning is shaping up as High pressure continues to move offshore. This has allowed for return southerly flow an a increase in low level moisture. Several locations are already reporting fog with the Vineyard and Nantucket down to 1/4 of a mile. A few other sites along the immediate south coast are also falling before 5 SM. Latest guidance from the NARRE-TL shows that the fog will continue to develop and spread north and west this morning. Thus will have to monitor if dense fog adv will need to be expanded. Aside from the fog, the other focus is on scattered showers early this morning. Stalled front to the south of the region, combined with overrunning moisture within the WAA pattern has resulted in the development of showers across Long Island and south of the Nantucket. Approaching upper level jet has also developed showers across eastern PA/Northern NJ with the expectation that they will become more widespread as they reach southern New England. Thus went ahead and increase precipitation chances early this morning. Shower will linger as we approach the morning hours which could make it a slip AM commute. QPF amounts will generally be light, but with PWAT values reaching above 1.5 inches south of the Pike, may need to watch for nuisance flooding.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Tomorrow... As upper lvl jet-streak becomes more refined it shifts its equatorward entrance region mainly SSE of New England later in the day even as secondary shortwave energy moves out of the Great Lakes and into central and N New England. This should allow initial round of SHRA to dissipate through the morning and shift offshore, focused more intently on the strengthening Gert and the front offshore which looks to capture it. Will maintain slight chance POPs mainly along the immediate S coast with particular focus on the Islands, closer to where the best synoptic support will focus later in the day. PWATs remain in the 1.50-2.00 inch range, supporting this area. Otherwise, with the secondary shortwave moving in from the W initially rising heights will give way to slight cooling aloft, enhancing mid lvl lapse rates from around 5.0C/km to start, closer to 6.0C/km by late afternoon mainly across NW MA. With modest moisture plume (still focused only on the S coast), this could lead to a period where CAPE values reach 700-1200j/kg but with a very tight gradient toward the E due to influence of mid lvl dry air and capping. This should limit focus for late day convection to mainly the most extreme W/NW zones, and most likely in N and central New England where diurnal heating will be better maximized. Shear will be the once enhancing component, as 0-6km shear values during the afternoon reach 45kt. Not expecting much in the way of severe due to these limiting factors, particularly the fact we remain split between two areas of synoptic lift to the NW and S. This agrees with latest SWODY2 which keeps marginal risk focused mainly across N New England. Highs a bit cooler than normal given a fair amount of early cloudiness. Tomorrow night... Weakening shortwave/front and upper lvl jet axis shift further offshore and become entangled in the future ET process of Gert. This will allow W flow and drier air to entrain through the remainder of the column even where mid lvl drying has not already occurred. Although a mild/moist start, trend will be toward clearing and gradual fog dissipation toward the early morning hours. Although dwpts will begin dropping, they are likely to remain in the 60s through sunrise, and with early cloud cover and continually W flow, lows should not dip below the low-mid 60s even in coldest spots. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Pleasant summer weather Wednesday and Thursday * Risk for showers/Thunderstorms Friday and Saturday * Improving trend Sunday into Monday Synoptic overview... Persistent and anomalous northeast Pacific jet continues to eject numerous short wave troughs across the Canadian/US border resulting in the risk of showers/T-storms at times. In between systems, many hours of dry weather. As for temps, active northern stream jet suppresses subtropical ridge and its associated excessive heat and humidity across the southern states, resulting in temps here near normal for mid Aug. Temperatures... As mentioned above, active northern stream jet suppresses subtropical ridge and its excessive heat and humidity south of New England. Overall this pattern supports temps near normal thru the period. The warmest day in this stretch is likely Wednesday in the post frontal airmass with WNW winds providing some additional downslope adiabatic warming and keeping seabreeze offshore. Thus most beaches will warm into the low 80s Wed with mid to upper 80s in the coastal plain /including Boston-Providence corridor/ and the CT river valley. The core of this dry airmass settles over the area Wed ngt into early Thu, which will result in cooler but very pleasant temps Wed ngt into Thu morning with lows in the 50s, possibly upper 40s across northwest MA. Given this airmass and potential radiational cooling sided with the cooler MOS mins Wed ngt/Thu morning. Seabreeze and cooler airmass Thu will keep highs in the upper 70s and low 80s. In fact dew pts could be in the 40s! Quite comfortable. Cloud cover and convective chances Fri and Sat will also limit highs to the upper 70s and low 80s. A drying trend along with post frontal WNW flow Sun and Mon may result in slightly warmer temps. Precipitation... Post frontal airmass overspreads the region Wed and lingers into Thu. Thus dry weather both days. Then next short wave approaches Fri with increasing clouds and warm frontal rain/showers likely. Trough is slow to exit so risk of convection Sat ahead of the approaching cold front. PWATs climb to +1 to +2 standard deviations above climo so some heavy downpours are possible. Ensembles and deterministic guidance both suggest trough begins to exit around Sunday so a drying trend is possible Sun/Mon. Beach Forecast... Swells from tropical storm Gert are expected to arrive into our coastal waters Tue ngt and then peak Wed with southerly swells up to 7 ft possible. Therefore surf will become quite rough Wed along with increasing rips. Beachgoers will definitely need to exercise caution especially at south facing ocean beaches where surf and rips will be strongest. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... Before 12z...Moderate confidence. VFR away from the Cape and Islands, but many eastern terminals will drop to MVFR cigs by sunrise. LIFR conditions will continue across the Cape and Islands as IFR occurs along the south coast. Widespread showers across the region after 06z. Today...Moderate confidence. Lingering fog burns off after sunrise. Sct showers will continue until late this morning. Otherwise, VFR with a few showers and isolated T-storms mainly W MA during the afternoon. Weak sea breezes along the east coastline. Tonight...Moderate confidence. VFR gives way to another mix of IFR/MVFR mainly CT/RI and SE MA while the remainder stays mostly VFR, then as winds shift around to the W, transition back to VFR everywhere toward the morning. Wednesday...High confidence. VFR with dry weather. KBOS Terminal...Moderate confidence in TAF. Low probability for a few hours of MVFR conditions after 09z. KBDL Terminal...High confidence in TAF. Outlook /Wednesday Night through Saturday/... Thu...VFR likely along with dry weather. Light WNW winds Wed becoming onshore Thu. High forecast confidence. Fri and Sat...marginal VFR-MVFR in scattered showers/T-storms both days. Low risk for IFR conditions Fri night into early Sat. Forecast confidence high on trends but lower on specific details.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Through tomorrow... S winds develop with gusts to around 20-25 kt on the E waters during the day tomorrow, but brief. Otherwise, they remain below small craft thresholds through the day tomorrow. Seas will increase late tomorrow, but should remain below 5 ft through tomorrow evening. Otherwise, mix of fog and showers, particularly on the S ocean waters and waters surrounding Cape Cod during the overnight and early morning. Tomorrow night... Continued risk for showers early, but then dissipating late. The main issue will be a gradually increasing southerly swell which could reach as high as 7 ft on the southern open ocean waters by daybreak on Wed. Therefore, will likely a round of small craft advisories for mainly the southern waters. Outlook /Wednesday Night through Saturday/... Wed and Thu...fine boating weather both days, good vsby and dry weather. Only issue will be across the ocean waters especially south shore where large southerly swells up to 7 ft Wed, then eroding Thu. Forecast confidence is moderate to high. Fri and Sat...scattered showers/T-storms expected along with light to modest southerly winds. Vsby may lower in precip and esp Fri night in areas of fog. Mainly southerly wind waves with swells from Gert likely well offshore.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Dense Fog Advisory until 10 AM EDT this morning for MAZ022>024. RI...None. MARINE...None.
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&& $$ SYNOPSIS...Nocera/Doody NEAR TERM...Dunten SHORT TERM...Doody LONG TERM...Nocera AVIATION...Nocera/Dunten MARINE...Nocera/Doody

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