Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201958 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 358 PM EDT Sun Aug 20 2017 .SYNOPSIS... High pressure builds over the region tonight and then offshore Monday, providing dry and comfortable weather. The high moves farther offshore Tuesday bringing warmer and more humid air into the region. The returning warm air triggers the risk for showers and thunderstorms late Tue, Tuesday night into early Wednesday. Later Wednesday the threat of showers and storms ends from west to east as a cold front sweeps through New England. High pressure brings dry weather Thursday through Saturday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 4 PM update... Thru sunset... Very pleasant conditions with temps in the low to mid 80s at 330 pm along with downsloping WNW winds providing comfortable dew pts in the upper 50s. Diurnal clouds erode as sunset approaches along with WNW winds diminishing as blyr layer begins to decouple. Overall, ideal late summer weather. After sunset... High pressure over Pennsylvania builds eastward into region tonight. This will provide light winds, mostly clear skies and a relatively dry airmass to promote temps falling fairly quickly with and after sunset. The most noticeable temp drop will be outside the urban areas. Given this followed the cooler MOS temps to derive mins overnight. Most locations outside the urban areas dip into the upper 50s tonight, thus very comfortable. For the cities temps fall back into the low and mid 60s. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Updated 4 PM... Monday... After a cool start to the day return flow (SW winds) becomes established by late morning as high pres slips offshore. Low level WAA results in slightly warmer and more humid conditions tomorrow with highs in the mid to upper 80s away from the south coast. Dew pts still tolerable (U50s to L60s) as warm sector airmass remains west-southwest of New England. Dry weather prevails along with mostly sunny conditions. Thus good viewing of the solar eclipse (just remember to wear safe glasses)! Could be some mid and high clouds arriving late in the day across western MA/CT, but by then the eclipse will have ended. Monday night... Low amplitude lead short wave and attending warm sector approach from the west. Convection over PA/NY/NJ late Monday likely weakens/dissipates as it approaches southern New England given loss of daytime heating and weak mid level lapse rates. Nonetheless low pops will be included in the forecast to account for any leftover showers especially western MA/CT. Much warmer and humid than previous nights given warm sector overspreads the area. This may be accompanied by stratus/patchy fog and possibly spotty light drizzle. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Big Picture... Zonal flow amplifies over the Northern USA and Canada during mid and late week, replacing warm air early in the week with cooler than normal air late week. GFS and ECMWF mass and thermal fields are similar through the period, although with small differences on Saturday. Meanwhile the GGEM develops a tropical system that moves up the USA East Coast mid to late week. Satellite imagery does show a disorganized system north of Puerto Rico moving west, but the GGEM is the only model to develop it into a threat to the East Coast while the other models develop no system. This will be monitored, but for now left out of our long range forecast. Details... Monday night and Tuesday... High pressure shifts offshore with a developing west to southwest flow over Southern New England. Moist layer between 800 and 850 mb suggests some low cloud formation under a cirrus canopy. The lack of deep moisture, including dry air below the moist layer, will work against shower formation. Data does show a developing theta-e ridge along the New York border, and this may provide a focus for a few showers to form late Tuesday in our far west. Precipitable water values rise through the day. The core of high values will remain west of us in New York, but values of 1.5 to 1.75 inches arrive during the afternoon. Expect increasing humidity during the day, with dew points climbing to the upper 60s and near 70. Temps at 850 mb reach 18C, which with full heating would support max sfc temps either side of 90. Tuesday night and Wednesday... Upper shortwave moving up the St Lawrence Valley will swing a cold front across the Northeast USA. Consensus of timing would bring the front across New York State Tuesday night and across Southern New England on Wednesday. Deep moisture along with PW values of 2+ inches will be acted on by favorable dynamics in the right entrance region of the upper jet. This suggests potential for strong thunderstorms along with locally heavy downpours, especially Wednesday just ahead of the front. Thursday through Saturday... High pressure builds in at the surface while an upper trough digs south from Quebec. The trough will carry a cold pool with cold advection aloft on Thursday and Friday. Temperatures at 500-mb will minimize at -16C to -18C early Saturday morning. Moisture fields show a moist layer between 850 and 700 mb during this period, but dry air above and below. This looks like a partly cloudy and dry pattern, but the instability shows some potential for a few showers. The mixed layer reaches to between 850 and 800 mb, with temps at the top of the layer supporting max sfc temps in the 70s. && .AVIATION /20Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... 2 PM update... Thru sunset... VFR with any cigs BKN050. Dry weather and modest WNW winds. After sunset... Diurnal clouds erode with VFR prevails. Dry runways and light WNW winds becoming calm. Patchy MVFR fog possible across interior valleys. Monday... VFR and winds becoming SW. Afternoon T-storms across NY/PA should diminish before entering western MA/CT late in the day. Monday night... VFR to start but lowering to MVFR and/or IFR as low clouds and fog overspread the area after midnight and especially toward Tue morning. Low risk of spotty light drizzle late. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Tuesday through Friday/... Generally VFR through the period. Areas of IFR cigs/vsbys in fog late Monday night and early Tuesday. Areas of MVFR cigs/vsbys in showers/thunderstorms Tuesday night and Wednesday. Areas of IFR cigs/vsbys in fog Wednesday night and early Thursday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. 4 PM update... Tonight... Modest west winds become light and variable overnight. Good vsby and dry weather prevails. Monday... High pres moves offshore yielding modest SW winds. Dry weather and good vsby continue. Monday night... Modest SW winds continue and will likely be accompanied by low clouds/patchy fog along with a low risk for spotty drizzle. Thus vsby may be reduced at times especially southern waters. Outlook /Tuesday through Friday/... Tuesday...High confidence. High pressure shifts east of the waters. Winds 20 knots or less, and seas 4 feet or less. Tuesday night-Thursday...Moderate confidence. A cold front approaches from the Great Lakes, crossing the waters late Wednesday or Wednesday night. Showers and scattered thunderstorms will move across the waters Tuesday night and Wednesday. Southwest winds increase with gusts 20 to 25 knots. Winds shift from the northwest Wednesday night and diminish to 15 knots or less. Seas build Tuesday night and Wednesday reaching 5 to 6 foot heights on the outer waters. The southwest flow may also nudge those higher seas into RI Sound as well. Once the cold front moves through and winds become northwest, seas will subside to 3 feet or less. A Small Craft Advisory may be needed on some of the waters Tuesday night and Wednesday. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...WTB AVIATION...WTB/Nocera MARINE...WTB/Nocera is the U.S. government's official web portal to all federal, state and local government web resources and services.