Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232101 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 501 PM EDT Sun Apr 23 2017 .SYNOPSIS...
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Weak high pressure builds across the area through Monday bringing dry and seasonably warm weather. Low pressure will slowly track up the east coast Tuesday and Wednesday, bringing periods of moderate to heavy rain, gusty winds and the risk of coastal flooding. Drier and warmer conditions will briefly return Thursday. A frontal system will bring scattered showers and possibly a few thunderstorms Thursday night into Friday. The front will waver close to the region, which could bring chances for showers at times into next weekend.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... High pres in control will bring mainly clear skies and light winds although some high clouds will move up from the south overnight. Good radiational cooling tonight so leaned toward cooler MOS temps. Radiation fog threat appears low given the low dewpoints in the interior. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Monday... High pres drifts offshore but the column remains quite dry except for increasing high level moisture moving up from the south. Sunshine will gradually give way to increasing high clouds in the afternoon, especially southern areas. Low level temps moderating so highs should be a few degrees milder than today, reaching the upper 60s and lower 70s interior but seabreezes will keep temps around 60 in the afternoon along the immediate coast. Monday night... Mid level confluence lifts north allowing deeper moisture well in advance of sfc low to slowly move north into SNE. Models have slowed the onset of moisture a bit as sfc ridging to the north hangs tough. Some light rain may overspread the region overnight with best chance south of the Pike. Increasing NE winds developing along the south coast toward daybreak. && .LONG TERM /TUESDAY THROUGH SUNDAY/...
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Overview... Tuesday through Wednesday night... Period of moderate to heavy rain is forecast especially Tuesday night into Wednesday morning. In brevity, energy presently sweeping over the SE U.S. lifts N into the mid latitude flow ahead of which there is a decent sub-tropical connection. Per GEFS/ECens/SREF there remains anomalous signals on the order of +1-2 SD with respect to precipitable waters and H925-85 winds converging into New England along with increasing probabilities of a decent slug of rain within a 6-12 hour window, again mainly around Tuesday night into Wednesday morning. Subsequent low-mid level ascent ahead of main H5-7 vortmax of rich theta-E sub- tropical air and H85 dewpoints exceeding +10C, with freezing levels pushed above 11 kft making this practically a warm-rain process, very atypical as noted per sounding climatology for Chatham which has a record max of 1.42" 12z Wednesday April 26th when the GFS is forecasting a 1.63" precipitable water. There are a number of concerns and a couple of unknowns discussed below. 1.) Fresh water flooding. Potential 1.0 to 1.5 inches of rain in a 6- 12 hour period centered around Tuesday night into Wednesday morning. Per USGS most streamflows in S New England are within their normal capacity based on climatology. Latest ensemble river forecasts per GEFS/SREF show slight rises but no indications of exceeding minor flood stage. So present thinking is that rivers will remain within their banks, yet there remains a risk of typical nuisance flooding such as ponding of water on roadways, urban / poor drainage issues. We`ve seen a fair amount of rain in the last 72-hours and prior to the heavier rain, beginning Monday night there will be some initial warm advective rains that`ll likely saturate soils leading to more runoff. Focus of risk especially within the I-95 corridor and areas S/E with the upslope onshore flow and signals discerned from CIPS analogs. There is also some additional concern up against the E slopes of higher terrain. 2.) Coastal flooding. Persistent E fetch beginning Monday night and through Wednesday morning with the strongest winds mainly due to the gradient flow between the N/E wedge of high pressure up against the approaching low. Fairly stout inversion above the surface which will limit mixing. Still can`t rule out 0.5-1.0 foot surge brought about by the persistent E flow around 15 to 20 mph sustained with gusts up to 30 mph during astronomically high tides Tuesday into Wednesday, the Boston tides listed below. Keeping in mind of minor flood stage in Boston is 12.5 feet and we`re dealing with multiple tide cycles, with a 1.0 foot surge, considering minor coastal flood issues with inundation of vulnerable shore roadways in addition to splashover, perhaps even some beach erosion. Particular focus on Tuesday night into Wednesday morning high tides. Please see the COASTAL FLOODING section below for additional details which also highlights the threat of poor drainage flooding for coastline communities during periods of heavy rain coincident with high tides. 3.) Uncertainty. Still some question on the strength and timing of the low center as it lifts up the coast, perhaps complicated by the wedge of 1035 high pressure N/E off of Newfoundland. There is some spread beginning Tuesday night. With this forecast will go with a non-NAM consensus. Still advertise categorically high PoPs as well as coincident moderate rainfall. More confident of embedded heavier showers, but still no indication of instability as we remain capped and thus no mention of thunder. Heaviest rainfall amounts forecast over S/E New England and along the E-slopes of high terrain. Will continue to highlight potential threats within the hazardous weather outlook. As the low passes across S New England Wednesday - Wednesday night there will be a lot of moisture associated trapped within the low levels with a measure of ascent associated with the main mid-level vortmax as winds become light. Practically textbook, expecting a period of low clouds, drizzle, and fog, quite soupy. Gradually improve going into Thursday, clearing. Thursday through Sunday... Back and forth pattern. Sweeping cold frontal boundaries ahead of which warm-moist, more unstable air surges N yielding the potential for showers and thunderstorms. Immediately followed by cooler air associated with high pressure out of Canada sliding E, winds turn onshore ahead of a warm front approaching from the S/W lifting back N before another surge of warm-moist, unstable air ahead of another cold frontal boundary. Looking at an active weather pattern with periods of wet weather and unseasonably warm temperatures followed by cool, dry, more seasonable conditions. Going with a blend of the global models, showers and thunderstorms possible Friday, turning cooler and dry into Sunday, return of wet weather into early next week.
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&& .AVIATION /21Z SUNDAY THROUGH FRIDAY/...
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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 Monday Night/...High confidence. Through 00z... VFR. Light winds with seabreezes along the coast. Tonight and Monday... VFR. Light winds. Seabreezes developing along the coast around midday Mon. Monday night... VFR with cigs lowering overnight. Chance of light rain developing after midnight, mainly south of the Pike. KBOS TAF...High confidence. Seabreeze redeveloping 15-16z Mon. KBDL TAF...High confidence. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday and Tuesday night... Expect CIGS to lower to MVFR-IFR with areas of -RA/RA with areas of LIFR likely Tue night especially along the coast and over the higher terrain. VSBYS lowering to MVFR with IFR developing Tue afternoon/night in areas of fog. E winds increase, sustained at 15 to 20 kts along the coast with gusts up to 30 kts possible. Wednesday and Wednesday night... Mainly IFR CIGS Wed with isolated -SHRA/-DZ and patchy fog. Areas of LIFR conditions Wed night in any leftover showers, -DZ and fog. S-SE winds up to around 20 kt on Cape Cod and the islands Wed morning become light/variable inland and light S along the coast Wed night. Thursday into Thursday night... Conditions start out MVFR-IFR early, then should improve to VFR. CIGS/VSBYS may lower to IFR again Thu night in areas of low clouds and patchy fog. Light SW winds inland, up to 10-15 kt along the coast. Low risk of -TSRA Thu night across interior. Friday... IFR conditions early, improving to mainly VFR. May see brief MVFR CIGS during the day in -SHRA. Low risk of TSRA as cold front slowly moves across.
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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 Monday Night/...High confidence. Tonight and Monday... High pres south of New Eng will result in mostly light winds 15 kt or less with SW flow tonight turning SE on Monday. Easterly swell over eastern waters will erode. Monday night... Increasing pressure gradient between low pres to the south and high pres to the north will lead to increasing E-NE winds developing overnight over the south coastal waters with gusts to 25 kt after midnight. Outlook /Tuesday through Friday/...Moderate confidence. * SMALL CRAFT ADVISORIES likely for a majority of the period. Tuesday and Tuesday night... Low pressure slowly pushes N along the eastern seaboard. Long easterly fetch remains in place. Sustained winds expected from 15-25 kt, highest Tue night, with gusts up to 30 kt. Wave action across the eastern outer waters will build up to 8 to 11 feet. Rain and patchy fog will bring reduced visibilities over the waters. Wednesday into Wednesday Night... There is a low risk for gales on the outer waters for a time Wed. Winds veer to SE with with gusts up to 35 kt through midday Wed, then will begin to diminish as they shift to S late Wed/Wed night. Expect easterly swells to continue, with seas up to 11 feet on the eastern outer waters through most of the day Wed before slowly subsiding. Rain during Wed will taper to showers along with patchy drizzle and fog Wed night with reduced visibilities continuing. Thursday-Friday... Winds shift to S-SW and diminish Thu and continue Fri. Expect gusts up to 20 kt at times Fri-Fri night. Seas will remain at or above 5 feet on the outer waters through Fri.
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&& .TIDES/COASTAL FLOODING...
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...Confidence continues to increase for potential minor coastal flooding during high tides from Monday night through Wednesday along the Massachusetts east coast... Expect persistent easterly wind fetch to remain in place from Monday night through Tuesday night, then will start to shift to S-SE and slowly diminish during Wednesday. The strongest sustained winds at 15 to 25 mph with gusts up to around 30 mph along the Massachusetts east coast. Astronomical high tides due to the new moon will occur during this timeframe. With the continued onshore winds building the seas up to 8 to 11 feet, the possibility continues to see up to around a 1 foot surge on top of the astronomical high tides. Along with wave action, an increasing confidence of minor flooding impacts, if not at least splashover. Inundation of vulnerable shoreline roads is possible. Some minor beach erosion is also possible along the east facing beaches. Boston High Tides... 10.88 feet / Monday 10:33 pm 10.87 feet / Tuesday 10:59 am 11.47 feet / Tuesday 11:20 pm 11.15 feet / Wednesday 11:50 am With the rain falling heavily at times, there is also the possibility of poor drainage flooding issues along the entire Mass and RI coastline.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 6 AM EDT Monday for ANZ250-254. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT TIDES/COASTAL FLOODING...Staff

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