Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 220200 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1000 PM EDT Sun May 21 2017 .SYNOPSIS... Monday will feature much cooler temperatures along with periods of showers, especially during the afternoon and evening. Mainly dry and seasonable weather is expected Tuesday. Unsettled conditions return Wednesday through Friday as slow moving upper level system brings scattered showers at times. Spotty showers may linger into the early portion of the upcoming holiday weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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10PM update... Low lvl ridging associated with slowly exiting high pres has left its mark across S New England. Areas E of the Worcester hills have dropped in temp much more quickly than areas W thanks to at least some clearing, although CI now dominates the skies across the region. This has allowed temps across the E to dip into the 50s and even upper 40s in spots already, but suspect with the continued push of warm advection and deeper cloud cover further cooling will be limited. Further W temps are only dropping into the low-mid 60s. We are likely only a couple of degrees away from the overnight mins. Otherwise, noting some returns on radar this evening, although with dwpt depressions between 20-30F it`s unlikely much of this is wetting the sfc yet, especially given echos are weak. As an example, higher reflectivities associated with showers in NH are yielding limited precip at the sfc with virtually no dip in vsbys. Therefore, will continue this trend for much of the night, only increasing POPs significantly during the early morning hours. Previous Discussion... High pressure slowly moves off the coast tonight as shortwave energy approaches from the west. This should allow mid/high level cloudiness to gradually lower and thicken from west to east tonight. Low temps should mainly be in the 40s to around 50. Many locations may record their overnight lows around midnight, before temps hold steady or even rise a bit. The coolest lows will probably occur across southeast New England, where the clouds will be slowest to thicken allowing for some lower 40s. A few showers may develop after midnight in the warm advection pattern. The best chance for this will be across the interior, but any showers will be light and scattered given upper level ridging/dry low level air hanging tough.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... ***Scattered showers overspread the region through mid afternoon Monday with the steadier/heavier showers later Monday afternoon and evening*** Monday and Monday night... Weak warm advection aloft coupled with an approaching shortwave will result in scattered showers overspreading the region from west to east Monday morning into mid afternoon. The forcing looks rather disorganized, so the activity may be more hit and miss showers through early afternoon with the greater focus across the interior. Later Monday afternoon and evening, the southerly low level jet will strengthen along with increasing Pwats. This should allow for showers to become more numerous with perhaps some brief locally heavy rain possible. Still difficult to locate this axis without a well defined synoptic system, but favor the south coast near the axis of higher Pwats/stronger low level jet. In fact, may see the axis of heaviest rain stay over the open ocean. Regardless, not expecting any flooding. There is the low risk for an isolated thunderstorm or two across southeast New England, but not looking for anything widespread. The bulk of the showers should come to an end by late evening across the interior, but may linger well after midnight along the southeast New England coast. Some patchy fog is also possible Monday night, especially toward daybreak if partial clearing occurs back across the interior. High temperatures on Monday will only be in the 50s to around 60 with onshore flow, clouds and showers. Low temps Monday night will only drop into the upper 40s and lower 50s, but again will have to watch for some patchy fog. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Dry and seasonable Tuesday * Areas of showers at times Wednesday through Friday * Chance of thunderstorms Thursday afternoon/evening * Low confidence on potential spotty showers next weekend Overview... H5 low cuts off across the mid and upper Mississippi valley while long wave trough digs down to the Gulf of Mexico through around mid week. Will likely see periods of precip during the middle and latter portions of this week, but timing of these features will be in question with steady SW flow in place. With the tropical connection from the Gulf in place, could see some decent rainfall as each short wave works up the eastern seaboard, especially as the upper low slowly shifts E across the SE U.S. while continuing to dig late this week. Quite a bit of individual model solution spread in play from Wednesday onward. Noting two camps, the U.S. models keeping most energy from approaching short waves offshore while the international models bringing stronger low pres along the coast with a pretty good slug of precip across most of the region. At this point, looks like best shot for precip will be Thu/Fri timeframe as medium range models continue to signal the cutoff mid level system working NE. As the holiday weekend approaches, a lot of questions in place as wide model solution spread lends to a low confidence forecast. Details... Tuesday-Tuesday night...Moderate to high confidence. Last of leftover showers and patchy fog will push off Cape Cod and the islands by mid to late morning. Otherwise, weak ridging will move into the region. With the weak ridge comes light pressure gradient, so expect light and variable winds to become late morning and afternoon sea breezes along the coast. Temps will run close to seasonal normals, with highs in the lower-mid 60s along the immediate coast with the sea breezes, ranging to the lower-mid 70s inland. Clouds may linger through most of the day, however. With SW flow developing aloft, may see first in series of weak low pressure areas work up the coast. For now, brought chance POPs early Tue night. Placement of best moisture/shower activity still in question, but at this point think best shot will be toward during the early part of the Wednesday morning rush hour mainly S of the Mass Pike. Looks like QPF amounts should from less than 0.1 inches N of the Pike up to 0.2 to 0.3 inches along the S coast and islands from 00Z-12Z Wed. Wednesday through Friday...Low to moderate confidence. With the developing SW mid level flow around the H5 cutoff low and digging central U.S. trough, will see a couple more low pressure areas work close to or just offshore during this timeframe. Will see best shot for precip with passing low pres Wed into Wed evening. PWATs look to be from 1.2 to 1.4 inches, highest across N CT/RI/SE Mass during Wed, though light onshore winds and decent low level moisture in place, could see QPF of 0.25 to 0.5 inches during Wed. Will also see patchy fog, with the best chance across E Mass and RI where temps will be coolest. Noting a brief break in the action Wed night, though could still see lingering spotty showers and more patchy fog as temps linger in the lower-mid 50s. Another stronger short wave and low pressure will work NE Thu morning, and will bring another shot of higher QPF, depending upon which model one believes. Will see likely POPs, and a decent amount of instability. The mid level cold pool/cutoff low works NE toward the NE U.S. Thu afternoon/night, which lend to fairly good mid level lapse rates of 7-8C/km crossing the region Thu afternoon/evening. Also noting LIs of zero to -2 and TQ values in the upper teens. Still have pretty cool sea surface temps which could temper things a bit, but have carried slight chance POPs for thunderstorms Thu afternoon/evening mainly near and S of the Mass Pike. May see a few downpours with any convection as well. Big question will be whether the cutoff low lingers across northern New England late Friday/Friday night. Does look like showers will linger Thu night/Fri, then could slowly weaken from S-N Fri night. Still some uncertainty on this timing. Expect temps during this timeframe to run close to or slightly below seasonal normals, except up to 10 degrees below normal along east coastal Mass on Wed with the onshore flow. Saturday-Sunday...Low confidence. 12Z model suite continues to push the H5 low into the Maritimes, though appears to be slower on the GFS vs. the GGEM and ECMWF, which could mean a weak short wave or two could move across in the developing NW flow, which may cause some isolated showers to linger off and on this weekend. Timing and placement is uncertain, though. At this point, have carried slight chance POPs for now. && .AVIATION /02Z MONDAY 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/... Tonight...High confidence. VFR. Mid level clouds will lower and thicken. A few showers are possible after midnight mainly across the interior, but VFR conditions should persist through daybreak Monday. Monday...Moderate confidence. Scattered showers will overspread the region from west to east through mid afternoon. The steadiest/heaviest of the showers will likely hold off until mid afternoon across the interior and probably the late afternoon/early evening along the coastal plain. VFR conditions may prevail for much of the Mon morning even across the interior, before lowering to MVFR with localized IFR conditions during the late morning or afternoon. Across the coastal plain, VFR conditions may prevail until early afternoon before MVFR conditions arrive by late afternoon/early evening. Monday night...Moderate confidence. MVFR to localized IFR conditions expected Monday evening in periods of showers. Some LLWS is also possible along the southeast New England coast during the evening. Improvement may occur from west to east after 6z, but low clouds and fog patches may hang tough through 12z along the southeast New England coast. KBOS TAF...Moderate to high confidence in TAF. VFR conditions may prevail into mid afternoon Monday. KBDL TAF...Moderate to high confidence in TAF. VFR conditions may prevail into mid to late Monday morning before deteriorating by afternoon. Outlook /Tuesday through Friday/... Tuesday...Moderate to high confidence. Areas of MVFR-IFR CIGS/VSBYS in patchy fog and leftover showers Cape Cod and the islands, improving by mid to late morning. Otherwise mainly VFR. Light/variable winds with sea breezes developing by midday. Patchy fog developing late Tue night with MVFR-IFR VSBYS. Scattered showers with local MVFR CIGS/VSBYS possible after midnight. Wednesday through Friday...Moderate confidence. Areas MVFR CIGS/VSBYS at times in scattered showers. Patchy late night/early morning fog each day with local MVFR-IFR VSBYS. May see -TSRA Thu afternoon/evening mainly S of the Mass Pike. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Tonight...High confidence. High pressure slowly moves off the coast tonight. Weak pressure gradient will keep winds/seas well below small craft advisory thresholds. Monday...High confidence. As the high moves further east of the region southerly winds of 10 to 15 knots with gusts to 20 knots should develop by afternoon. However, feel that winds/seas will generally remain below small craft advisory thresholds through Monday afternoon. Monday night...Moderate to high confidence. Southerly low level jet will likely result in a period of south wind gusts of 25 knots especially across our southern waters where some 5 foot seas may also develop. Since it is marginal and mainly a 3rd period event, opted to hold off on small craft headlines for now. Nonetheless, appears that one will likely be needed for some of our waters. Lastly, showers and fog patches may reduce visibilities for mariners. Outlook /Tuesday through Friday/... Tuesday...Moderate to high confidence. Light winds and calm seas with weak high pressure over waters. Sea breezes along the nearshore coasts expected during the afternoon. Wednesday and Thursday...Moderate confidence. Persistent E-SE winds Wed/Wed night gusting up to 20 kt on the outer waters with building seas as high as 6-7 ft on the southern waters. Winds shift to S Thu. Scattered showers through the timeframe. Patchy late night/early morning fog with visibility restrictions. Chance for thunderstorms Thu afternoon/evening. Friday...Moderate confidence. S winds shifting to W, gusting up to 25 kt on the southern outer waters. Seas up to 6-8 ft. Lingering showers and patchy fog with visibility restrictions. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/EVT NEAR TERM...Frank/Doody SHORT TERM...Frank LONG TERM...EVT AVIATION...Frank/EVT MARINE...Frank/EVT

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