Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 242057 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 457 PM EDT Mon Apr 24 2017 .SYNOPSIS...
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Low pressure will slowly track up the east coast Tuesday and Wednesday, bringing periods of rain, gusty winds and the risk of minor coastal flooding. Drier and milder conditions will briefly return Thursday. A cold front may bring scattered showers and possibly a few thunderstorms Friday into early this weekend, but may waver across the region into early next week.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Deep moisture plume will slowly lift northward into SNE tonight resulting in lowering cigs. Band of light rain across northern NJ and NYC area will struggle to move north through this evening due to presence of dry layer in the low levels so expect dry conditions into this evening. Eventually expect some spotty light rain to move up from the south and west after midnight as low level jet moves into the region along with increasing low and mid level frontogenesis which induces modest forcing for ascent. Best chance of rain will be from western New Eng to the south coast region where low levels eventually moisten up. However, it may remain dry through the night further north across northern and especially NE MA where low level dry air persists through 12z. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday into Tuesday night... Closed/vertical stacked low off the SC coast lifts slowly northward to the Mid Atlc region Tue night. With high pres locked over the north Atlantic, this results in a deep moist easterly flow into New England with low level wind and PWAT anomalies increasing to 2-3SD above normal. This will bring periods of rain, heavy at times through Tue night. The rain will gradually spread north across SNE through the morning and into the afternoon as low levels moisten and easterly low level jet moves north across the region. Rainfall during Tue should be mostly light to moderate. But a second period of heavier rainfall is likely Tue night, as a SSE low level jet approaches with increasing PWATS over 1.5". Expect a period of heavy rain along with chc of a few thunderstorms along the coast after midnight as elevated instability increases at the nose of the low level jet. Rainfall of 0.75-1.5" is expected with locally up to 2 inches in any heavier convective rain/t-storms. This will not produce river flooding but minor urban and poor drainage flooding is possible, especially Tue night. It will be a chilly and raw day Tue with gusty NE winds and temps in the mid 40s to lower 50s, mildest near the south coast. NE winds will occasionally gust to 20-30 mph near the coast with highest gusts over the Cape/Islands. Temps should rise into the mid 50s Tue night across RI and SE MA, with slowly rising temps overnight in the interior. Gusty easterly winds will continue Tue night along the coast. 50+ kt low level jet approaches the south coast late Tue night but inversion will likely prevent these winds from mixing down. && .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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Overview... Interpreting the N Hemispheric pattern, the GFS / GEFS continues to forecast a stronger MJO event in the Central Pacific (phase 8) while the EC / ECens is more subdued. Aside, still think there is a sub- tropical connection from the Pacific within the low-levels out ahead of broader H5 troughing over the N Central Pacific that contributes to a stronger H5 ridging pattern into the NE Pacific and subsequent downstream amplified mid-latitude trough-ridge-trough pattern across the CONUS into the Atlantic. A clockwise-gyre over the Atlantic with a counter-clockwise across the CONUS, most obvious is the persistent SW flow over the NE CONUS aloft, a pattern supporting an environment across New England of warmer temperatures, higher dewpoints. Coupled with any forcing mechanisms, i.e., trailing cold fronts ahead of lee- cyclogenesis, would expect shots of showers and thunderstorms. Small- scale details in the discussion below, most importantly the midweek coastal low with sub-tropical connections. Details... Wednesday... As weak low moves up the eastern seaboard, heaviest precip will occur during the morning into midday hours. Moderate to heavy rain forecast. Potential flood impacts, interior and coastal which will be highlighted below. There is still an air of uncertainty. The 24.0z NAM continues a slower trend while there is spread in individual members of the 23.21z SREF. Global models have slowed a bit though continue to exhibit consistency with over- all trends. Their accompanying ensembles show decent clustering of individual members. However, some concern expressed in earlier for- casts that the building mid-level ridge ahead of deeper troughing upstream may result in sub-tropical energy remaining cut-off from the mid-latitude flow, that it hangs off the E CONUS rather than lifting N as model solutions around late last week were suggesting in their long-range. While all solutions are succinct that with the building ridge the sub-tropical low deamplifies, it may be the case that the low hangs out well through Thursday till it is kicked E by stronger synoptics Friday. With this forecast, will go with a non- NAM consensus. Energy presently sweeping over the SE CONUS lifts N with the mid-latitude flow along the eastern seaboard having latched into deep subtropical moisture. GEFS / ECens / SREF continues to signal around +2 SD anomalies of H925-85 convergent inflow and precipitable waters as well as strong probabilities of a 1-2" rainfall event, surface dew points exceeding 60F, and E onshore flow, mainly along the immediate coast, 15 to 20 mph sustained with gusts 30 to 35 mph. Interrogating operational models for more detail, still seeing that with low-mid level ascent of the warm-moist conveyor belt ahead of the H5-7 vort- max of rich theta-E sub-tropical air, H85 dewpoints exceed +10C as freezing levels are pushed above 11 kft over much of S New England beneath which the column is completely saturated. With deep- layer forcing of precipitable waters exceeding 1.6" (notably the daily max from SPC precipitable water climatology for 0z and 12z on April 26th is around 1.2"), looking at an efficient warm- rain process. However, per consensus of forecast guidance, the low deamplifies as it remains progressive. A good slug of rain is forecast, but perhaps impacts will remain isolated. While uncertainty of exact impacts remain, will continue with high categorical PoPs. Negative showalters and a conditionally unstable airmass aloft, can`t rule out a rumble of thunder, but won`t prevail in the forecast. Feel such an environment supports embedded heavier showers. Within likely to high categorical PoPs will highlight a moderate rainfall event. Will go with a widespread 1-2" event with particular focus S/E with convergence of low level inflow a higher moisture content as well as along the E-slopes of high terrain. Can not rule out higher amounts up to 3". Wednesday night through Thursday night... Dreary, drizzly at times. Not an overwhelming amount of confidence. While expecting lots of trapped low level moisture beneath a measure of low level ascent, near-stable temperatures with areas of drizzle and fog, there is some concern the sub-tropical low ends up slower, lingering longer. Some of the heavier rain may fall early on with the low drifting N/E later. Not entirely sure we`ll mix out during the day Thursday, however if we do it should end up mild beneath more than likely scattered to broken cloud decks. Friday into Friday night... A shot at warmer weather prior to a sweeping cold front with some shower activity. Given strong synoptics pushing in from the W, S flow should amplify and aid in mixing out the low-level environment. Clearing out, we should warm into the 70s. The cold front as well as showers later on dissipate in their progression E, thinking as a result of more parallel low to mid level flow along the front and beneath enhanced ridging out ahead of deeper upstream troughing. Chance PoPs. Some elevated instability so will keep with the mention of thunder, however confidence is low. If any, mainly over the CT River Valley. Decent SW-NE unidirectional shear, a weak cap around H85. Saturday-Monday... A busy pattern. A consensus of forecast guidance signals that the pick of the weekend is Saturday. A warm front slowly lifting Sunday possibly into Monday. Shower and thunderstorm chances appear likely though in greater detail with respect to timing and impacts remain uncertain. Will keep with chance PoPs for now.
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&& .AVIATION /21Z MONDAY 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 Tuesday Night/... Through 00z...VFR with high clouds. Coastal seabreezes. Tonight...Moderate confidence. VFR cigs trending downward to MVFR/IFR along the south coast towards daybreak with patchy -RA developing. Tuesday and Tuesday night...Moderate confidence. Conditions lowering to IFR/LIFR in the morning with widespread rain, stratus and patchy fog, lingering through Tue night. E wind gusts to 25 kt developing over Cape/Islands Tue with winds veering to SE Tue night with similar gusts. LLWS developing Cape/Islands after midnight as SE/S low level jet at 50+ kt develops. KBOS TAF...Moderate confidence. KBDL TAF...Moderate confidence. Outlook /Wednesday through Saturday/... Wednesday...Moderate confidence. MVFR-IFR CIGS with -RA/RA and patchy fog. Reduced VSBYS likely, down to IFR in spots. May see areas of LIFR CIGS across the higher inland terrain and along the coast. SE winds gusting up to around 25 kt mainly across Cape Cod and the islands Wed, should diminish Wed night. Low risk of TSRA along S coastal terminals. LLWS impacts for coastal terminals with 40-50 kt S winds 2 kft agl, possibly approaching 60 kt on the outer Cape and Nantucket, shifts offshore late in the day. Wednesday night...Moderate confidence. Mainly IFR in areas of -RA, -DZ and FG. Areas of VLIFR along the coast and across higher terrain. Light/variable winds, except SE-S up at 5-10 kt across Cape Cod and the islands. Thursday-Thursday night...Low to moderate confidence. Drizzle and fog move offshore early Thu, but isolated showers linger with local MVFR VSBYS. CIGS should improve to VFR across western terminals, but will likely remain MVFR-IFR for central and eastern areas. IFR-LIFR conditions may return in patchy fog and low clouds Thu night with light winds. Friday-Saturday...Low confidence. Brief MVFR-IFR VSBYS early Fri, then improving. May see scattered -SHRA across N central and W Mass early Fri. CIGS should be mainly VFR, though areas of MVFR return Fri night and early Sat. SW winds shift to W late Fri or Sat, though timing very much in question.
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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 Tuesday Night/...High confidence. Tonight and Tuesday... Increasing pressure gradient between low pres to the S and high pressure to the N will lead to increasing easterly winds spreading north across the waters late tonight and especially Tue. Gusts to 25-30 kt expected with building seas. Reduced vsbys in rain and fog Tue. Tuesday night... E winds veering to SE overnight. Wind gusts may briefly diminish for a time Tue evening but increase again after midnight with gusts 25-30 kt, especially south coastal waters as SSE low level jet approaches. Reduced vsbys in rain and fog. Outlook /Wednesday through Saturday/... * Small craft advisories likely for a majority of the period. Wednesday...Moderate to high confidence. E-SE winds at 15-20 kt will continue, with gusts up to around 30 kt during the day. Seas continue to build up to 8-12 ft during Wed, highest along the outer waters south and east of Cape Cod and Nantucket. Winds and seas will slowly diminish Wed night, though wind will remain onshore. Reduced visibilities possible during the day, more likely at night in patchy rain, drizzle and fog. Low risk of thunderstorms on the southern waters through midnight Wed. Thursday-Thursday night...Moderate confidence. Light winds in place, mainly S across the southern waters and E-SE on the eastern waters. Seas remain high, up to 6-8 ft on the outer waters early and slowly subsiding but remain at or above 5 ft over the open waters. May see local visibility restrictions in patchy fog mainly after dark. Patchy rain and drizzle may also linger. Friday-Saturday...Low to moderate confidence. Light S-SW winds early Fri will increase, gusting up to around 20 kt. Winds may shift to W as a cold front may approach sometime Sat. Seas remain around 5 ft on the outer waters Friday, though may subside Fri night or early Sat. Patchy fog with locally lower visibilities early Fri. May see a few showers late Fri into Sat if cold front approaches.
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&& .TIDES/COASTAL FLOODING...
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* Potential minor flooding impacts during astronomically high tides Tuesday night through Wednesday along the east and south coasts Not anticipating any flooding impacts for the Tue high tide as winds, surge and seas will not have built up enough. Minor flooding possible during the Tue night and Wed high tide cycles along both coastlines. A storm surge around 1 ft with low risk of 1.5 ft surge on top of high astronomical tides and building seas may lead to minor inundation of vulnerable shoreline roads. Some beach erosion is also possible, especially for east and SE facing beaches. Boston High Tides (flood stage 12.5 feet)... 11.47 feet / Tuesday 11:20 pm 11.15 feet / Wednesday 11:50 am Providence High Tides (flood stage 7 feet)... 5.98 feet / Tuesday 8:11 pm 5.53 feet / Wednesday 841 am
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 AM Tuesday to 8 AM EDT Wednesday for ANZ232>235-237-255-256. Small Craft Advisory from 8 AM Tuesday to 8 AM EDT Wednesday for ANZ231-250-251-254. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT TIDES/COASTAL FLOODING...KJC

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