Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 242014 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 414 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 warmer conditions will briefly return Thursday. A frontal system will bring scattered showers and possibly a few thunderstorms Friday. A more active, warmer weather pattern beginning Saturday continuing into early next week.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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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.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
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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.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/... */ Highlights... - Heaviest rain Tuesday night thru Wednesday - Accompanying fresh water and coastal flooding concerns - Areas of fog and drizzle for Wednesday night, soupy mess - Possible improvement Thursday, possibly soupy again overnight - Sweeping cold front and some shower activity into Friday night - An active, warmer weather pattern Saturday onward */ 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. */ Discussion... Tuesday night through Wednesday... 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. In brevity, energy presently sweeping over the SE CONUS lifts N with the mid-latitude flow along the E CONUS having latched into deep sub- tropical 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. Will highlight those briefly below. 1.) Fresh water flooding. USGS streamflows are in normal capacity based on climatology. Latest ensemble forecasts continue to suggest that rivers remain within their banks. Forecasting a 1 to 2 inch rainfall event, the heaviest within the forecast period, there is still a risk of nuisance flooding such as ponding of water on roads, urban / poor drainage issues. Individual CIPS analogs waver on the favored area of heavy rain, thus thinking all of S New England is at risk of seeing 1-2" though have greater concern of higher amounts along E-slopes of higher terrain with E onshore flow. 2.) Coastal flooding. E fetch during multiple astronomically high tide cycles, coupled with heavy rain, looking at the possibility of minor coastal flooding along vulnerable shoreline roads considering wave action plus a 0.5 to 1.0 foot surge, at the minimum splashover. Could see inundation of such shoreline roads. Can`t rule out beach erosion. In addition, for shoreline communities, during high tide cycles during heavy rain, there could be drainage issues which could lead to flooding. Particular focus on Tuesday night into Wednesday morning high tides. As reference, Boston tides listed below. Boston High Tides (flood stage 12.5 feet)... 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 Please see the COASTAL FLOODING section for additional details. Will continue to highlight potential threats within the hazardous weather outlook. In closing, while there`s still uncertainty, 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. Not clear cut. Saturday onward... 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, is quickly followed up by a cold front into Tuesday. 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. && .AVIATION /19Z 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. Likely visibility impacts with RA. High confidence of lower cigs over high terrain. It is possible over the Cape and Islands we`ll see some possible improvement for a period. E- SE winds 15 to 20 kts sustained. Gusts up to 35 kts. Possible LLWS impacts for S/E coastal terminals with 40-45 kt S winds 2 kft agl. Wednesday night...Moderate confidence. IFR - VLIFR. Areas of FG and DZ. Winds becoming light out of the E. Lingering LLWS early on with continued 40-45 kt S winds 2 kft agl. Thursday into Thursday night...Low confidence. Gradual improvement, slowest N/E, during the day. Cigs lifting VFR. SCT-BKN cigs. Filling back in overnight lowering back down to IFR - LIFR. Possible patchy DZ and FG. Continued light E winds turning S. Friday into Friday night...Low confidence. Improving during the morning, cigs lifting. SCT-BKN VFR. Chance of SHRA ahead of a wind shift with S winds turning W. Low risk TSRA.
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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 - Friday night/... * SMALL CRAFT ADVISORIES likely for a majority of the period. Wednesday.. Weakening coastal low slowly approaching from the S. Accompanying heavy rain and breezy E winds sustained around 15 to 20 kts with gusts up to 35 kts. Waves building 8 to 10 feet on the outer waters. Visibility impacts with heavier rain, possibly improving at times. Wednesday night through Thursday night... Light winds overall, E initially and slowly becoming S. Seas remain high around 8 to 10 feet on the outer waters, diminishing slowly into Friday morning but remaining above 5 feet on the outer waters. Some visibility impacts along the near-shore possible with fog. Friday into Friday night... Increasing S winds ahead of an approaching cold front expected to sweep the waters during the overnight period with some possible shower activity. May have some issues with S flow over remnant E swell from earlier in the week. Waves on the outer waters remain above 5 feet.
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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 astronimical 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...
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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.
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&& $$ SYNOPSIS...KJC NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell TIDES/COASTAL FLOODING...

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