Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 241744 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 144 PM EDT Mon Apr 24 2017 .SYNOPSIS... Weak high pressure remains over the region today bringing another day of dry and seasonably warm weather. Low pressure will slowly track up the east coast Tuesday and Wednesday, bringing periods of 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 Friday. A more active, warmer weather pattern beginning Saturday continuing into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 955 am Update... Low clouds were still affecting outer Cape and Nantucket late this morning, but satellite imagery shows back edge pushing offshore, so clearing is expected by early afternoon. Northern edge of high cloud shield continues to lift into southern New England, with thicker cloudiness confined south of Mass Pike. Still plenty of sunshine expected this afternoon, although these clouds will be more noticeable across northern CT, RI, and SE MA. Sea breezes have already begun along South Coast and will get underway along East Coast by midday, where temperatures will hold around 60. Farther inland, highs will reach lower 70s, if not mid 70s in parts of Merrimack and CT Valleys. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Tonight... Mid level confluent flow lifts northward allowing deep layer moisture to advance into southern New England. Thus trend will be for increasing clouds and the chance of spotty light rain/drizzle toward Tue morning from south to north. Increasing easterly gradient between 1035 mb surface high over southeast Quebec and 1001 mb low over the Carolinas will support an increasing chilly onshore wind. Tuesday... Closed/vertical stacked low off the SC coast lifts northward to the Mid Atlc region. Low and mid level onshore wind anomalies begin to increase into southern New England. Thus firehose of moisture off the Atlantic will result in periods of rain...mainly light but becoming more widespread Tue along with increasing onshore winds. Raw/damp day with a chilly ESE wind with strongest winds along the eastern MA coastline with gusts up to 30 mph. Onshore winds and precip will result in a chilly day with highs only in the 40s...low 50s south coast. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... */ 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 /18Z 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/... 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 /Tuesday Night through Friday/... 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... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Today... Tranquil boating weather with high pres overhead. Low clouds may limit vsby across the S waters early this morning. Tonight... Increasing pressure gradient between low pres to the S and high pressure to the N will lead to increasing E-NE winds developing overnight over the S coastal waters with gusts to 25 kt after midnight. Tuesday... Pres gradient continues to tighten between low pressure entering the Mid Atlantic region and high pressure over SE Quebec. E winds of 20-30 kt likely along with limited visibility in periods rain. Outlook /Tuesday night - Friday night/... * SMALL CRAFT ADVISORIES likely for a majority of the period. Tuesday night through 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. && .TIDES/COASTAL FLOODING... * Continued confidence of potential minor flooding impacts during astronomically high tides Monday night through Wednesday * Greatest impact expected along the E Massachusetts coastline Persistent easterly fetch along with astronomically high tide cycles from Monday night through Wednesday, with the height of the winds Tuesday into Wednesday. Strongest sustained winds, mainly along the E MA coast, of 15 to 20 mph, gusts up to 30 mph. This presents the possibility of around a 1.0 foot surge on top of high astronomical tides. Along with wave action, with seas building 8 to 11 feet immediately offshore of S/SE Atlantic-facing beaches, an increasing confidence of minor flooding impacts, if not at least splashover. Inundation of vulnerable shoreline roads is possible. Beach erosion is also possible, especially for Atlantic-facing beaches. 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 A final word, can`t rule out additional impacts as a result of heavy rain. Combined with high tides there is the potential of poor drainage issues along all coastlines, including the S coast of MA. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 AM to 8 PM EDT Tuesday for ANZ232>235-237-255-256. Small Craft Advisory from 2 PM to 8 PM EDT Tuesday for ANZ231- 250-251-254. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...KJC/JWD SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...Nocera/Sipprell TIDES/COASTAL FLOODING...

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