Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190246 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1046 PM EDT Mon Sep 18 2017 .SYNOPSIS... Tropical Storm Warnings continue for coastal Rhode Island and Southeastern Massachusetts as hurricane Jose continues to move north. The impacts from Jose will be felt through Wednesday, with gradual dissipation thereafter. High pressure brings drier and seasonable weather Thu- Fri across the interior. However, Jose`s proximity may allow for windy conditions with a continued chance for rain across southeast MA Thu-Fri. High pressure brings dry conditions for next weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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No major changes to the forecast this evening. Tweaking timing of measurable rainfall based on regional radar trends. Could see some spotty drizzle out of the low clouds before that approaches the south coast late tonight. Clouds continue all night, too. Brought temperature forecast back in line with observed trends. Previous discussion follows... Speaking in terms of a synoptic overview, ridging remains a dominant player today into tonight, as the mean flow N of Jose remains anticyclonic with a ridgline poised from the middle Appalachian region into N New England, just to our west. As a result, subsidence inversion remains in place in soundings across the region from this morning, and BUFKIT model soundings. The onshore flow from the Gulf of Maine (sfc and low lvl easterlies) continues to draw marine airmass, and the resulting marine moisture remains trapped beneath this subsidence inversion. Marine stratus/fog and drizzle continue across much of S New England as a result, and guidance continues to be clear out these low clouds much too fast. Therefore, the overnight hours tonight will ultimately remain similar to the last several nights, low clouds and fog developing beneath the subsidence inversion. In fact, could see another round of localized dense fog with a focus on east coastal locations, where temps this afternoon are coolest (faster reaching crossover temperatures). The one caveat here, will be, as a combination of a strengthening mid lvl baroclinic zone and increasing pres gradient as Jose shifts N toward the +1020hPa high pres, which could lead to a slight increase in winds, limiting the ability for fog development away from the immediate coastline. Otherwise a mild night with mins mainly in the 60s ans moisture increases ahead of a PRE-like outer rain band during the early morning hours.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/... ***Tropical Storm Warnings continue for south coastal RI including Block Island, south coastal MA, Cape and Islands, and southeast coastal MA including Plymouth MA*** Overview... Tropical Storm Warnings continue for all waters save for Boston Harbor and the near shore land zones of SE MA and RI. Latest guidance and NHC track both indicate a slightly earlier turn to the E than previously forecast, suggesting Jose, when it makes its closest pass mid-day Wed, it could be a but further offshore, and S of the 40/70 benchmark. However, the gradual extra-tropical transition (ET) process will allow the pres gradient to strengthen further out away from the storm, with the strongest wind radii increasing with time. Meanwhile the first band of rainfall arrives during the morning hours tomorrow, with an almost, but not quite PRE like setup. Stay tuned as the track gets updated over the next 24-48 hours. Winds... As the ET process continues and the strongest pres gradient and low LVL jet sets up on the NW quadrant following the sharp curve to the left late tomorrow and tomorrow night, expect gradual increase in winds regionwide, but with particular focus toward SE MA and RI. Gusts may reach tropical storm magnitude as early as late tomorrow evening across the open waters and the islands, slowing shifting N to the Cape and extreme SE MA during the overnight. At this time, given current guidance mean track and NHC forecast, sustained winds of +34 kt, arrive across S coastal locations by early Wed morning, with gusts beginning to exceed 40 kt (at times 50 kt) continuing through the day and into the early evening, slightly shifting E with time. Gusts may linger well into the overnight Wed night even as sustained winds wain as the overall wind radius once again tightens with the weakening storm. The focus remains predominantly across areas where current Tropical Storm Warnings continue, mainly coastal RI and SE MA, so will maintain these headlines through this evening`s NHC advisories and likely beyond. However, one thing that will need to be considered, is the hoisting of interior SNE wind headlines (wind advisories most likely) for the late tomorrow into Wed period as the NW quad LLJ makes its pass across interior RI and E and central MA (possibly E CT as well). Low- mid lvl gradient is strong enough to support gusts to nearly 40 kt at times across this area and given fully-leaved trees there is a risk for some tree damage and power outages. Will not be hoisting these for the time being due to the recent slight SE shift in the track guidance suggesting some uncertainty, but this is something that will need to be considered. Rainfall... Strongest mean upper jet remains across S Quebec into N New England although a weak synoptic jet associated with the very subtle vort-max which will interact with Jose. Noting modest increase I mid lvl baroclinic zone as this N stream jet and the weak vort-max related jet streak setup late in the overnight hours as the first plume of higher theta-e air from Jose shifts into S New England. This suggests a PRE-like scenario, although given it is not fully disconnected from Jose it is not a true PRE. Still the setup is such that a band of rainfall, occasional mod-heavy as PWATs reach nearly 2.00 inches beginning during the early morning hours and continuing into early Tue. This is likely to be region- wide as the rain bands strictly associated with Jose remain to the S. In fact, it could be with this initial banding early tomorrow, that interior portions of S New England see the bulk of their rain related to Jose. After what may be a brief lull, rain bands begin to narrow and focus along the convergence within the NW quadrant of the now turning Jose. This will begin to focus the highest moisture and lift mainly along RI and SE and even most likely E of the I-95 corridor for Tue night into Wed night. Given this, continue to tightening the NW-SE QPF gradient with highest totals of 3-6 inches now relegated to the Cape and Islands, dropping to 1-2 inches along the I-95 corridor and then less to the NW. This will continue to be refined as the end-movement of Jose`s remnants remain a bit of a question mark at this time. Will be issuing a Flash Flood Watch for Nantucket and Cape Cod due the combination of storm surge, the inability to fully drain each successive tide cycle. High surf... Long period southerly swell from Hurricane Jose will continue to increase, with swells possibly reaching 10 ft over the southern waters through tonight resulting in high surf and dangerous rip currents. Thereafter, swells will transition to more of a wind wave as winds increase. High surf advisories will be issued for those areas not currently covered with a Tropical storm Warning. For more information on the coastal flooding risk, see the section below. && .LONG TERM /THURSDAY THROUGH MONDAY/... Thursday thru Sunday... Jose is forecast to move east of 40N/70W Wed night, but then may meander over the waters during Thu, Fri and Sat. During Thu-Fri we could have a scenario where ridging brings mostly sunny skies to the interior, yet along southeast MA and especially the Cape/Nantucket the proximity of Jose could allow for additional rainfall and lingering strong winds/gusts. Since at this point we are several days out, there is much uncertainty on the outcome. Sat-Sun is our best shot at Jose finally moving further for our area, allowing high pressure to bring dry conditions and mostly sunny/mostly clear skies. && .AVIATION /03Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... High confidence in trends. Lower confidence in timing. No significant changes with 00Z TAF update... Tonight... IFR/LIFR conditions in lowered CIGS and fog once again as conditions diminish this evening. Some improvement in vsbys toward morning from S-N (mainly after 08Z) as rain develops. Winds E-NW with gusts approaching 30 kt on the islands by 12z Tomorrow and tomorrow night... Influence from Jose begins to be felt. Winds remain E-NE then shift more to the N early Wed morning. Wind gusts increase to 40-50 kt during the overnight hours, slowly increasing during the afternoon and evening, peaking mainly on the Cape/Islands, but still 30-40 kt expected across interior RI/SE MA and E CT. Rain/fog will lead to continued lower vsbys, mainly MVFR but IFR also likely at times. Mainly IFR CIGS except across extreme W MA, where some MVFR/VFR is possible at times. KBOS Terminal...High confidence in trends, lower confidence in timing. IFR once again expected to go to LIFR, timing may be a bit off in TAF. Strongest winds begin tomorrow afternoon. KBDL Terminal...High confidence in trends. Lower confidence in timing. Outlook /Thursday through Saturday/... Tuesday-Wednesday...Moderate confidence. MVFR with areas of IFR in low clouds/rain/fog. Northeast winds with potential for 35-40 kt speeds, gust around 60 kts possible near KACK. Poor visibility on Cape and Islands in rain and fog. Conditions are dependent on the track and speed of Hurricane Jose. Some improvement possible from west to east Wednesday afternoon. Thursday and Friday...Moderate confidence. MVFR/IFR cigs/vsbys across the coastal plain of SE MA and RI. Mainly VFR elsewhere. NE wind gusts 30 to 40 kts possible SE MA on Thu. Gusts over 40 kts possible on the Cape and Islands Thu, gusts 30-35 kts possible Fri. Saturday...Moderate confidence. VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence ***Tropical Storm Warning posted for much of the southern New England coastal waters*** Jose is the main driver of the winds and seas in the short term period, and even into the outlook period. Final impacts will depend on the exact track of Jose, which still has time to change over the next day or so. Mariners are strongly urged to monitor the latest forecasts regarding Jose. Tonight...Light northeasterly flow for much of the night, though towards daybreak Tue increasing winds along the southern outer coastal waters may bring some gusts around 25 kts. Increasing long period south swells will result in seas continuing to build, reaching 6 to 12 feet on the southern outer coastal waters and RI/BI Sounds. Poor vsbys in fog. Increasing chances for rain. Tuesday...Seas steadily build Tue and Tue night as Jose works its way towards its closest approach to the waters. By Tue evening seas build to 10-17 feet on the southern outer coastal waters, and 9-14 feet RI/BI Sounds. Winds generally ENE increasing to 20-30 kts sustained, gusts 30-40 kts, with the highest values on the southern outer coastal waters. Reduced vsbys in rain and areas of fog. Tue night, seas potentially build to 20-25 feet on the southern outer coastal waters, 15-20 ft possible in RI/BI Sounds. Seas will continue to build into Wed morning east of Cape Cod and along Mass/Ipswich Bays and east to 10-15 feet. NE Winds 25-35 kts with gusts 35-45 kts, except gusts 40-50+ kts possible along the southern outer coastal waters and possibly Nantucket Sound. Continued reduced vsbys in rain and areas of fog. Outlook /Thursday through Saturday/...Moderate confidence. ***Tropical Storm Warning posted for much of the southern New England coastal waters*** Wednesday... Winds and seas expected to peak Wednesday morning, with Jose at its closest approach to the waters. Sustained winds 25-40 kts with gusts 40-50+ kts, with the strongest gusts near/SE of Nantucket. Southern outer coastal waters and waters east of Cape Cod: 20-25 ft seas, with the potential for 15-20 ft seas in RI/BI Sounds. Waters east of MA/Ipswich Bay: seas building to 15-20 ft. Seas and winds start to subside during Wed night. Thu-Fri...If Jose doesn`t make a quick eastward departure, it is possible that gusts to Tropical Storm force may linger on some of the coastal waters during Thu and even into a portion of Friday. Seas will also be gradually subsiding, but it may be a very slow process. && .TIDES/COASTAL FLOODING... Jose will affect the coastline in ways typical of a strong nor`easter but with especially large swells coming into our south coast. The slow movement of Jose will result in a long duration event of tides above normal and erosion from Tuesday night through Thursday and possibly still later. For the ocean exposed south coast from Westerly to Westport, we anticipate the potential for moderate to severe erosion from large swells that will be propagating into Block Island and Rhode Island Sounds later tonight and Wednesday. There could be areas of minor coastal flood from wave overtopping for the Tuesday evening and Wednesday morning high tides. For the rest of the Tropical Storm Warning area from Hull to Plymouth, Cape Cod, Nantucket, and Martha`s Vineyard, we anticipate generally minor to moderate coastal flooding for the Wednesday midday, Wednesday night, and Thursday early afternoon high tides, except there is a risk for high end moderate coastal flooding for the Nantucket Harbor area for the Wednesday late night and Thursday early afternoon high tides. We are anticipating a storm surge of 2 to 3 feet in these areas with 3 to 3.5 feet a plausible worst case scenario for Nantucket. We continue to forecast areas of severe beach erosion over multiple high tide cycles for the outer Cape (mainly Eastham to Chatham, east and south side of Nantucket, and south side of Martha`s Vineyard where we may see waves near 20 feet and long periods enter the near shore waters. A particular concern is the probable long duration of this erosion event that may take its toll over multiple high tide cycles. There is the risk that we will see some residual erosion taking place through Friday and even possibly into the weekend in a few spots. For the Coastline from Boston to Salisbury, we anticipate a risk of minor coastal flooding for the Wednesday midday and Wednesday late night high tides. We may need to issue a Coastal Flood Advisory for this stretch of coastline as we get a little closer to the event. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Tropical Storm Warning for MAZ019>024. Dense Fog Advisory until 7 AM EDT Tuesday for MAZ005>007- 013>022-024. High Surf Advisory from 5 AM Tuesday to 2 AM EDT Thursday for MAZ007. Flash Flood Watch from Tuesday evening through Wednesday afternoon for MAZ022-024. RI...Tropical Storm Warning for RIZ006>008. MARINE...Tropical Storm Warning for ANZ231>237-250-251-254>256. Small Craft Advisory from 11 AM Tuesday to 2 AM EDT Thursday for ANZ230. && $$ SYNOPSIS...Doody/NMB NEAR TERM...Belk/Doody SHORT TERM...Doody/NMB LONG TERM...NMB AVIATION...Belk/Doody/NMB MARINE...Doody/NMB TIDES/COASTAL FLOODING...Staff

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