Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 242300 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 700 PM EDT Mon Apr 24 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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650 PM Update... Northern fringe of high cloud deck moving slowly across southern areas at 22Z as seen on latest IR satellite imagery as well as OBS. Thin band of precipitation seen across S coastal areas on NE regional 88D radar mosaic, but no rain is reaching the ground due to continued very dry low level layer across the region. Dewpts at 22Z were in the lower-mid 30s across N central and NW Mass with temps hovering at 70-75 degrees. Very high dewpt depressions indeed. With E-SE winds in place, T/Td spreads along the S coast are much lower, though still dry enough to allow for evaporation of the precip aloft before reaching the ground. Have updated to bring conditions current and incorporated trends into the evening forecast. This did lower dewpts a bit, but should start to see them rise during the night. Previous Discussion... 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/... 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/... Overview... As cutoff H5 low pres and associated surface system moves up the eastern seaboard Wed, it will slow and tend to weaken near or just S of the region through Thu night. Band of heavy rain moves across through midday Wednesday ahead of this system, along with gusty E-SE winds that will slowly shift to S as remnants of the surface low linger. Will likely see lingering light showers, areas of drizzle and fog. Can not pinpoint when or where the showers will occur, but suspect mainly across central and eastern areas with the good low level moisture will continue to feed into these areas with the onshore winds, albeit diminishing as the low weakens. Digging H5 trough across the Plains states will slowly shift E as large subtropical high pres (Bermuda high) looks to set up off the SE U.S. coast by the end of the week. This will make the progression of the H5 trough difficult while mainly zonal flow sets up across New England N of the high. A cold front may try to make a run across the region, but appears will get hung up in the W-E flow. Not seeing a whole lot of moisture or instability with this front, especially as it wavers across the region by this weekend. Will likely see S-SW winds late this week through the weekend, which may shift to W at times depending where the front lies. A lot of uncertainty with this portion of the forecast. Details... Wednesday...Moderate confidence. As weak low moves up the eastern seaboard, heaviest precip will occur during the morning into midday hours. Some question as to how quickly the precip exits the region late Wed into Wed night. 12z NAM/GFS tend to be quickly in pushing heavier precip out, while the GGEM/ECWMF linger it a bit later as the low lingers S of the region. With the low lingering, could see some marginal instability work into S coastal areas. Noted LIs around zero to -1 along the S coast and over the southern waters, as well as K indices around 30. Have kept mention of isolated thunder across the immediate S coast and adjacent coastal waters through midnight, but not confident. The more certain part of this forecast will be the lingering showers, areas of drizzle and fog through Wed night. May see visibilities locally reduced below 1 mile, less along the immediate coast early Wed morning and again Wed night. Will also see E-SE winds gusting up to 25-30 mph mainly across Cape Cod and the islands where a strong low level jet will pass across. Winds at 2Kft running in the 50-60 kt range, but not seeing a lot of good mixing with an inverted layer aloft. Winds will diminish as pres gradient slackens and winds start to shift to more southerly during the afternoon or Wed night. Thursday...Low to moderate confidence. As weakening surface and upper low meander near or just off the coast, will see some more scattered showers, patchy drizzle and fog in place. The drizzle and fog will mainly occur during the overnight and early morning hours, then return late Thu night. With the lack of a strong onshore flow, though, looks like temps should become milder as winds start to shift to a more southerly direction, though they will remain light. Just seeing a lot of low level moisture lingering through Thu night. Friday...Low confidence. Will start to see a more S-SW wind flow in place as surface and upper low finally flattens out and a cold front starts to approach. However, noting the SW H5 steering currents tend to become more W-SW during the day and especially Fri night. So, as the front approaches, will get caught in this flow and slow its progression. Some showers may move into N central and western areas ahead of this front as it tries to sag S across the region Fri night or early Sat morning. Saturday-Monday...Low confidence. A lot of uncertainty with remnants of the front lingering across the region. At this point, have kept slight chance to low chance POPs across the region. Exact timing and position of the precip still in question though. As the H5 long wave trough tries to shift E, may see the front lift around the late Sunday or Monday timeframe, but this timing is uncertain. Temperatures should run warmer than seasonal normals for late April. && .AVIATION /00Z TUESDAY 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 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... 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. && .TIDES/COASTAL FLOODING... * 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 && .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/EVT SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT TIDES/COASTAL FLOODING...

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