Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 230627 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 227 AM EDT Sun Apr 23 2017 .SYNOPSIS... Low pressure moves out to sea tonight with high pressure building into New England late tonight through Monday. As high pressure ridge noses across central and northern New England, low pressure will slowly move NE along the SE and mid Atlantic coast. This will bring a prolonged period of wet weather from Monday night through Wednesday night, along with gusty easterly winds. There looks to be a short dry spell sometime early Thursday. However a cold front will move across late Thursday bringing some scattered showers and possible an isolated thunderstorm well inland. Another period of showers is possible into early next weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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2 AM Update... Winds have now shifted to the NNW as mid level trough moves across the region. This will continue to result in low clouds lifting across eastern MA and back edge of cloud shield over western-central MA/CT to continue moving east. Previous Discussion... Otherwise a dry night with clearing developing from west to east late tonight as mid level shortwave moves to the east and high pres builds in from the west. Low temps will settle into the mid 30s to around 40 degrees.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... Sunday... High pres in control will bring sunshine, light winds and seasonably mild temps. Seabreezes are likely along the coast during the afternoon. Highs will range from the upper 50s along the immediate coast to the upper 60s in the CT Valley. Sunday night... High pres remains in control. Clear skies and light winds although some increase in high clouds from the south is expected overnight. Good radiational cooling with lows ranging from mid 30s to lower 40s. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Overview... Development of H5 cutoff low pres off the Carolinas early next week will slowly progress northward up the eastern seaboard while high pres ridge noses W-SW across central New England through around Wed night. Will likely see periods of precip and gusty onshore winds. Further west, noting digging H5 long wave trough moving slowly SE out of the eastern Rockies into the Plains states. This will develop a responding downstream SW flow as ridging builds over western Quebec into eastern Ontario by late next week. This should help to lift the east coast cutoff low, but timing is in question. The upper ridging over eastern Canada looks to be short lived as the midwest upper trough shifts slowly eastward late next week. Southern stream ridge builds off the Carolinas/Florida coast, so SW flow aloft should remain in place. Model solution spread increases late next week into next weekend as to where the approaching front may push, or stall in the upper flow if the E coast ridging up the eastern seaboard. Details... Monday... High pressure near or just off the S coast early Monday will slip offshore. This ridge may be just strong enough to keep dry conditions across most of the region, with dewpts mainly in the upper 30s to lower 40s. However, developing low pressure of the SE U.S. coast. Winds will start to shift to E-SE along S coastal areas during the day as the leading edge of precip tries to shift N. Only kept slight chance POPs into the immediate S coast and Islands toward daybreak. Daytime highs will be close to seasonal normals. Monday night through Wednesday night... With slow movement of low pressure along eastern seaboard as its associated mid level system cuts off from the southern stream flow. along with increasing easterly flow thanks to ridging over the Maritimes, will see prolonged period of precipitation across the region. This, along with long fetch moisture plume working up the coast, with PWATs up to 1.4 to 1.6 inches Tue night into Wed. So, expect to see periods of moderate rainfall especially during the late Tuesday into Wednesday timeframe. The onshore flow will also help with higher amounts across E Mass into RI. Expected QPF amounts range from 0.75 to 1 inch across central and western areas, and 1.25 to 1.5 inches further E. Will not help with the slow movement of the low up the eastern seaboard. With this expected rainfall, went along with guidance and have CAT POPs for central and E areas. Kept high LKLY for now across the W. Will also see persistent onshore E flow beginning Monday night thru Wednesday. This, along with building easterly swells as well as astronomical high tides, will have to monitor potential for possible coastal issues during this timeframe. Details are in the Tides section below. Thursday through Saturday...Low confidence. As the weakening H5 low shifts NE to the Maritimes by early Thu, may see a brief respite to the wet conditions early Thursday. However, upper level pattern remains progressive. Noting H5 heights slowly fall as another cold front approaches. Big question during this timeframe is how far E will the front move as H5 ridge builds off the SE U.S. coast. May see some showers move into N central and W Mass by midday or early afternoon Thu. Some decent instability also along this front, so can not rule out an isolated thunderstorm. Models showing a wide variance of solutions for late next week into next weekend. What`s left of the front may stall somewhere across the region. For now, have carried mainly dry conditions on Fri, but may see another low moving along the remnants of the front toward the region Fri night/Sat. Tough call as the 12Z ECMWF indicating some possible briefly heavy showers as a weak wave of low pres moves across. Way too early to lock in on this timing. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/...
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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 Monday/... 2 am update... Thru 12z... IFR over eastern MA lifts to MVFR-VFR as NNW usher in drier air across the region. Elsewhere, MVFR will lift to VFR by 12z. High forecast confidence. After 12z... Any lingering IFR-MVFR over Cape Cod and Nantucket at 12z quickly improves to VFR. Elsewhere, VFR with light NW winds becoming west. However near the coastline light NNW winds become onshore around midday. Dry weather prevails. High forecast confidence. Tonight...High confidence. VFR. Monday...High confidence for VFR and light winds. KBOS TAF...High confidence in trends, moderate confidence in timing. Seabreeze developing 15-16z. KBDL TAF...High confidence in trends, moderate confidence in timing. Outlook /Monday Night through Thursday/... Monday night through Wednesday night...Low to moderate confidence. Expect CIGS to lower to MVFR-IFR with areas of -RA/RA. Patchy fog develops Mon night and Tue morning and again Tue night/early Wed morning with low T/Td spreads. May see LIFR CIGS/VSBYS at times during both nighttime periods. Expect E winds to increase, gusting to around 25 kt Tue, then become E-SE Tue night. Low chance for 30 kt gusts at times. Winds shift to S on Wed and diminish. May see IFR CIGS/VSBYS return in low clouds and patchy fog Wed night, but may improve from W-E after midnight as drier air may work in. Thursday...Moderate confidence. Should be mainly VFR. May see local MVFR conditions across central and western terminals in sct showers. Low risk of isolated afternoon 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 Monday/...High confidence. Tonight and Sunday... Some northerly gusts to 20 kt over the eastern waters this evening, otherwise diminishing winds later tonight and especially Sunday as high pressure gradually builds over the waters. Winds becoming onshore around 10 kt Sun afternoon as seabreezes develop. Lingering swell on the eastern waters will lead to SCA seas which will gradually subside during Sunday. Sunday night... Mainly southerly winds 15 kt or less as high pres gradually moves offshore. Some gusts to 20 kt possible over NE MA waters where seas may build to 5 ft. Outlook /Monday Night through Thursday/... Monday...High confidence. Expect winds and seas below small craft criteria. Light S-SE winds back to E during the afternoon. Monday night through Wednesday night...Moderate confidence. Coastal low slowly pushes NE during this timeframe. Will see easterly winds in place for most of the period. Small Craft headlines will likely be needed Tue through Wed at least. Sustained winds of 15-20 kt with gusts up to 30 kt likely. Long easterly swells will develop especially Tue through Wed as average seas up to 6-10 kt on the outer waters. Winds shift to S-SE and diminish during Wed as the low moves across or just W of the waters, but will take time for the seas to subside. Thursday...Moderate confidence. W winds shift to SW during the day but remain below small craft criteria. May see gusts up to 20 kt Thu night. Leftover high swells of 6-9 ft will only slowly subside so headlines may continue.
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&& .TIDES/COASTAL FLOODING... Continue to see potential of an easterly wind flow beginning late Monday through around midday Wednesday. Could see sustained winds up to 15 to 20 mph with gusts up to around 30 mph, but the question is whether the higher winds above the surface will mix down due to low level inversion. Looks like there could be a chance for the stronger winds to mix down around the late Tuesday into Wednesday timeframe, as the H925 jet of 30-35 kt moves across. Here are the times of heights of the high tide for Boston Harbor... 10.9 feet / Monday 10:33 pm 10.9 feet / Tuesday 10:59 am 11.5 feet / Tuesday 11:20 pm 11.2 feet / Wednesday 11:50 am With a surge of up to 1 foot brought about by onshore flow, particularly at the time of the Tuesday night high tide, there could be some splashover issues along vulnerable E MA roadways. Have continued to mention this potential in the Hazardous Weather Outlook. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 6 PM EDT this evening for ANZ250-254. Small Craft Advisory for hazardous seas until 2 PM EDT this afternoon for ANZ251-255.
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&& $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC/Nocera/EVT SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/Nocera/EVT MARINE...KJC/EVT TIDES/COASTAL FLOODING...

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