Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 251417 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1017 AM EDT Thu May 25 2017 .SYNOPSIS... Low pressure and its attending warm front will bring a steady rain and drizzle to the region today along with cool northeast winds. The rain will be heavy at times tonight into Fri as the low tracks along the southern New England coast to just east of Cape Ann Friday. Big improvement to start of the holiday weekend as weak high pressure delivers dry weather with mild days and cool nights both Sat and Sun. However by Sunday night and into Monday another coastal low may bring the risk of showers, although a washout is not expected. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 am update... */ Highlights... - Rain into the midday period, heaviest S/E, mainly offshore - Fog / drizzle developing 3 to 4 pm, roughly - N/NE winds, potential for some rogue 30 to 40 mph wind gusts */ Discussion... A cool, wet day on tap. Mature low pressure beginning to undergo occlusion as it becomes negatively tilted across the Mid-Atlantic, lifting over the NE CONUS overnight. Into this evening, ahead of the low, will continue to see isentropic ascent of the warm-moist conveyor belt with sub-tropical connections beneath broad diffluence aloft. Widespread rain across the region with embedded heavier rain showers aided by convergent low-level forcing of higher theta-E air. The deep column moisture present, as seen per morning 12z soundings out of Upton NY and Chatham MA plus intrusion of warmer air that contributes to higher freezing level heights with the southerly sub- tropical connection sets up efficient warm-rain processes. But with evolving dynamics and negative-tilting, would appear that the crux of deeper moisture and strongest forcing stays S/E of S New England, the heaviest rain into this evening confined over the Outer Cape and Islands from now till roughly 3-4 pm. It is then that dryer air aloft associated with conveyor belt motions wrapping into the low, dries the column within the mid to upper levels. Still a measure of lift and forcing upon the moist boundary layer, expect areas of fog and drizzle to encompass the region as the surface warm front slowly drags N, stalling roughly SW to NE along the immediate S coast of New England where moisture will pool. Previous forecast fairly spot on and there were very little changes other than accounting for the latest trends. Agree with what has been said prior: cooler with greater rain intensity as the column saturates towards the respective wet-bulb, winds out of the N/NE with the surface warm front situated S slowly lifting N. A cool, raw, damp day overall. Still monitoring low level winds for the afternoon period as they ramp up in response to the deepening low center with the negative- tilt occlusion. Agree with the previous forecaster that a stout inversion N of the warm front should limiting the mixing of such winds to the surface. Again, can`t rule out some rogue wind gusts but nothing to the effective of widespread warranting a wind advisory.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight into Friday... Anomolous low pressure system will continue its eastward progression during the overnight hours. A potent pv-anomaly will round the base of the trough and develop a surface low over the middle Atlantic region. This low will trek right over southern New England bringing widespread rainfall. Some of it will be heavy at times as the system becomes vertically stack resulting in deep moisture through the entire column. PWAT values are also near an inch to 1.5 inches with increasing LLJ aiding in moisture advection. This will result in another chance for locations south and east of the I-95 corridor of seeing an inch of rainfall. This much rain combined with the King high tide may result in urban flooding as the water will have trouble receding. Guidance is also hinting as some elevated convection with a few models having some surface CAPE late tonight into tomorrow morning. Therefore continued with the mention of isolated thunder. This may also enhance precipitation amounts. Widespread heavy rainfall will begin to wind down by Friday morning as the low begins to move offshore. However, pretty good comma head signature with a deformation zone in the mid-levels could set up. This will keep the dreary weather going through much of the day before beginning to let up by nightfall. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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*/ Highlights... * Mainly dry weather w/mild days and cool nights Saturday and Sunday * Risk of rain returns Sun night into Monday * Seasonably warm Tue & Wed but low risk of scattered showers */ Details... Friday night... Improving weather as closed mid level low over the Gulf of Maine exits seaward and allows rising heights along with mid level anticyclonic flow to move across New England. This will promote a drying trend overnight. Temps will be seasonably cool with lows in the upper 40s and low 50s. Saturday... WNW flow aloft with mid level mean ridge axis across the eastern Great Lakes will provide dry weather and seasonable temps across southern New England. Models do differ on cloud cover Sat with GFS showing much more low level moisture. Given the WNW flow aloft thinking at least partial sunshine will verify. This combined with light winds will provide very pleasant conditions with highs 70-75 CT river valley and 60s elsewhere. Weak pressure gradient will support cooling afternoon seabreezes for the coastline. Sunday... Deep layer ridging lingers across the area promoting dry weather much of the day. However model time sections and moisture profiles indicate lots of clouds likely on Sunday. 00z deterministic and ensemble guidance in good agreement that rain/showers associated with upstream system should arrive into southern New England either late in the day or after sunset. So as of now the bulk of the daylight hours of Sunday appear dry, especially across RI and eastern MA. Given the expected cloud cover highs should be a few degs cooler than Saturday. Fortunately SSE winds should be fairly light but will keep the immediate coastline cooler. Monday... Weak coastal low may pass south of New England along with its precip shield impacting the area. Doesn`t look like a washout but enough support to keep showers in the forecast. Somewhat cool with highs in the 60s given cloud cover and light onshore winds. Tuesday/Wednesday... Not as cool as parent low pressure over the Great Lakes provides warmer southwest flow into New England. With the milder temperatures comes the risk of scattered showers as mid level flow becomes increasingly cyclonic.
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&& .AVIATION /14Z THURSDAY THROUGH MONDAY/...
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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 Friday/...Moderate confidence. 15z update... 7 AM update... Today... Into midday -RA/RA transitioning to DZ with increasingly lower visibility with FG. Lowering CIGs S to N with time towards IFR. N/NE winds becoming more E/NE, gusty at times potentially as high as 30-35 kts though thinking isolated. Mainly 20-25 kts. Tonight... IFR-LIFR CIGs and VSBYs. RA/+RA potential mainly from 6-12z with embedded TSRA possible. E winds continue with mainly 20-25 kt gusts. Friday... IFR/MVFR CIGs with SCT-BKN -SHRA, on and off through the day. Conditions may improve to VFR across western terminals in the late afternoon. Northwest winds for all terminals. KBOS TAF... Lower conditions moving in as low pressure passes over the terminal. So overnight winds are going to be a challenge, gusty at first but becoming light while turning counter-clockwise NE to NW with low passage. Closely watching the 6-12z Friday time- frame for +RA potential and possible embedded TSRA. Whether impacts to the AM push. KBDL TAF... Will hold winds more N funneling through the CT River Valley, turning NW with low pressure passage overnight. FG/DZ into this afternoon, more RA and possible +RA/TSRA overnight 6-12z time- frame. Lowering IFR into midday, LIFR into evening and holding so, especially with CIGs through Friday morning. Outlook /Friday Night through Monday/...moderate confidence. Friday night... A mix of MVFR-VFR but a trend toward dry weather overnight as surface winds become WNW. Saturday and Sunday daytime... VFR. Light winds Sat with seabreezes at the coast. Winds Sunday more from S-SE direction at fairly light speeds. Sunday night-Monday... Cigs/vsbys trending to MVFR/IFR in showers. Winds from the southeast, trending from the south Monday.
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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 Friday/...High confidence. 10 am update...No major changes. Today... Approaching system from the west will help increase winds and seas through the day. SCA continues where confidence is highest for seas above 5 feet. Also went ahead an issued SCA for near shore waters for the potential for gusts near 25 kts. Rain and fog will limit vsbys through the day. Tonight... Poor vsbys in rain and fog Thursday night. Easterly winds around 25 knots eastern coastal waters with strongest winds during the evening. These winds then lift north of the waters. Seas building up to 10 feet across the Cape Ann waters, less elsewhere. SCA continue. Cannot rule out an iso thunderstorm over the waters tonight. Friday... Potent low pressure system still over the region will keep seas and winds up through the period. SCA will be needed. Outlook /Friday Night through Monday/... Friday night... Low pressure over Gulf of Maine moves seaward with WNW winds developing across the MA/RI waters resulting in a drying trend and improving vsby. Saturday and Sunday... Fairly light winds and tranquil seas as high pres crest over the area. Dry weather and good vsby too. Sunday night and Monday... A series of weak lows may develop and pass south of New England, near the 40N/70W benchmark.
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&& .TIDES/COASTAL FLOODING... Combination of a very high astronomical tide and onshore flow will result in widespread minor coastal flooding tonight. Persistent easterly flow yesterday and today will help aid in a storm surge around .7 to 1.0 which will push many sites above flood stage. The Salisbury to Cape Ann stretch of coastline continues to be the most at risk, since the low level ENE jet would have just moved north of this area just a couple of hours before high tide with a moderately significant NE wave fetch in place. However, the expected total water level looks to be high enough to support areas of minor coastal flooding along the coastline south of Gloucester to Plymouth as well as Provincetown. Thus, went ahead an issued a Coastal Advisory for much of the MA east coast including Cape Cod and Nantucket for tonight`s high tide. Waves will not be a major factor but looks like enough low level wind gradient to produce 6 to 10 foot waves in the near shore waters in Ipswich Bay, about Cape Ann, and Massachusetts Bay. Some erosion is likely along the Salisbury and Plum Island shorelines where wave action will be somewhat more significant. Elsewhere along the MA and RI coasts, the combination of the high astronomical tide and a tidal departure near a half foot may be enough to cause pockets of minor coastal flooding as has become more common during such king tide cycles. A statement may be needed for the Islands, and both the MA and RI south coasts, including Narragansett and Buzzards Bay shorelines, for the Thursday evening/night high tides. Right now confidence is to low. Little to no storm surge is expected at the time of the late Friday night or very early Saturday AM high tide. This as well as offshore flow may limit the potential. Will have to continue to watch as we cannot rule out the potential for isolated pockets of minor coastal flooding. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Coastal Flood Advisory from 10 PM this evening to 3 AM EDT Friday for MAZ007-015-016-019-022-024. RI...None. MARINE...Small Craft Advisory until 4 PM EDT this afternoon for ANZ231>234. Small Craft Advisory until 6 PM EDT Friday for ANZ235-237-250- 251-254>256.
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&& $$ SYNOPSIS...Nocera/Dunten NEAR TERM...Nocera/Dunten/Sipprell SHORT TERM...Dunten LONG TERM...Nocera AVIATION...Nocera/Dunten/Sipprell MARINE...Nocera/Dunten TIDES/COASTAL FLOODING...WFO BOX Staff

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