Area Forecast Discussion
Issued by NWS Boston, MA

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780 FXUS61 KBOX 260231 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1031 PM EDT Thu May 25 2017 .SYNOPSIS... Low pressure and its attendant warm front will bring a steady rain and drizzle to the region through tonight along with cool northeast winds. The rain will be heavy at times tonight into Friday morning as this low tracks across Southern New England. Big improvement to start of the holiday weekend as weak high pressure delivers dry weather with mild days and cool nights both Saturday and Sunday. 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 AM FRIDAY MORNING/...
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*/ Highlights... - Widespread light rain / drizzle - Moderate-heavy rain towards midnight into early morn Friday - Increasing onshore easterly winds, cooler air off waters - This all prior to a warm front and low pressure sweeping over S New England Friday morning */ Latest Discussion... 1030 pm Update... A lot going on over the past hour. Area of thunderstorms is developing south of Block Island and will continue to move northward. Model soundings support elevated instability so anticipate the thunderstorms to hold together as the move onto the south coast. Other focus is on the coastal flooding. Many locations along the MA east coast is running about a foot surge, resulting in widespread minor coastal flooding. Unfortunately the Gloucester gauge went out about 8pm so cannot truly evaluate Essex county, which is where confidence is highest. The due northeast flow with increase wind gusts as 925 mb jet begins to increase, will help push the surge into the Cape Ann bowl. Cannot rule out erosion from the building seas as well. Otherwise showers and drizzle will continue through tonight before the heavy rain overspreads the area. Areas of dense fog will also be a concern along the Cape and Islands. Previous Discussion... Moderate to heavy rain expected with the possibility of a rumble of thunder. Low occlusion sweeping NE across S New England overnight. Crux of deep-layer synoptic forcing out ahead of the low through a fairly moist profile well up to H3 beneath the left front quadrant of the upper level cyclonic jet streak. Strong omega forcing not out of the question that will easily and efficiently wring out moisture, rather the nature and magnitude of said moisture wrapping into the occlusion, its associated precipitable waters / theta-E. Confident as to moisture pooling along the low to mid level warm front lifting gradually N across the region. Conditionally unstable moist profile aloft, and given strong omega, can not rule out thunder. The system itself is quick-moving, short duration, the bulk of the rain falling around midnight into the 6a timeframe. But there`s still spread, even between the GEFS and SREF members, with the SREF exhibiting members with higher amounts up around 2.0 to 2.5 inches. Quite an anomalous system with winter-like characteristics, taking nothing for granted, and as such am leaning more towards the high- res guidance as consensus of ensemble probabilities along with CIPS Analogs signal a low-prob of greater than 3-inches storm total rain- fall over E/SE MA and RI. Can not rule out an inch of rainfall in the roughly 6-hour period noted earlier. Potential urban and poor drainage issues, mainly inland as low tide proceeds after midnight into morning along the shores. Not thinking any impact to the rivers rather nuisance issues along area roadways, low-lying spots and culverts notorious for having issues. Focusing on the I-95 corridor and immediate roadways surrounding.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday... Scattered shower activity lingering within the wrapping comma-head occlusion. Influx of cooler air rearward as the pressure gradient tightens with the low lifting E with a weak ridge of high pressure building from the W, will see a steepening boundary-layer profile allowing for both moisture and momentum to mix out. So thinking bulk of wet-weather activity will be early on, with the later-half mostly dry as winds increase out of the NW. Clouds breaking, more sunshine, given the time of year and the fact that the low lacks a more pole- ward connection of colder air, with the cooler air an artifact of being dragged down from aloft, should turn out near-seasonable with highs around the mid to upper 60s, warmer SW CT while cooler NE MA. Friday night... Drying out. Occlusion continues to lift E out to sea. Still a N flow on the backside with cooler air, the boundary layer remains well- mixed beneath rising heights from the W and an inversion around H8. Some scattered to isolated shower activity initially, diminishing through the overnight hours. Low clouds linger given boundary-layer mixing and moisture pooling beneath the inversion. With the warm warm blanket in place, keeping it mild with lows down into the upper 40s to low 50s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... */ Highlights... * Mainly dry weather w/mild days and cool nights Saturday and Sunday * Risk of rain returns Sun night into Monday * Slightly above normal temperatures Tue & Wed, with a risk for scattered showers */ Details... Saturday and Sunday... Near to slightly below normal temperatures. Weak high pressure centered just off to the east. Limited moisture within the vertical column should keep most of southern New England dry. Will have to watch out for diurnal showers west of the CT River each afternoon, with Sunday being the more likely day. Weak pressure gradient will support cooling afternoon seabreezes for the coastline. Sunday Night and Monday... Parent low pressure expected to remain north of the Great Lakes, while a triple point low traverses south of New England. Expecting an increasing risk for showers. While there should be periods of showers, not expecting a washout. Some elevated instability, but otherwise not a great setup for strong thunderstorms. Included at least a low risk for isolated thunderstorms though. Near to slightly below normal temperatures continue. Tuesday into Thursday... Lingering fronts finally should move away from our region. High pressure moving into the central Appalachians should provide some sunshine and at least a light south to southwest flow. This should mean slightly above normal daytime temperatures, with near normal low temperatures through this period. A cold pool aloft centered just south of James Bay should mean a cyclonic flow for our region. Will have to be wary of diurnal showers each day. && .AVIATION /03Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...Moderate confidence. Tonight... IFR-LIFR CIGs and VSBYs. RA/+RA potential mainly from 06-12Z 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, becoming gusty into the later half of the day. Friday night... SHRA dissipating while improving VFR. NW winds blustery initially, relaxing. CIGs becoming BKN to SCT. KBOS TAF... IFR-LIFR through overnight. Winds a challenge. Strong easterly sustained at first, becoming light overnight while turning counter-clockwise NE to NW with low passage. Closely watching the 06-12z Friday timeframe for +RA potential and possible TSRA, whether there will be impacts to the AM push. Adverse weather may impact prior to that time, too. KBDL TAF... Hold N winds funneling through the CT River Valley, turning NW with low pressure passage overnight. FG/DZ into evening, more RA and possible +RA/TSRA overnight 06-12z. IFR-LIFR, especially with CIGs through Friday morning. Outlook /Saturday through Tuesday/...moderate confidence. Saturday and Sunday...Mainly VFR. Light winds Saturday, with seabreezes at the coasts. Light S-SE winds Sunday. Sunday night through Monday...Cigs/vsbys trending to MVFR/IFR in showers. Winds from the southeast, trending from the south Monday. Tuesday...Mainly VFR, with scattered MVFR in leftover showers. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. Tonight...Rain and fog, possible thunder. This as low pressure lifts N/E across S New England ahead of which along a warm frontal boundary E/NE winds will be sustained around 15 kts initially, becoming somewhat light with low passage and beginning to turn N/NW. Seas 6 to 8 feet on the outer waters. Friday into Friday night... Low pressure exiting behind which showers will linger as winds will be initially blustery out of the NW with sustained flow around 15 kts, gradually diminishing with time as a weak ridge of high pressure builds in from the W. Outlook /Saturday through Tuesday/... Relatively light winds and seas through this period, once seas subside across the outer waters Saturday morning. && .TIDES/COASTAL FLOODING... */ Tonight... Combination of a very high astronomical tide and onshore flow will result in widespread minor coastal flooding tonight, mainly for the E MA coast. Persistent easterly flow around 15 mph will result in a surge of 0.8 to 1.0 feet along the E MA coast. In addition to very high astronomical tides will result in minor flooding. Greatest concern continues to be from Salisbury to Cape Ann. This is because this is where the strongest easterly flow is forecast along with a combination of E swell and wind wave direction. Greatest risk of both minor coastal flooding and beach erosion. A Coastal Flood Advisory continues for the MA E 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 8 foot waves in the near shore waters in Ipswich Bay, about Cape Ann, and Massachusetts Bay could be an issue. 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. But our confidence continues to be low and will not be issuing a statement for the remaining coastline. */ Friday night into very early Saturday morning... A surge of 0.2 to 0.4 feet is forecast and combined with the peak of astronomical tides (Boston at 12.41 feet), should see at a minimum coastal flooding of low-lying locations notorious for becoming inundated during such tides (i.e., Morrissey Boulevard in Boston). Additional coastal flood statements may be necessary. Will continue to monitor and make updates after this evenings tide cycle. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Dense Fog Advisory until 3 AM EDT Friday for MAZ022>024. Coastal Flood Advisory until 3 AM EDT Friday for MAZ007-015- 016-019-022-024. RI...Dense Fog Advisory until 3 AM EDT Friday for RIZ008. MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ235-237- 250-254>256. Small Craft Advisory until 6 PM EDT Friday for ANZ251. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Dunten/Sipprell SHORT TERM...Sipprell LONG TERM...Belk AVIATION...Belk/Sipprell MARINE...Belk/Sipprell TIDES/COASTAL FLOODING...staff is the U.S. government's official web portal to all federal, state and local government web resources and services.