Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191952 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 352 PM EDT Wed Apr 19 2017 .SYNOPSIS...
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Weak low pressure area will approach New England this evening, crossing the region overnight. A weak area of high pressure will bring a brief period of dry conditions Thursday. Low pressure from the Plains redevelops along the New England coast Friday afternoon/evening. This means rain Thursday night and Friday. Weak high pressure builds drier weather for Saturday and early Sunday. Another low pressure area moves to the Carolina coast and passes well offshore of Nantucket Sunday night. High pressure in Canada brings dry weather early next week.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 1255 PM Update... Low clouds finally lifting since around 16Z and tending to dissipate away from the S coast as seen on latest visible satellite trends as well as OBS. Still have a couple of spots along the S coast where clouds linger with the onshore wind flow. Mainly clear conditions continue from east of the CT Valley to around the I-495 beltway early this afternoon. S-SE winds keeping temps cooler than normal, though noting several spots away from the immediate coast with readings into the lower 50s at 17Z. Mid and high clouds moving into central and western areas over the last couple of hours ahead of approaching cold front out of western NY state. Area of rain seen on NE regional 88D radar mosaic at 17Z, with some heavy rain falling across portions of central NY. Should see leading edge of precip approach the Berkshires by around 4 PM, then push into the CT valley by around 5 PM as previously forecast. Have updated near term forecast to bring conditions current. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Tonight... Flat upper shortwave trough moves across New England tonight. Impressive low level jet of 50 KT across central New York State weakens to about 30 KT in southern New England. Associated surface low weakens as it approaches New England. Have gone with likely POPs but believe QPF values will be generally less than .25 inches due to the weakening dynamics of this system. Temperatures will not drop much tonight given the southerly wind and cloud cover. Thursday... With the exception of a few lingering showers possible along the south coast in the morning, anticipate a weak short wave ridge to keep most of the region dry. Trapped moisture under a mid level inversion will likely result in considerable cloudiness lingering across the area, especially during the morning. Temperature guidance is fairly bullish on warming most locations even with considerable cloud cover. This seems reasonable over the interior given the warmer air mass across the area on Thursday but think sea breezes along both coasts will hold temperatures down some there. Suspect Boston and other coastal locations may experience a late morning high and then cool during the afternoon in response to the sea breeze coming off a water with sea surface temperatures in the mid to upper 40s. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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Big Picture... Longwave flow pattern trends from zonal late this week to weakly cyclonic over the Central and Eastern USA by early next week. Contour heights remain at or above normal values. The shortwave scale shows one shortwave moving across Canada and passing north of New England Friday night and Saturday. A southern stream shortwave moves ashore from the Pacific today, forms a closed low over the Plains Friday night, and then shifts to the Florida coast late Monday. A portion of this shortwave eject east through the Mid Atlantic states Monday. Mass fields are similar among the models through early next week, which increases confidence. Details... Friday... Northern stream shortwave starts the day over the Great Lakes with supporting 135-knot jet aimed at Buffalo. These move east, with the upper jet reaching the coastline in the afternoon. Meanwhile low level southeast jet feeds in off the ocean into Eastern MA, riding up over the stalled front south of New England. All models show the surface system redeveloping near NYC and Long Island late afternoon and moving off to our east Friday night. The resulting lift combined with precip water values over an inch and southeast flow should bring rainy weather to most of Southern New England until the system moves past on Friday evening. The high precip water values suggest potential for locally heavy downpours embedded in the rain area. Stability parameters are marginal (TT is 46-48) and mostly favor CT. We note that guidance pop values diminish quickly by midday while the moist southeast flow lingers through the afternoon and evening. We have manually maintained likely pops through the afternoon and diminish west-to-east as the surface system moves past. Winds shift from the north- northwest bringing in partial clearing during the night. One other change regards the temperatures. With a steady onshore flow over water temps in the mid 40s, we lowered temps a couple of degrees with max values along the coast in the upper 40s and low 50s and temps inland in the mid to upper 50s. Saturday... Weak high pressure in Canada builds south with drier air. Meanwhile, a second trough axis associated with the northern shortwave moves through New England. Most of the forcing and moisture with this system is focussed over Northern New England, but close enough to us to expect areas of daytime clouds, especially in Northern MA. Sunday... Southern stream upper low and trough feed moisture up the coast and generate another area of overrunning south and southwest of New England. The GFS and ECMWF show the surface level easterly jet and low level southwest jet remaining south of our area. This brings into question whether we would get any rain. Meanwhile cross sections show deep lift and deep moisture over southern areas such as Providence and less moisture and lift over northern areas such as Boston. Worcester and Hartford are in between. We will feature chance pops for most areas with the bulk of pcpn Sunday afternoon and evening. South Coastal areas, closest to the offshore low, will feature likely pops at the height of the storm. The shortwave Monday through Wednesday... Canada high pressure reasserts control from the Maritimes much of this period. The ECMWF and GGEM take the low off Florida and eject it northeast and pass it just off Nantucket Tuesday night or Wednesday. The GFS keeps the system well offshore. Not much confidence in the solutions at this time. We will feature slight chance pops in RI and Eastern MA Tuesday and await a consensus to form.
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&& .AVIATION /20Z WEDNESDAY 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 Thursday Night/...Moderate confidence. 17Z update... Through 00Z...Mainly SCT-BKN VFR CIGS through most of the afternoon. Clouds will continue to increase through 00Z. May see local high end MVFR CIGS push into the CT valley around 21Z-22Z with sct -SHRA. SE winds shift to S, gusting up to 20-25 kt along immediate coast. Tonight... Much of the area should remain mainly VFR, may see areas of MVFR ceilings in the 2-3Kft range. Best chance for -SHRA through 08Z with low chance for brief IFR conditions. Any -SHRA should end by 10Z-12Z near and N of the Mass Pike, but MVFR CIGS will likely linger. Thursday... A SE to S onshore flow will likely result in considerable cloudiness with areas of MVFR ceilings possible at least in the morning. Scattered showers may approach the western portion of the area late in the day. KBOS TAF...Moderate confidence. SCT-BKN VFR CIGS through late afternoon. CIGS may lower to MVFR around 01Z-03Z. Low risk for period of IFR CIGS/MVFR VSBYS. Scattered -SHRA move across from 02Z-06Z, then push S of KBOS around 10Z-12Z Fri. S winds around 15 kt this aftn will diminish tonight. Cigs lifting into Thursday with lighter winds and potential sea breezes. KBDL TAF...High confidence. SCT-BKN low-end VFR cigs, becoming MVFR along with increasing chances of -RA towards 20z. Increasing S winds with gusts up around 20 kts. Outlook /Friday through Monday/... Friday... Moderate confidence. IFR in rain and fog, improving to VFR from west to east Friday night. East to southeast winds may gust near 30 knots at times along the east coast of Massachusetts Friday. Winds shift from the north and diminish during Friday night. Saturday...Moderate-high confidence. VFR Saturday with potential for MVFR cigs along the east coast of Massachusetts. Sunday-Monday...Low-Moderate confidence. VFR cigs Sunday morning lower to MVFR by Sunday afternoon. Aras of MVFR/IFR in rain and fog late Sunday and Sunday night. Best chance of lower conditions will be near the South Coast and Islands. Conditions improve to VFR 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 Thursday Night/...High confidence. 1 PM Update... Noting some close to 20 kt on near shore waters E of Boston Harbor and in Nantucket Sound at 17Z. May still see gusts up to 25 kt on the outer waters so will continue Small Crafts. Noting seas marginally up to 5 ft on the southern outer waters, but could reach the eastern outer waters during the afternoon on persistent E-SE swell. Elsewhere, anticipate conditions to remain below Small Craft Advisory conditions but will need to keep an eye on seas over the outer coastal waters south of New England as they may approach 5 feet tonight. Also, there is a possibility of wind gusts close to the Small Craft Advisory threshold in Cape Cod Bay. Outlook /Friday through Monday/... Friday...Moderate confidence. East to southeast winds to the north of a warm front Friday. Some southern waters may slip south of the front, with winds becoming more from the south. Wind speeds will increase east of Massachusetts during the day with gusts 25 to 30 knots and a low potential for 35 knot gusts. These winds will help build seas east of Massachusetts with 5-8 feet in the afternoon/evening. Poor visibility at times in rain and fog during the day. A cold front swings across the waters early Friday night, with winds shifting from the north and northwest behind the front with speeds to 20 knots. Seas will diminish overnight as winds become offshore. Visibility will improve with the wind shift. Saturday... Moderate confidence. Winds from the north-northwest with speeds up to 20 knots. Low potential for 25 knots. Lingering 5 foot seas on the eastern outer waters will diminish later in the day. Winds diminish Saturday evening. Sunday-Monday... Moderate confidence. Winds turn from the east Sunday as low pressure passes well offshore. Winds up to 20 knots are possible with the best chance on the southern outer waters. Seas remain below 5 feet on our waters, but build to 5 feet just south of our southern outer waters. Winds turn from the north on Monday but remain below 20 knots.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Thursday for ANZ250. Small Craft Advisory until 2 AM EDT Thursday for ANZ251. Small Craft Advisory for hazardous seas until 8 AM EDT Thursday for ANZ254. Small Craft Advisory for hazardous seas until 5 AM EDT Thursday for ANZ255.
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&& $$ SYNOPSIS...WTB/Thompson NEAR TERM...EVT SHORT TERM...Thompson LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT

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