Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191902 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 302 PM EDT Mon Sep 19 2016 .SYNOPSIS...
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Tropical moisture ahead of a cold front will result in widespread showers and a few thunderstorms today, which will linger along the coast tonight and possibly into early Tuesday. High pressure brings dry weather for midweek. A cold front sweeps south from Canada Friday with showers, followed by cooler weather for the weekend.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 2 PM Update...Showers are diminishing somewhat in intensity, but continue across much of southern New England. Additional showers are developing over NW MA and parts of eastern NY that will likely move into the area later this afternoon. Main concern remains the heavy rainfall with PWATs over 2 inches just moving into CT at this time. KBOX VWP indicates the LLJ may be picking up over the region. If that continues to increase, we may see a re- intensifying of the showers, particularly across RI and SE MA. Previous discussion... Southern New England continues to be in a squeeze play between the north Atlantic ridge and approaching upper level trough across the midwest. This has resulted in moist, southwesterly flow for the northeast. Main focus for today will be the approaching trough from the west and several shortwaves moving along the stalled frontal boundary. The right rear quad of the upper level jet is moving over NW MA and the nose of the 850 mb LLJ will start to move over SE MA and RI. This should provide enough dynamics within this moist environment to keep showers and thunderstorms going through much of the day. Heavy downpours continue to be the main threat through the rest of the afternoon. There is the potential for urban and poor drainage flooding as these tropical downpours could quickly drop well over an inch of rain in an hour. While CAPE values will be meager today due to lack of daytime heating, the 0-6 km shear values will be between 30-40 kts which could be enough for a strong storm or two. Marginal helicity values are enough to have the potential for a quick spin-up, especially within the moist tropical environment. Still a low potential. Lastly, southerly flow will continue to pump in tropical moisture keeping dewpoints in the upper 60s to low 70s. This will result in another muggy day with highs reaching into the mid to upper 70s as cloud cover will limit heating for today. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... Tonight into tomorrow... Frontal boundary will stall somewhere across eastern MA which could keep showers and thunderstorms moving across the south coast, Cape and Islands during the overnight hours. Heavy downpours continue to be a possibility as dewpoints remain in the upper 60s to lower 70s. Appears dry air won`t begin to move into southern New England until Tuesday night. Low clouds and stratus will also develop and be a nuisance for tonight as soundings show a low level inversion setting up, trapping the moist airmass at the surface. A few showers could continue across the Cape and Islands Tuesday morning, otherwise anticipate a changing airmass. Low levels will begin to dry out as frontal system will finally get kicked offshore. Westerly flow aloft will bring in a dry airmass and increased mixing potential. 850 mb temps remain near 16C which could result in surface temps in the low to mid 80s, well above normal. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
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Big Picture... Longwave ridge remains in place over the West Atlantic through midweek. The pattern then goes through a transition late week as a ridge builds over the Plains and Western USA while a Hudson Bay trough digs over the East Coast and Maritimes. Shortwave pattern shows flat flow midweek along the Northern Tier of states while subtropical ridge extends from the West Atlantic to Texas and low pressure sits along the Southeast Coast. Upper low digs over the Western USA with high pressure slipping over the top into Western Canada. This creates a brief Rex Block configuration that shifts energy into Northern Canada, and which in turn dislodges closed upper low from the Hudson Bay area and nudges it southeast over the Maritimes and Northeast USA. Normal 500 mb contours for September are about 5770-5790 meters. Expect above normal heights through midweek and below normal heights over the weekend into Monday. So a warm midweek followed by much cooler temps starting this weekend. Details... Tuesday night through Thursday... Signs of a weak surface front slipping through on Tuesday night, but none of the longer range models show more than a few clouds with this feature. Any showers look to remain along the Canadian border near favorable jet dynamics. High pressure builds with dry weather through Thursday. Light pressure gradient expected both days, which would allow sea breezes along the coastlines. Dew points will start in the 60s Tuesday night and drop into the 50s inland as winds turn northwest. Dew points in the upper 40s and 50s will infiltrate most of the region...maybe not Nantucket...by Wednesday. Min temps are based on expected dew points. Max temps are based on mixing temps aloft equiv to 13-14C at 850 mb. Friday... As high pressure moves east, a stationary front forms from Northern New England through the Great Lakes. South of the front, moisture pools late in the week with Precip Water values around 1.5 inches. SBCAPE values also build by Friday afternoon with 500-600 Joule values forecast in interior MA and CT. Totals climb to around 50 by Friday. Meanwhile, the upper flow as noted above turns more northwest and pushes the front south into our area. This should bring showers/thunderstorms as the front moves through Friday afternoon and night. Saturday through Monday... Upper trough digs over the Maritimes with a northwest upper flow and colder temps aloft across New England. Temps at 500 mb will reach -20C. Cold pool moisture may support partial sky cover. Temps at 850 mb will be 4-6C Saturday, 0C Sunday, and 2C Monday. This will mean max sfc temps in the upper 50s and 60s during this period.
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&& .AVIATION /19Z MONDAY THROUGH SATURDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Through 00Z...Moderate confidence. A mix of conditions is expected with areas of MVFR/IFR in heavier showers, stratus, and patchy fog throughout much of southern New England. Greatest likelihood of VFR conditions is across NW MA. Tonight...Moderate confidence. MVFR/IFR in areas of stratus and patchy fog. Showers linger across SE MA tonight but should see some improvement in the interior. Could see some fog and stratus in the interior due to radiation. Tuesday...Moderate confidence. Areas of MVFR/IFR stratus and patchy fog possible in the morning especially near the coast, then improving thereafter. Showers may linger near the coast for a time. KBOS TAF...High confidence in trends, lower confidence in timing. KBDL TAF...High confidence in trends, lower confidence in timing. Outlook /Tuesday night through Saturday/... Moderate confidence. Mainly VFR through the period. Areas of MVFR cigs adn vsbys in showers and thunderstorms Friday afternoon and night. Winds increasing from the north on Saturday.
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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. High confidence through Tuesday. S-SW flow through tonight as seas will gradually build on outer waters but are expected to remain below 5 ft. Dry weather over waters for today, with the potential for showers and thunderstorms by tonight and lasting into Tuesday. Winds may begin to switch to the west by late Tuesday. Patchy fog through the period until dry airmass moves in by late Tuesday. Outlook /Tuesday night through Saturday/... Moderate confidence Tuesday night through Thursday... Cold front moves off to the south. High pressure builds over the waters. West to northwest winds generally under 10 knots. Seas 4 feet or less. Sea breezes possible along the coasts Wednesday and Thursday. Friday-Saturday... Cold front drops south across the waters Friday afternoon and night. Showers and isolated thunder possible late Friday and Friday night with brief poor vsbys. Southwest winds Friday 15 knots or less, becoming North to Northeast Saturday with gusts to 20 knots. Seas 4 feet or less Friday. Seas near 5 feet possible on the exposed waters Saturday.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...Dunten/RLG SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/RLG MARINE...WTB/Dunten

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