Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191953 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 353 PM EDT Thu Apr 19 2018 .SYNOPSIS...
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Low pressure that passed well southeast of Nantucket this afternoon tracks south of Nova Scotia tonight. An upper level disturbance behind the low brings rain and snow showers to the region this evening. Drier air moves in on Friday as high pressure over the Great Lakes Region moves eastward. Building high pressure will bring gradually warming temperatures and dry weather into early next week. Storm system expected for mid to late week.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Tonight... Surface low pressure was centered well southeast of Nantucket late this afternoon, and will pass south of Nova Scotia overnight. However the upper trough and shortwave energy will be passing overhead. Accompanied by lingering low level moisture, showers are expected well into this evening. Based on trends on radar and surface observations, started out this evening with likely pops, transitioning to mainly chance pops after 00Z. Mainly rain as primary precip type to start, except over the east slopes of the Berkshires where it was still cold enough for light snow even as of late this afternoon. Elsewhere across the interior the light rain showers will transition to a chance of light snow showers, with up to a coating of snow accumulation possible on grassy surfaces and up to a few tenths of an inch of accumulation along the east slopes of the Berks. After 06Z much of the area is drying out as the upper trough axis starts moving off to the east, however with NW flow light snow showers may linger over the east slopes of the Berkshires. Kept a slight chance pop there for the overnight.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
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Friday... Cyclonic flow around the upper low continues on Friday. Models continue to show a decrease in overall moisture over the course of the day, however sufficient moisture in the low levels for diurnal cumulus/stratocu. Mainly dry conditions expected, though could see some spot showers along the east slopes of the Berkshires, and a few sprinkles elsewhere. The cold advection and cold temperatures aloft should allow mixing to at least 850 mb. Models continue to show winds in this layer around 25 knots, so NW gusts of 25-30 mph anticipated for the afternoon. Highs generally in the mid 40s to low 50s, except for low 40s along the east slopes of the Berkshires. For most of the region this is a good 10 degrees or so below normal for mid April. Friday night... Upper trough lifts east of the area while surface high pressure over the Great Lakes Region moves eastward. This allows for drier air and skies becoming mostly clear. Diminishing NW winds become light, with lows in the upper 20s and 30s.
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&& && .LONG TERM /SATURDAY THROUGH THURSDAY/...
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Highlights... * Moderating temperatures and dry weather will prevail through the weekend and into next week * Next batch of rainfall will occur during the mid-week Overview... Longwave wave trough over the Northeast will lingering through the weekend before mid-level ridging pushes through for early next week. At the surface, strong high pressure leaning towards 2-3 STD above normal will lead to dry weather. However with the upper level trough, temps will slowly moderate out through next week. By Tuesday/Wed, both the GFS and EC indicate a cut-off 500mb low associated with the southern stream. This southern stream could interact with an approaching northern stream wave, developing a system for mid next week. This is supported by ensemble guidance which has between 50-70 percent probs of seeing 0.5 inches of rainfall. Details... Temperatures...Moderate confidence. Spring will make an appearance during this medium range forecast as temperatures will begin to moderate through the weekend and into next week. Longwave trough over the region will slowly begin to exit towards the Maritimes by early next week. This will keep the weekend below average for temperatures as both 925 and 850mb are near 1-2 STD below normal. Still plenty of sunshine across the area for the weekend with fairly light winds so it will feel pretty warm compared to this past week. Mid-level ridge will build into the area by Monday/Tuesday. This will help increase temperatures back into the low to mid 60s. Near the coastline, temperatures will feel cooler due to potential sea breeze develop as high pressure is situated right over the area. Approaching system for the mid-week will drop temperatures back down below average, especially behind the system. Precipitation...High confidence. Dry weather will prevail from Saturday into Tuesday thanks to surface high building in from the west. Model guidance indicates this high will near 2-3 STD above normal. A good break to dry out the soils. However, approaching surface low that develops near the Carolinas will track towards southern New England between Wednesday and Thursday bringing wet weather to the area. As mentioned above, ensemble guidance has between 50-70 percent probability of seeing 0.5 inches of rainfall. Chances are this amount will go up especially since this event is still 6+ days out and there is strong agreement amongst the ensembles and deterministic guidance.
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&& .AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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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 Night/... This evening...Moderate confidence. Areas of MVFR CIGs/localized reduced vsbys with -SHRA/-SHSN. CIGs/VSBYs improving to VFR this evening, though scattered SHRA/-SHSN remain possible and could bring brief MVFR conditions. Exception to this is along the Cape/Islands where IFR conditions expected to prevail into early this evening, then improving to MVFR. Winds becoming predominantly from the NW. Overnight...Moderate confidence. Conditions improving to VFR. Friday...High confidence. VFR. Northwest winds will gust to 25 knots during the day. Isolated -SHRA/-SHSN possible along the east slopes of the Berkshires, this could briefly cause MVFR cigs/vsbys. Elsewhere patchy sprinkles possible. Friday night...High confidence. VFR. KBOS Terminal...Moderate confidence in TAF. Mainly light -SHRA and VFR into this evening, then VFR. Gusty NW winds on Friday preclude sea breeze development. KBDL Terminal...Moderate confidence in TAF. MVFR CIGs this afternoon with scattered SHRA, improving to VFR this evening with showers diminishing. Outlook /Saturday through Tuesday/...High confidence. VFR.
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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 Night/...High confidence. Tonight... A low pressure system passing southeast of Nantucket moves moves south of Nova Scotia tonight. Rain showers will lower vsbys at times into this evening. Increasing northwest winds expected tonight, gusting 25 to 30 kts at times. Seas will build overnight, with 5-8 foot seas on the southern outer coastal waters overnight. Small craft advisory continues for tonight for all waters. Friday...Northwest winds gusting to 25 knots, with seas 5-7 feet on the outer coastal waters. Small Craft Advisory in effect. Friday night...Northwest winds gust 20-near 25 kts with seas gradually subsiding. A Small Craft Advisory may be needed for the outer coastal waters. Outlook /Saturday through Tuesday/...Moderate Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday Night through Tuesday: Winds less than 25 kt.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 AM to 3 PM EDT Friday for ANZ231>234-251. Small Craft Advisory from 4 AM to 6 PM EDT Friday for ANZ230. Small Craft Advisory from 4 AM to 11 AM EDT Friday for ANZ236. Small Craft Advisory from 9 PM this evening to 5 PM EDT Friday for ANZ235-237. Small Craft Advisory from midnight tonight to 6 PM EDT Friday for ANZ250. Small Craft Advisory from 9 PM this evening to 8 PM EDT Friday for ANZ254>256.
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&& $$ SYNOPSIS...Doody/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...Doody AVIATION...Doody/NMB MARINE...Doody/NMB

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