Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221744 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 144 PM EDT Tue May 22 2018 .SYNOPSIS... Low pressure will move east from the eastern Great Lakes today with its warm front skirting the southern New England coast. Cloudy skies, cooler temperatures, and showers will be the rule as the day progresses. Scattered showers will linger overnight as the low exits the region. Skies will clear most of Wednesday but a secondary cold front could bring an isolated shower to eastern sections late. High pressure will bring dry weather Thursday and Friday. Another cold front will drop south and bring a chance of showers later Saturday through Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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145 pm update... Light rain showers have overspread much of the region as of mid afternoon and will enter southeast MA/RI shortly. These light rain showers were a result of an approaching warm front/modest low level jet. However...cloud bases were quite high so much of this rainfall will be rather light. A bit better elevated instability arrives this evening...so some locally/brief heavier showers will be possible at that time generally south of the MA Pike. There is even the low risk for a rumble or two of thunder in this region. Temperatures will generally remain in the 60s this afternoon with cloudy skies and some showers.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... The bulk of the showers should exit southeastern areas by midnight, but a few showers will remain possible after midnight. Some fog may develop, given the ample low level moisture. ample low level moisture. Overnight lows will be in the low to mid 50s. Wednesday... Much drier air filters in aloft on NW-N winds, behind the departing low pressure area. K Indices drop to below 10 for much of the morning and early afternoon. Early morning cloudiness will give way to mainly sunny skies through the early afternoon. Temperatures are forecast to rise to the upper 70s, except near 80 in the Connecticut and Merrimack Valleys. The tricky part comes late Wed afternoon. A cold front, mainly aloft, will move from north to south across the region. A wind shift to the northeast behind the front will be most pronounced along the eastern MA coast. It will also likely be accompanied by cloudiness (best handled by NAM), a band of K Indices to 32 and Total Total Indices to 50. Surface dewpoints will still be in the 50s. As a result, have forecast a chance of a shower in eastern sections late in the day. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Big Picture... The flow over North America will continue to feature northern and southern streams. Trough in the southern stream will continue to linger over the Gulf of Mexico through the week. Meanwhile the northern stream will feature one shortwave moving east of New England Wednesday night and a second shortwave ejecting from the Western USA, then phasing with another shortwave from northern Canada over the weekend as it moves east toward New England. The solutions show agreement through Friday, then diverge in details over the weekend and early next week. So confidence is good through Friday, then diminishes over the weekend. Contour heights remain above normal through the period. Expect much of this time to feature near or above normal temperatures. Concerns... Wednesday night... Upper trough and cold pool aloft may continue to generate showers in Eastern Mass early in the night before everything shifts offshore. High pressure then builds in with clearing skies overnight. Thursday through Saturday... High pressure center moves overhead Thursday. It then settles to our south Friday and Saturday, bringing a westerly flow to our region. The weak flow Thursday will allow sea breezes. Mixing will reach 850 mb with temps aloft supporting max sfc temps in the low to mid 70s but cooler along the coast. The westerly flow Friday and Saturday will be an offshore flow most places and bring warm temperatures all the way to the coast. Mixing should reach to 800 mb Friday and Saturday, with temps at that height supporting max sfc temps in the 80s. A cold front drops south from Canada on Saturday, with a chance of showers in the afternoon and at night as it moves through Southern New England. Best chance of showers will be in Northern Mass. Sunday-Monday... Cold front moves south of the region Sunday, but stalls close enough to support showers during this time. Meanwhile, high pressure tries to nudge south over the Maritimes and bring drier weather to New England. Confusing? Consider the GFS which also feeds Gulf moisture up the coast while the ECMWF keeps it well to our south. Low confidence in the forecast for these two days. We will show a low- end chance of showers both days. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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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 Wednesday night/... Remainder of the afternoon...High confidence. Generally VFR conditions despite light rain showers. Tonight...Moderate to high confidence. Ceilings and visibilities should gradually lower this evening and especially after midnight. Timing uncertain but widespread MVFR conditions should develop with locally IFR/LIFR focused across the south coast, Cape and Islands where the greater risk is for lower clouds and fog. Scattered showers mainly this evening with even the low risk for a rumble or two of thunder near the south coast. Wednesday...High confidence. Lingering lower cigs/vsbys should improve to VFR by late morning/early afternoon in all locations. These conditions will be slowest to improve across the Cape and Islands. Wednesday night...High confidence in VFR conditions. KBOS Terminal...High confidence in TAF. Sea breeze should come to an end early this evening. KBDL Terminal...High confidence in TAF. Outlook /Wednesday Night through Saturday/...Moderate confidence. Wednesday Night through Thursday Night: VFR. Friday through Friday Night: VFR. Breezy. Saturday: Mainly VFR. Local MVFR possible in fog and afternoon showers. Breezy.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... Today and tonight...Weak pressure gradient will keep winds and seas well below small craft advisory thresholds. Direction will be mainly S to SW. Showers likely this afternoon and tonight. Biggest concern for mariners will be the potential for some fog developing tonight, along with a slight chance of some thunderstorms across the southern coastal waters. Local visibilities 1 to 3 nm in patchy fog tonight. Wednesday...Winds shift to more of a northerly direction behind departing low pressure. But local onshore sea breezes will likely dictate direction in the afternoon, until a secondary cold front arrives from the north late. Local visibilities 1 to 3 nm in patchy fog early in the morning. Outlook /Wednesday Night through Saturday/...Moderate confidence. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday through Thursday Night: Winds less than 25 kt. Friday: Low risk for Small Craft Advisory with seas approaching 5 feet. Wind gusts near 20 kt. Friday Night: Winds less than 25 kt. Local rough seas. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of afternoon rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/GAF NEAR TERM...Frank SHORT TERM...GAF LONG TERM...WTB AVIATION...Frank/WTB MARINE...WTB/GAF

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