Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 220803 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 403 AM EDT Tue May 22 2018 .SYNOPSIS...
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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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 310 AM update... Thickening high cloudiness had overspread western MA and northwest CT early this morning. Showers and a few thunderstorms have been steadily progressing across southwestern NY and western and central PA overnight. Low pressure will be moving eastward from the eastern Great Lakes today. Its warm front will skirt the southern New England coast. We will be on the cooler side with some showers. The timing is somewhat slower than we thought yesterday, but not much. Winds at 400-500 mb increase to 50 knots from the west this morning, so am expecting the rain and showers to move steadily eastward across the forecast area. SPC-HRRR has a good handle on this timing. The precipitation will mainly be light, falling from middle level cloudiness through the early to mid- afternoon hours. Late in the afternoon and into the evening, some elevated instability arrives with K indices approaching 33 in CT, RI, and southeast MA. Cannot rule out an isolated rumble of thunder in those areas. Highs today will reach the upper 60s to lower 70s...cooler Cape and Islands. && .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 fornt 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 /08Z TUESDAY THROUGH SATURDAY/... 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...Moderate to high confidence. Mainly VFR conditions into this afternoon. Rain showers arrive in western MA and CT mid-morning, spreading eastward to the coast by early to mid afternoon. Initially, rain will be falling from mid cloudiness. But expect ceilings to lower to MVFR late this afternoon and to IFR tonight most areas. Areas of fog developing tonight, especially southern and eastern sections. Cannot rule out an isolated thunderstorm, mainly in CT, RI, and southeast MA. Wednesday...Moderate to high confidence. Early morning fog/IFR breaks out to VFR by mid morning. Clouds redevelop in northeast and eastern MA mid to late afternoon with a chance of a shower. Still should be VFR. KBOS Terminal...High confidence in TAF. 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. && .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...GAF SHORT TERM...GAF LONG TERM...WTB AVIATION...WTB/GAF MARINE...WTB/GAF is the U.S. government's official web portal to all federal, state and local government web resources and services.