Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 242314 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 714 PM EDT Sat Jun 24 2017 .SYNOPSIS... Weak high pressure brings mostly clear skies and light winds tonight, as well as lower humidity. A fast moving disturbance moves across New York and Northern New England late Sunday, bringing scattered showers to northern parts of our area. A weak cool front moves across the area late Tuesday. Then a warming trend begins for the latter half of the week with a return to summer heat and humidity by Friday. Thunderstorm chances will also increase late in the week as a slow-moving cold front moves across southern New England Friday into Saturday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 710 PM update... Drier air, with dewpoints in the lower to mid 50s has overspread the western half of the region. Dewpoints were still in the upper 50s and lower 60s in RI and eastern MA, with 70 still reported at Chatham as of 7 PM, but the wind shift to the west was about to kick in there. Winds were already beginning to diminish. Expect mostly clear skies overnight with lows mainly 55 to 60. Previous discussion... Weak high pressure builds over the northeast tonight, coincident with a shortwave ridge racing through the upper flow. The diminishing pressure gradient will bring lighter wind. This opens the potential for some radiational cooling overnight. With dew points falling into the 50s, expect the cooler locations away from the cities to reach the low to mid 50s overnight, while the rest of us have mins in the upper 50s and lower 60s. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Broad cyclonic flow across the northern USA, but with a strong 120-knot jet moving through the flow and driving features quickly through the flow. Shortwave over the Great Lakes Sunday morning races northeast, crossing Upstate NY in the afternoon and Northern New England Sunday night. This shortwave will carry enough cold air at 500 mb to destabilize the airmass across these areas Sunday. The instability parameters are consistant with this, and favor thunder to our north and west but not in Southern New England. Even so, we will be in the right entrance region of the upper jet, which will favor upper venting and generate some lift. We will forecast a chance of afternoon/evening showers, primarily across Western and Central Mass. The mixed layer is forecast to reach to at least 800 mb and possibly 750 mb. Winds in layer will be 15-20 knots, so expect southwest winds gusting to those speeds in the afternoon. Temperatures at 800 mb will be 7-9C, equiv to 12-14C at 850 mb. This would suggest max sfc temps in the low to mid 80s. After the shortwave passes to our north, any showers should diminish. For most folks, it should be a fair night with temps in the 50s. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Cooler with a few showers/isold t-storm possible Mon into Wed * Warming trend toward end of the week with return to summer heat/humidity by Fri Details... Monday into Tuesday night... An upper level trough moves eastward from the Great Lakes on Monday and exits New England Tuesday night. Temperatures cool aloft Monday, making for somewhat unstable air. The column will be lacking a lot of moisture, as indicated by low K indices. The best chances for any showers or an isolated thunderstorm will be in western and northern MA Monday afternoon/evening. On Tuesday, with the axis of the cold pool aloft over New England, there is a better chance of scattered showers and thunderstorms across the entire region. Total Totals Indices reach the lower 50s across the northwestern two-thirds of southern New England. Highs mainly 75 to 80 both days over the interior with comfortable overnight lows in the 50s except near 60 over Cape Cod. Wednesday into Thursday morning... Looks like a dry pattern Wednesday into Thursday morning with the upper trough having exited and a surface high pressure ridge building over the Mid-Atlantic states. K Indices will be very low and surface dewpoints will be in the upper 40s to lower 50s. Highs Wednesday will warm a bit...into the lower 80s. Minimum relative humidities will approach 30 percent but winds should be light, preventing fire weather concerns. Thursday afternoon and Friday... On Thursday, heat and humidity begins to return around the western periphery of high pressure near Bermuda and in advance of an approaching low pressure/cold front system in the eastern Great Lakes. High temperatures are forecast to reach the mid and upper 80s. A strong southwesterly low level jet will develop during the day, with models indicating 40-50 kts at 925 mb. Mixing should allow wind gusts to reach 25-30 mph at the surface, especially in eastern MA and RI. Have raised winds above guidance values. With south-southeast surface winds and strong southwest to west winds aloft, forecast 0-3km helicities are above 300 m2/s2 across our region with significant 0-1km values as well. The setup is there for potential severe weather. However, the day is dry to start. But both the GFS and ECMWF show a rapid influx of 34-36 K indices into western sections between 18-22Z Thursday late afternoon. The implication is that convection forms over the eastern Great Lakes, moves across New York state and into CT/western MA late in the day. The source of the convection...the cold front...will remain well to our west so it should weaken as it heads east Thursday night, but with a high shear/high helicity environment in place here, we will need to watch for isolated severe thunderstorm potential. On Friday, summertime heat and humidity will have peaked with highs reaching the upper 80s and lower 90s and dewpoints reaching the upper 60s to near 70 in the CT River Valley by evening. Depending on model solutions, the cold front will be either approaching or moving through on Friday afternoon. There should be strong CAPE and still some 0-3km helicity near or above 150. Am forecasting 40-50 percent chances of thunderstorms. The potential does exist for severe storms Friday into Friday evening. Saturday... The Saturday forecast depends on the timing of the aforementioned cold front. The 12Z model runs have slowed this front down and consequently, another very warm day is forecast, with highs in the mid and upper 80s and continuing chances of showers and thunderstorms. Cooler weather may wait until late in the weekend, beyond this forecast period. && .AVIATION /23Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/...Moderate-High confidence. Tonight... VFR. Mostly clear skies with diminishing wind, becoming calm in the interior. Sunday... VFR. Increasing cumulus clouds late morning and afternoon especially northwest of I-95. Scattered afternoon showers ORH- HFD-BAF and points north. Could be brief MVFR cigs/vsbys in any afternoon shower. Sunday night... VFR. West winds. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Monday through Thursday/... Mon-Tue...High confidence. Mainly VFR. A few diurnally driven showers and perhaps an isold t-storm northwest sections Monday and across much of the region Tuesday. Wed... High confidence. VFR. Thu... Moderate to high confidence. VFR to start, although some patchy fog possible along the south coast early. Areas of MVFR cigs and vsbys in scattered showers and t-storms developing in western MA and western CT late in the afternoon and spreading eastward Thu night. && .MARINE... Tonight... 710 PM update... The buoy southeast of Block Island still was reporting 8 ft seas, so will allow the High Surf Advisory to expire as forecast at 8 PM. Previous discussion... West winds become gusty from the northwest early and then diminish overnight as high pressure builds over the waters. The high will bring drier air. Rough seas linger on the outer waters and on RI Sound and Block Island South with 5-7 foot heights this evening, then diminish overnight. Small Craft Advisory will linger on the RI waters this evening, and they will continue on the outer waters overnight. Sunday and Sunday night... Winds become south-southwest as the high pressure moves off to the east. Afternoon winds should remain 20 knots or less. Rough seas linger during the morning on the southern outer waters and possibly parts of RI Sound, but will trend below 5 feet by mid afternoon. Winds and seas remain below Small Craft thresholds Sunday night. Outlook /Monday through Thursday/... Monday through Wednesday...High confidence. Quiet boating weather with winds and seas below SCA. A few hours of nearshore lower 20 knot wind gusts are possible each afternoon. Thursday... Moderate confidence. Southwest winds will gust to near Small Craft Advisory levels as our region is in between high pressure over Bermuda and low pressure over the eastern Great Lakes. Seas will be increasing to 4 to 6 ft over the outer waters, too. Chance of showers and thunderstorms Thursday night. && .TIDES/COASTAL FLOODING... Astronomical tides are rather high through this weekend during the night time cycles. Boston has a 12.4 ft high tide forecast around midnight tonight and 12.2 ft just after midnight Sunday night. While most places had their highest astronomical tide of the year last month, a few spots along our coast will have it with this June set of highest tides. Either way, the difference between the May and June highest tides is small. While offshore winds are forecast, current conditions suggest a 0.4 surge which will result in minor splashover. Thus will continue a coastal flood statement for tonight`s high tide. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory until 8 PM EDT this evening for MAZ020-023- 024. RI...High Surf Advisory until 8 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ231>234. Small Craft Advisory until 11 AM EDT Sunday for ANZ235-237. Small Craft Advisory until 2 AM EDT Sunday for ANZ250. Small Craft Advisory until 6 PM EDT Sunday for ANZ254>256. && $$ SYNOPSIS...WTB/GAF NEAR TERM...WTB/GAF SHORT TERM...WTB LONG TERM...GAF AVIATION...WTB/GAF MARINE...WTB/GAF TIDES/COASTAL FLOODING...STAFF

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