Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 231908 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 308 PM EDT Wed May 23 2018 .SYNOPSIS...
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Other than a few brief showers possible this evening across eastern MA, dry weather will continue for the remainder of the work week with very warm temperatures for Friday afternoon. Saturday will also be warm, just before a cold front sweeps south across Southern New England generating a few showers and scattered thunderstorms. Although it will not be a washout, the potential for showers will linger through the remainder of the Memorial Day weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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305 pm update... A secondary cold front across northern New England will drop southward into our region this evening. This may trigger a few brief spot showers across eastern MA during the evening hours with even a low risk for a rumble of thunder. Limited moisture though and diminishing instability will keep any of this potential activity short-lived and isolated. This potential activity across eastern MA should wind down by late evening. Otherwise...dry weather with overnight lows bottoming out in the upper 40s to the middle 50s.
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&& .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
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Thursday... Large high pressure in control will result in plenty of sunshine and a beautiful day. Somewhat cooler mid level temps behind tonight/s cold front will keep high temps mainly in the 70s. However...weak pressure gradient will allow for sea breezes holding high temperatures in the 60s along the coast. Thursday night... High pressure will move off the coast Thu night allowing for a return south to southwest flow of milder air. Initially boundary layer may decouple enough to allow for overnight lows to drop into the upper 40s to the lower 50s near or just after midnight in some locations. However...some boundary layer mixing may occur resulting in rising temps toward daybreak in locations that are able to decouple for a time.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/... Big Picture... Upper ridge over the Northeast USA de-amplifies with time by the weekend, leaving a more zonal flow over the weekend and early next week. A shortwave trough ejects from the Western USA into this zonal flow, eventually moving over the Northeast USA early next week. Southern stream upper trough hovers over the Gulf of Mexico and brings tropical moisture to the Gulf coast. The ECMWF keeps the tropical moisture south of Mason-Dixon through the period, while the GFS makes a stronger effort to move it up the East Coast but then stalls out south of Long Island. Contour fields from Thursday onward remain above normal, although not excessively so. Thermal fields are near normal Thursday, then above normal most of the time through next Tuesday. The thermal fields do dip close to normal Sunday and Monday especially over NE Mass. An easterly flow Sunday and Monday due to high pressure over the Maritimes suggests a cooler surface layer underneath the warm deep layer, thus supporting the thermal indicators. Model mass and thermal fields are in general agreement through Saturday, then show growing differences early next week. Forecast confidence is good through Saturday, then diminishes to low early next week. Concerns... Thursday night through Saturday... High pressure offshore provides a west-southwest surface wind, so most places should warm up. The only air conditioning will be where this flow is an ocean flow, such as the Outer Cape and parts of Cape Ann. Mixing is forecast to reach about 825 mb each day, temps at that level support max sfc temps in the mid to upper 80s. Can`t rule out a 90, especially if the mixing goes a little deeper than expected. A cold front moves south Saturday. Precipitable water values build ahead of the front with GFS values reaching 1.5 inches Saturday afternoon/evening. Model consensus shows Total-totals in the upper 40s at that same time; mid level lapse rates are forecast at 6 to 6.5 C/Km, and lifted indices away from the South Coast go sub-zero. So the potential is there for scattered thunder along with the showers. The front moves through early Saturday night, with winds shifting from the northeast behind the front. Sunday-Tuesday... The cold front stalls just south of the region Sunday, with northern stream high pressure building over the Maritimes. This should bring cooler temperatures Sunday and Monday, most notable in Eastern Mass. With the front stalled nearby and weak disturbances moving through the flow, a chance of showers will linger each day with best chance along the South Coast. As noted above, there remains uncertainty with how much tropical moisture works up the coast during the early week as well as if it reaches our area. This event, or non-event, remains in the low confidence part of the forecast. So we will maintain chance pops for this part of the forecast. && .AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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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 Thursday Night/... Tonight through Thursday night...High confidence in mainly VFR conditions right through Thursday night. A few brief showers are possible this evening across eastern MA. Otherwise...the main issue will be timing the arrival/departure of diurnal sea breezes along portions of the coast. KBOS Terminal...High confidence in VFR conditions. Sea breeze should come to an end by late afternoon/early evening. KBDL Terminal...High confidence in VFR conditions. Outlook /Friday through Sunday/...Moderate confidence. Friday through Friday Night: VFR. Breezy. Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, slight chance TSRA. Saturday Night: Mainly MVFR, with local IFR possible. Chance SHRA, slight chance TSRA, patchy BR. Sunday: Mainly MVFR, with local IFR possible. Chance SHRA.
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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 Thursday Night/... Tonight and Thursday...High confidence. A weak pressure gradient with high pressure in control will keep winds and seas below small craft advisory thresholds. Thursday night...Moderate to high confidence. High pressure moving off the coast will result in southwest winds developing. Enough of a low level jet may result in marginal small craft conditions with southwest wind gusts of 20 to 25 knots and marginal 3 to 5 foot seas across the outer-waters. Outlook /Friday through Sunday/...Moderate confidence. Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Sunday: Winds less than 25 kt. Local rough seas. Chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...Frank/Sipprell MARINE...Frank/Sipprell

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