Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 231351 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 951 AM EDT Wed May 23 2018 .SYNOPSIS... Isolated showers move offshore this morning, with a second set of showers possible this evening. Expect dry and warming weather Thursday and Friday. 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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950 am update... Low clouds/fog patches earlier this morning have quickly given way to partial sunshine in most locations. 850T between +11C and +12C along with good mixing should yield highs in the lower 80s in many locations...except a bit cooler where localized sea breezes occur. Dry weather generally prevails...but a shortwave may trigger a brief spot shower or two late this afternoon/evening across eastern MA. Nothing more than than expected given lack of low level moisture.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... Isolated shower activity over N/E MA this evening, otherwise dry, turning cool. H3 jet streak amplified, diving S in response to enhanced upstream ridging, a N-stream vort-lobe and attendant colder H85 temperatures skirt by. N surface wind response, onshore along the coast. Drier airmass pushing across the region, lowering both dewpoints and temperatures. Push lows down into the upper 40s for many locations, potential impacts to sensitive vegetation. Thursday... High pressure in control, another pleasant day on tap. However beneath continued influence of skirting N-stream vort-lobe / cooler airmass. Light winds overall, allowance of sea-breezes along the coast. Mixing up to around H85, can`t rule out a few W wind gusts along the interior, especially over high terrain. Mixing drier air as well, dewpoints falling into the 30s for interior locations. && .LONG TERM /THURSDAY NIGHT 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 /14Z WEDNESDAY 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 Thursday/...High confidence. Today...Most locations have already become VFR and any lingering lower cigs/vsbys near the south coast should improve by lunch time. Generally looking at NW winds of 5 to 15 knots but localized sea breezes likely for a time this afternoon. Tonight...VFR. Thursday... VFR. Sea-breezes along the coast. KBOS Terminal...High confidence in VFR conditions. Potential sea- breeze around midday but probably is short-lived and done by early evening. KBDL Terminal...High confidence in VFR conditions. Outlook /Thursday Night through Sunday/...Moderate confidence. Thursday Night 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... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. A series of cold fronts later this morning and again this evening, high pressure thereafter which will shift gradually E into the end of the week. Low clouds, fog, visibility issues ahead of the front with scattered showers, some thunder possible over the S waters this morning. Clearing out, high pressure building in, a weak pressure gradient, winds for the most part will be light allowing near-shore sea-breezes during the day. Outlook /Thursday Night through Sunday/...Moderate confidence. Thursday Night: Winds less than 25 kt. Seas locally approaching 5 ft. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Frank SHORT TERM...Sipprell LONG TERM...WTB AVIATION...Frank/Sipprell/WTB MARINE...WTB/Sipprell

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