Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170742 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 342 AM EDT Tue Apr 17 2018 .SYNOPSIS...
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An upper level system will keep unsettled conditions across the region through mid week. A low pressure area will produce a period of chilly rain during Thursday. Lingering rain showers Thursday night may mix with or change to wet snow across northern Massachusetts. The weekend looks to be dry but with more below normal temperatures.
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&& .NEAR TERM /THROUGH TODAY/...
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Cold front moves off the east coast of Mass first thing this morning. Winds turn from the west, bringing in drier air at the surface. Upper low will be overhead of New England today, with a -24C cold core. Cross sections show a deep moist layer reaching to at least 600 mb. Low level lapse rates reach 9C/Km, while mid level lapse rates are around 5.5 to 6C per Km. Satellite images show lots of clouds. If any breaks develop this morning, the sunshine working on the cool temps aloft should generate clouds, leaving us with mostly cloudy skies. The shortwave and instability may also generate widely scattered showers. Mixed layer is forecast to reach 850 mb. Temps at the top of the layer at -5C to -7C suggest max sfc temps in the mid 40s northwest to lower 50s coastal plain. Winds in the mixed layer reach 20 to 25 knots, so expect gusty west winds late morning and afternoon.
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&& .SHORT TERM /TONIGHT THROUGH 6 PM WEDNESDAY/...
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Tonight... Second shortwave rotates around the upper low and across Southern New England during the first half of the night. Deep moisture lingers overhead as this takes place. Expect mostly cloudy skies with widely scattered showers through midnight, then partial clearing late. We stayed close to guidance min temp forecast in the 30s. Wednesday... Another shortwave moves through during the day. This one will be supported by a 100-knot jet, but with less moisture aloft and most of that will be shallow. Expect a partly cloudy day with mixing again reaching 850 mb. Temps at that level will be a couple of degrees higher than today, about -3C to -4C. This supports max temps in the upper 40s to mid 50s.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Chilly rain appears likely for Thursday, possibly lingering into Thursday night * The coming weekend looks to be dry but cooler than normal Discussion... The large scale pattern across greater North America is dominated by large closed low centers, typical of early spring. A closed low about northern New England early in this period will drift across the Canadian maritimes later in the week. For the most part, temperatures are expected to remain somewhat below normal for most of this period. One precipitation event is expected on Thursday and possibly into Thursday night as a fairly vigorous short wave trough approaches from the west and eventually becomes absorbed into the Canadian Maritimes closed low. As far as the surface track, the GFS and Canadian models bring it across southern New England. The ECMWF, both the 00Z and 12Z operational runs and their respective ensembles, tracks the surface low just south of New England and then NE into the eastern Gulf of Maine. The UKMET and NAM also keep the low track south of New England. The track differences would be reflected in stratiform versus showery precipitation and temperatures. For now, we are leaning toward the ECMWF solution with temperatures in the 40s during most of the day Thursday and a period or two of stratiform rain along with drizzle. The thermal profile looks like wet snow could mix with the rain or even have the precipitation change to a period of snow across northern MA before ending Thursday night as the boundary layer cools a tad per model 925 mb temperature projections. No accumulation is expected other than perhaps a coating or so across the higher elevations of the Berkshires and Worcester hills. A model consensus would point toward a dry but cooler than normal weekend ahead. A closed upper low pressure center will move across the southern USA and produce an anomalous coastal storm off the SE USA coast by early next week, but any impacts on southern New England would be after this forecast period. && .AVIATION /07Z TUESDAY THROUGH SATURDAY/...
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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/... Today...High confidence. Lingering MVFR/IFR cigs leading the overnight cold front, but conditions improve to VFR after the cold front moves through. Expect cloud bases 3500-5000 feet during the late morning/afternoon. A few isolated rain showers are possible during this time, mixed with snow over the higher elevations. West-southwest winds will gust 15-20 knots most places, and 20-25 knots Cape Cod/Islands. Tonight and Wednesday...High confidence. VFR. Winds gusting 20 to 25 knots Wednesday. KBOS Terminal... High confidence in TAF. MVFR early morning ahead of a cold front crossing the region. Winds shift from the WSW behind the front, and ceilings improve to VFR. This happens by 10Z/6 AM. Winds gust 20-25 knots this afternoon. KBDL Terminal...High confidence in TAF. VFR. Winds gust 20-25 knots this afternoon. Outlook /Wednesday Night through Saturday/...Moderate confidence. Wednesday Night: Mainly VFR, with MVFR conditions possibly developing west to east. Chance of light rain and slight chance of light freezing rain NW MA. Thursday: Mainly MVFR ceilings and visibilities, with local IFR conditions possible in rain and fog. Thursday Night...MVFR ceilings and visibilities in rain and fog improving to VFR by morning. Chance of precipitation mixing with or changing to snow across northern MA. Friday...Mainly VFR.
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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 Wednesday/...High confidence. Today... High confidence. Southerly winds turn from the west-southwest with gusts to 25 knots. Rough seas were already in place early, and with the expected winds we expect only a slow subsiding through the day. Small Craft Advisory continues for most areas. Tonight and Wednesday... High confidence. Winds diminish a little tonight, but then return to similar 25 knot gusts on Wednesday. Seas linger at or above 5 feet on the outer waters and RI waters, but subside below 5 feet on several nearshore waters including Mass Bay and Cape Cod Bay. Small Craft Advisory will continue for most waters. Outlook /Wednesday Night through Saturday/...Moderate Confidence. Wednesday Night: Winds less than 25 kt. Seas near 5 feet outer coastal waters. Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain. Thursday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers. Friday: Winds less than 25 kt. Seas up to 5 ft over the outer coastal waters.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 5 AM EDT Wednesday for ANZ232>234. Small Craft Advisory until 11 AM EDT this morning for ANZ231. Small Craft Advisory until 6 PM EDT this evening for ANZ236. Small Craft Advisory until 5 AM EDT Wednesday for ANZ235-237- 250-251-254>256.
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&& $$ SYNOPSIS...WTB/Thompson NEAR TERM...WTB/Thompson SHORT TERM...WTB LONG TERM...Thompson AVIATION...WTB/Thompson MARINE...WTB/Thompson

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