Area Forecast Discussion
Issued by NWS Boston, MA

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803 FXUS61 KBOX 241511 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1011 AM EST Tue Jan 24 2017 .SYNOPSIS...
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A powerful coastal storm east of the NJ coast will move northeast to Nantucket this evening then into the Maritimes Wednesday. Precipitation lingers through the day and into tonight. Pockets of freezing rain are possible into this evening across portions of interior Massachusetts. High pressure builds in behind the departing storm and brings dry mild weather Wednesday and Thursday. A cold front crosses New England Thursday followed by cooling temperatures and blustery winds through the weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 AM update... ...Headlines... *** Increasing chances for freezing rain this afternoon into tonight across the high terrain *** Winter Weather Advisory... While temps aloft are warming so the threat of snow across interior MA will continue to diminish today. Thus any snow this morning will change to rain by midday. However concern is regarding low level/shallow cold air. Temps across the Worcester Hills and East Slopes of the Berks temps are only in the upper 20s and lower 30s. This combined with dew pts in the mid to upper 20s, a few inches of fresh snow/sleet combined with N-NE winds continuing as low pres tracks from NJ coast to Nantucket this evening...all of this suggest shallow cold air will not be able to scour out or modify much. Thus will limit Winter Weather Advisory to the high terrain for any leftover snow changing to freezing rain. With comma-head moving across the region thru tonight may have to extend this advisory into tonight and change the headline to freezing rain advisory. So icing could become a problem this afternoon into tonight. Will reevaluate after reviewing entire 12z model guid. Flood Watch... Despite strong low pressure still off the NJ coast the main PWAT plume/warm conveyor belt rains have moved north into ME. Thus the risk of heavy rain/precip has ended. However with mid level closed low south of New England second moisture plume is beginning to form offshore northwestward into southeast MA. This will wrap cyclonically across the region today and tonight and form the comma-head precip. Thus periods of rain/showers today and tonight but not expecting heavy rainfall amounts to induce any flooding. However a few heavy downpours are possible as dry slot approaches and steepens mid level lapse rates. This combines with modest synoptic scale lift within the developing comma-head for a risk of isolated thunder along with a few downpours. Strong Winds... Core of strong low level winds has lifted north of the region. Thus will cancel the High Wind Warning and wind advisories. Will replace the high wind warning for Essex with a wind advisory as wind gusts up to 45 mph will persist until 11 am or noon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Tonight... Periods of rain likely persist much of the night given proximity of well developed low pressure near the southeast New England coast. Ptype still may be freezing rain across interior southern New England especially across the high terrain, so winter weather Advisories may need to be extended and will have to watch for a potential impact on the evening rush hour. Temps should remain above freezing along and southeast of the Boston to Providence corridor. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Big Picture... West Atlantic ridge in place midweek will shift out to sea. Pacific high pressure builds east into the Western USA, kicking an upper trough east to the Eastern USA over the weekend. Ridge west/trough east pattern continues early next week. Shortwave embedded in the upper trough will move northeast with the trough, crossing New England during Thursday. Upper jet associated with this shortwave sweeps east and crosses New England later Wednesday night and Thursday, then remains well to our south over the weekend. Contour heights at 500 mb will be above average during midweek, then below average Friday through Monday. This means mild weather midweek followed by colder weather over the weekend. Details... Wednesday-Thursday... Low pressure moves off through the Maritimes Wednesday morning and carries off any leftover clouds and showers. Atlantic high pressure builds over Southern New England with dry weather. Mixed layer reaches to at least 950 mb, possibly a little higher to 925 mb. Temps in this layer would support max sfc temps of 45 to 50. Not much sky cover in the evening, but the trend during the night will be for increasing clouds as the upper jet approaches and enhances upper venting. Previous model runs had weakened the jet as it moved overhead, but this 00Z suite maintains the jet strength while the moisture in the column increases. Expect a few showers from late night into Thursday morning. The upper jet drives a shortwave trough and surface cold front through our area. Current suite suggests timing during the afternoon but this may shift a little as the event gets closer. Colder air moves in behind the front, and the cold advection should increase mixing depth to 850 mb. Winds in this layer reach 25 knots, possibly 30 knots...which is higher than model guidance. We will adjust forecast values higher. Timing of the cold front will determine the high temperature. If the cold air arrives mid afternoon then max temps may again be in the mid 40s to lower 50s. Friday through Monday... Broad cyclonic flow with cold pool clouds and a cooling trend on temps aloft. Temps aloft trend from around -8C Friday to -11C or -12C Sunday and Monday, so max sfc temps trend from either side of 40F Friday to the 30s early next week. Another shortwave moves through the flow over the weekend and may increase the sky cover as it passes, but current forecast data shows moisture too limited for precipitation. && .AVIATION /15Z 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. 15z update... No major changes with this update. Main concern is for freezing rain/drizzle this afternoon and tonight across the high terrain of MA including KORH. Winds will continue to trend downward. Rain continues but of lesser intensity and becomes more showery with a few downpours possible. Regarding cigs and vsbys...not much change from current conditions into this afternoon. Thus persistence a good forecast for today. Earlier discussion below. ====================================================================== Today and tonight...Moderate to high confidence. MVFR to IFR conditions will persist in periods rain along the coastal plain although not as widespread/heavy as early this morning. Across the interior, sleet will transition to rain/freezing rain. Temps may not rise above freezing across parts of interior MA especially across the high terrain. Therefore, we are concerned about freezing rain persisting into this evening. Northeast wind gusts of 35 to 45 knots with the strongest on the coast will gradually diminish this morning, although it will remain breezy across northeast MA through the afternoon. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate confidence in TAF. Sleet will change to rain by 12z. Outlook /Wednesday through Saturday/...Moderate confidence. Wednesday-Thursday... VFR Wednesday. Ceilings and vsbys trend to MVFR after midnight in rain and snow as a cold front approaches from the west. Areas of IFR are likely Thursday morning. Conditions trend back to VFR after the cold front moves through Thursday afternoon. West winds gust to 20 knots Wednesday and 25 to 30 knots Thursday afternoon. Friday-Saturday... VFR. West winds gust to 25 knots each day.
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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. *** Dangerous seas continue today but diminishing winds provides some improvement *** 10 am update... Strong winds (Gale force) will be across the MA/NH waters remainder of this morning. However all waters will continue trending downward as strongest winds lift north into ME despite low pres remaining off the NJ coast. This low will move NE toward Nantucket by this evening. Will adjust headlines downward. Earlier discussion below. =================================================================== Today...Moderate to high confidence. ENE wind gusts of 45 to 55 knots with seas building to 20+ feet across our outer-waters this morning. Winds should drop below Storm Force from south to north this morning as low level jet lifts north of the region. In fact, other than our northern waters winds probably drop below small craft levels this afternoon as well developed low pressure moves into the region diminishing the pressure gradient. Seas will also diminish but will remain very high into the afternoon. Tonight...Moderate to high confidence. Northwest small craft wind gusts of 20 to 30 knots will re-develop behind departing low pressure. Seas gradually lower, but still between 7 and 12 feet for much of the night across the outer-waters. Outlook /Wednesday through Saturday/...Moderate confidence. NW winds diminish Wednesday and back from the WSW. Lingering gusts to 25 knots. A cold front sweeps across the waters Thursday, followed by colder air and gusty west winds 25 to 30 knots. West winds gusting to 25 knots continue through Saturday. Seas will diminish through the period, starting 5 to 10 feet on the exposed waters Wednesday and diminishing Wednesday night. Seas will rebuild reaching 5 to 10 feet Thursday in the west winds behind the cold front, then slowly diminish with 5 to 7 foot seas lingering on the outer waters through Saturday. A Small Craft Advisory will be needed for some of all of the waters through this time period.
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&& .TIDES/COASTAL FLOODING...
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10 am update... High tide has past but will continue with headlines for another hour or two especially northeast MA to the NH border as NE gales continue along with pres falls...delaying water from retreating seaward. This combined with seas of 20-25 ft across this area will result in minor coastal flooding thru noon-ish or so. Earlier discussion below. ======================================================================= Core of strongest winds affects East Coast until 10 am. Latest tide gauge readings show about a 2.0 to 3.0 storm surge early this morning, which continues to run above model storm surge guidance. It seems reasonable to still expect this morning`s storm surge to peak between 3.0 and 4.0 feet, although 4.0 feet is probably a good upper bound but not all locations will experience a surge that high. Seas were running close to 20 feet offshore and should build a little further this morning to around 25 feet. These high seas will result in strong wave action along coast with potential for significant beach erosion and possible damage to structures such as vulnerable sea walls. Upshot of all of this is we expect widespread minor coastal flooding along the East Coast. However, pockets of moderate coastal flooding are still likely along Eastern Massachusetts coast near Newburyport/Plum Island, Scituate, and Nantucket Harbor. Winds turn offshore later today which minimizes coastal flood threat in later high tide cycles.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Coastal Flood Warning until 11 AM EST this morning for MAZ007- 015-016-019. Winter Weather Advisory until 4 PM EST this afternoon for MAZ002>004-008-009-012-026. Coastal Flood Warning until noon EST today for MAZ022-024. Wind Advisory until 11 AM EST this morning for MAZ006-007. Coastal Flood Advisory until noon EST today for MAZ023. RI...None. MARINE...Small Craft Advisory until 10 AM EST Wednesday for ANZ231>237- 254-255. Gale Warning until 11 AM EST this morning for ANZ230-250-251- 256.
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&& $$ SYNOPSIS...WTB/Frank NEAR TERM...Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Frank/Nocera MARINE...WTB/Frank/Nocera TIDES/COASTAL FLOODING...staff

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