Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 241950 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 250 PM EST Tue Jan 24 2017 .SYNOPSIS...
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A powerful coastal storm east south of Long Island will move northeast to Nantucket this evening then into the Maritimes Wednesday. Showers with locally brief heavy rain lingers through tonight with areas of freezing rain confined to the high terrain of the Worcester Hills and the Berkshires of 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 /THROUGH 6 AM WEDNESDAY/...
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230 pm update... *** Areas of freezing rain likely lingers into tonight across the high terrain especially at elevations above 1000 ft *** Winter Weather Advisory... Temperatures continue to warm aloft as closed low east of NJ continues to advect Trof of Warm Air Aloft (TROWAL) into the region. This will result in predominantly rain this afternoon and tonight. Although as column cools later tonight there is a chance of some sleet mixing in at times with the rain. As low pressure tracks south of New England surface winds will remain from the NNE. This will keep the shallow cold air over interior MA in place thru today and likely into tonight. Thus freezing rain is expected especially at elevations above 1000 ft across the Worcester hills and Berkshires. Already received a report of 3/16" of ice accretion at 1130 am from Berlin MA and 0.25 inch of ice in North Adams MA. Therefore expect some icy roads this afternoon and tonight across the high terrain of MA. As precip and ice accretion continue there will be a low risk of down branches and isolated power outages across this region into tonight. Forecasting additional ice accretion of up to a tenth of an inch. This is supported by the 00z NCAR ensembles, 12z Euro/NAM and 09z SREF. Rainfall... Despite subtropical plume of moisture northeast of the region into ME and the Maritimes...closed low east of NJ is circulating a new plume of moisture off the Atlantic which is beginning to form a comma-head over southern New England. This radar will continue to fill in with additional showers. Precip shield is fragmented due to some drier air aloft from the dry slot offshore advecting into the region. However this dry air aloft is serving to steepen the mid level lapse rates and enhancing updraft potential. Thus low risk for isolated thunder this afternoon into tonight. But more importantly these steepening mid level lapse rates will support heavier showers with brief downpours. A model blend of global and Hi Res guid supports widespread qpf of 0.15 to 0.35 thru tonight with local amounts up to a 0.50 where heavier embedded showers develop. Winds... Core of low level jet is now along ME coast into the maritimes, however 40-45 kt low level NE jet continues to wrap cyclonically across the region as low pres remains south of New England. These winds will slowly weaken this afternoon and tonight...remaining below wind advisory criteria. However these gusty NE winds will continue the raw/damp chilly feel thru tonight.
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&& .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 /19Z TUESDAY THROUGH SUNDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 20z update... Thru tonight...high confidence. Not much change with mainly IFR conditions in showers and fog with marginal IFR/MVFR CT river valley. Main concern late this afternoon thru tonight is freezing rain for the high terrain of MA including KORH. Up to a tenth of an inch of ice accretion is possible. NNE winds up to 35 kt late this afternoon slowly slacken and become NNW overnight. Wednesday...high confidence. Any MVFR cigs over Cape Cod and island at sunrise quickly move offshore followed by VFR. Elsewhere VFR prevail with modest WNW winds, dry weather and good vsby. Low risk of leftover showers Cape Cod and Nantucket at sunrise. Wednesday night...high confidence. VFR to start then lowering to VFR-MVFR toward sunrise Thu. Low risk of IFR-MVFR across the high terrain of MA/CT and possibly northwest RI toward daybreak. Light WNW winds become SSW toward morning. Chance of showers toward daybreak as well. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Wednesday through Saturday/...Moderate confidence. 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. *** Winds slacken tonight but very rough seas continue *** 245 pm update... Thru tonight... Low pres south of Long Island at 3 pm tracks NE to Nantucket this evening and then across the Gulf of ME tonight...then south of Nova Scotia Wed. NNE winds 20 to 30 kt with a few gusts up to 35 kt late today will slacken slowly and become NNW overnight. Vsby lowering to 1 mile or less in showers and fog..but improving after midnight and especially toward daybreak. 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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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Winter Weather Advisory until 4 PM EST this afternoon for MAZ002>004-008-009-012-026. RI...None. MARINE...Small Craft Advisory until 10 AM EST Wednesday for ANZ231>237. Small Craft Advisory until 5 PM EST Wednesday for ANZ254-255. Small Craft Advisory until 4 PM EST Wednesday for ANZ256.
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&& $$ SYNOPSIS...WTB/Frank NEAR TERM...Nocera SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Nocera MARINE...WTB/Frank/Nocera

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