Area Forecast Discussion
Issued by NWS Boston, MA

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838 FXUS61 KBOX 250830 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 330 AM EST Sun Feb 25 2018 .SYNOPSIS...
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A fast moving low pressure system will bring rain today to the coastal plain and a mixture of sleet, snow and freezing rain changing to rain across interior Massachusetts. High pressure will bring mainly dry and mild temperatures Monday through Wednesday. There is the potential for a strong coastal storm late next week that if it impacts our region may bring significant rain and/or snow...strong winds on the coast along with potential coastal flooding. This storm is far from a certainty at this point...but given the ingredients in place does bear watching.
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&& .NEAR TERM /THROUGH TODAY/...
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*** Mixed wintry precip today across interior, especially northern MA *** Leading edge of precip shield is approaching SNE from the west and will overspread the region through 7 am. PWATs 2-3SD above normal combined with good low level isentropic ascent will bring a moderate QPF event with widespread precip today. Progressive system so much of the precip will be exiting the region by late afternoon or early evening. A mixed bag of wintry precip is expected given warm nose aloft and presence of low level cold air. In fact models are indicating temps in the 950-925 mb layer cooling to 0 to -2C near the south coast this morning and down to -5C near the NH border. With warming in the 850-700 mb layer advancing northward this will set up a good sleet signal this morning, especially across northern MA. In addition, with sfc warm front remaining south of New Eng and sfc wave tracking along the boundary, low level northerly winds will help to lock in cold air at the surface, especially higher terrain which will increase threat of icing after warming aloft changes precip to rain. North of the Mass Pike... Cold air is initially deep enough for some snow especially toward NH border and NE MA for at least a few hours after precip onset before changing to freezing rain and rain. Snow in NE MA may last until late morning as warmer air aloft coming up from the SW. An inch or 2 of snow accum is possible, mainly along and north route 2 into Essex county with low risk for 3 inches in portions of northern county. Some minor sleet accum is also possible down to the Mass Pike region. Regarding freezing rain, it appears greatest threat for appreciable icing will be over higher elevations in the Berkshires and Worcester hills where temps may hold at or below freezing much of the day. We relied heavily on 3km NAM and WRF-ARW which shows the cold air over higher terrain. Potential for localized 0.10-0.20" ice accretion over the Worcester hills and especially the Berkshires with less than 0.10" elsewhere. South of the Mass Pike... Precip will be mostly rain but will likely mix with sleet for a few hours this morning across northern CT, northern RI and along the I-95 corridor to BOS. Freezing rain is less of a threat south of the pike, but may still see a few hours of freezing rain over higher terrain in northern CT and central MA. Advisories largely unchanged but we dropped it for coastal Essex county and SE Middlesex county where temps expected to remain above freezing with just a mix of rain and sleet.
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&& .SHORT TERM /TONIGHT/...
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Precip will exit eastern New Eng by early evening, otherwise a dry night expected. Stratus and patchy fog in the evening may clear overnight, but any clearing may lead to more fog developing. Patchy black ice will also be a concern overnight with moisture near the ground and lows dropping to upper 20s to lower 30s away from the coast.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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*/ Highlights... - Quiet, dry, near-seasonable early week period - Possible stormy pattern late week, beginning of March */ Overview... Blocky N Atlantic pattern associated with an anomalous -NAO of 4 to 5 standard deviations (SD) below average, associated +3 SD H5 high retrograding W into SE Canada, parent 1040+ surface high, pressing the thermal wind axis S thru which N/S stream impulses merge having dug through a prevailing W CONUS H5 trof before ejecting, stretching E. Energy piling up, slowed, pressed S by high pressure building N, favorable storm development emerges off the E CONUS coast. Airmass juxtaposition key as to outcomes towards the end of the week after a quiet start. Only confidence is persistent NE winds of significant duration during high astro-tides, coastal flooding concerns. Lower confidence otherwise, consensus forecast guidance heavily weighted with ensemble means continues. Hit targets of opportunity below. */ Discussion... Thursday into the weekend... Battle of airmasses between an ejecting low out of the Central CONUS up against building high pressure into Canada. Slowed pattern, each building their own weight, juxtaposition between the two ultimately determining outcomes. Strong -NAO, -4 to -5 SD, would expect a cool, dry setup for NE CONUS, storm track lying further S per block. Mid- Atlantic getting crushed. Interrogating Washington DC top 10 snows versus the NAO time series after 1948, 5 out of 7 storms, 3 prior to 1948, were associated with a -NAO around -3. Recall February 2010? Washington DC "snowmageddon" of 17.8"? Monthly NAO averaged near -2. Cherry picking data? Perhaps. CIPS analogs? Interrogating where NAO indices <= -1, noting trend of sliding low S of New England, further so with a stronger -NAO. Ensemble members clustering off the Delmarva Peninsula while model deterministic solutions are hugging further N close to Long Island. Believe S trend is emerging per 25.0z EC. Echoing prior forecaster, strong -NAO, subsequent block, storms forced E, slowed, evolving into coastal storms, but storms could get suppressed S. It is all about timing. Simply wait and see with later forecasts. But per climatology, prefer ensemble clustering / 25.0z EC which are further S compared to all other guidance. Thursday into Friday lingering into Saturday, will continue with chance PoPs. Warm-core system, absent Arctic air, precip-type issues mainly as rain / snow first glance. Confident of persistent NE winds during high astro-tides, coastal flooding concerns.
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&& .AVIATION /07Z SUNDAY THROUGH THURSDAY/...
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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 Tonight/... Through 12z...Conditions lowering to MVFR in western/central New Eng 08-12z as precip overspreads the region. VFR likely holding until 12z E/NE MA before deteriorating. Ptype mostly snow and sleet at the onset north of the Pike, and rain mixed with sleet to the south. Today...Moderate confidence. Widespread IFR developing. North of the pike, mostly sleet with some snow NE MA changing to rain but pockets of freezing rain persisting over the Berkshires and Worcester hills. South of the Pike, rain mixed with sleet in the morning, with pockets of freezing rain in the morning over higher terrain in northern CT and central MA. Precip exits by late afternoon/ or early evening. East wind gusts to 25-30 kt developing along the coast. Tonight...Moderate confidence. IFR conditions with patchy fog in the evening, improving to VFR after midnight. Dry conditions. KBOS Terminal...Moderate confidence in TAF. Lowering to IFR today then improving tonight. Rain mixed with sleet in the morning. KBDL Terminal...Moderate confidence in TAF. Lowering to IFR today then improving tonight. Rain mixed sleet in the morning. Low risk of a few hours of freezing rain in the morning. Outlook /Monday through Thursday/...Moderate to high confidence. Monday through Monday Night: VFR. Breezy. Tuesday through Wednesday Night: VFR. Thursday: Mainly VFR, with local MVFR possible. Breezy.
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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 Tonight/...Moderate confidence. Today...A period of marginal easterly gales likely across much of the open waters today as low level jet moves over the waters. Cold air above the boundary layer will promote decent mixing this morning but inversion will strengthen during the afternoon. Rain and fog will reduce vsbys. Tonight...Winds diminishing and becoming NW during the night. Speeds dropping below SCA but hazardous seas will continue over outer eastern waters. Outlook /Monday through Thursday/...Moderate to high confidence. Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night: Winds less than 25 kt. Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday Night: Winds less than 25 kt. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
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&& .TIDES/COASTAL FLOODING... Thursday through Saturday... The potential for coastal flooding and beach erosion exists on several high tide cycles late next week. This will depend upon the development of a potential strong/long duration coastal storm. While it is too early to give more specific information...high astronomical tides and the potential for a strong coastal storm will need to be watched especially along the eastern MA coast. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory until 1 PM EST this afternoon for CTZ002>004. MA...Winter Weather Advisory until 1 PM EST this afternoon for MAZ005-006-009>012. Winter Weather Advisory until 4 PM EST this afternoon for MAZ002>004-008-026. RI...None. MARINE...Gale Warning from 9 AM this morning to 3 PM EST this afternoon for ANZ231>234. Small Craft Advisory from 6 AM this morning to 6 PM EST this evening for ANZ230. Small Craft Advisory from 6 AM this morning to 3 PM EST this afternoon for ANZ236. Gale Warning from 6 AM this morning to 1 PM EST this afternoon for ANZ235-237. Gale Warning from 10 AM this morning to 6 PM EST this evening for ANZ250-254. Small Craft Advisory from 6 AM this morning to 5 AM EST Monday for ANZ251. Gale Warning from 6 AM this morning to 3 PM EST this afternoon for ANZ255-256.
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&& $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell TIDES/COASTAL FLOODING...

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