Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221133 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 633 AM EST Thu Feb 22 2018 .SYNOPSIS... A wave of low pressure tracking along a stalled frontal boundary to the south will bring a period of snow and sleet along and north of the Pike with rain and sleet to the south. Active weather pattern Friday through the weekend as a series of frontal boundaries usher wet weather across the region with the threat of freezing rain across the N/W interior, high terrain. A near-seasonable, quiet pattern for early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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630 am update... No major updates to the forecast which remains on track. Previous discussion... Sct showers moving east across the south coast region within the deep moisture axis near and south of the coast. This shower threat will continue through daybreak before low level dry air begin to push south toward the coast. Otherwise, colder air has been a bit slower to arrive across SNE with temps still in the 40s north and 50s south, but 30s to the N/W and good low level dry advection with dewpoints falling through the 20s. Temps will fall into the 30s north and lower 40s south around daybreak. Strong high pres to the north with flat wave tracking along stalled frontal boundary to the south today as temps gradually fall through the 30s. Low level colder air undercutting somewhat milder air aloft will bring a decent shot of mid level omega within a deep moisture plume which will spread precip across SNE. Soundings show a rather pronounced low dry layer which will have to be overcome and this will likely delay onset of precip until around midday and especially during the afternoon. Biggest challenge remains the ptype forecast as cooling aloft is a bit slower than low level cooling. Pronounced low level cold air with 925/950 mb cooling to -3 to -5C this afternoon with a warm layer above 800 mb which will set up a zone of sleet across the region. Cold air deep enough far north with soundings suggesting mostly snow north of the Pike, mixed with sleet at times, with rain mixing with or changing to sleet to the south Precip could even change to a period of snow/sleet across N CT, N RI and interior SE MA. Precip may begin as a brief period of rain northern MA but will quickly change to snow given evaporative cooling processes. Not really expecting any freezing rain given the depth of the low level cold layer, but there is a low risk near the MA/CT and northern RI border. Decent shot of omega through DGZ noted across northern MA this afternoon so a brief period of heavier snow is possible. Expecting an inch or 2 accum north of the Pike, mainly on colder non-paved surfaces with locally up to 3 inches possible over the Berkshires. Less than an inch elsewhere away from areas near the south coast.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Precip will exit the region by early evening with dry conditions most of the night as surface ridging builds south into SNE. However, sufficient moisture for cloud cover to persist through the night, especially south of the Pike. Partial clearing is possible across NE MA late. Lows mostly in the 20s to lower 30s. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... */ Highlights... - N/W interior freezing rain late Friday into Friday evening - Once again Saturday night into Sunday - Dry, near seasonable into early next week */ Overview... Upstream Pacific flow flattening, SE CONUS sub-tropical H5 ridge weakening, SW-NE thermal wind axis and storm track along the better baroclinic zone shifts S/E along which mid-level impulses eject out of a preferred H5 trof pattern across the W CONUS. An active weather pattern with a series of disturbances delivering mixed precipitation events thru the weekend. Building up over the N Atlantic signaled by a strongly -NAO, in going with ensemble means, we fall within a near-seasonable, dry pattern W of favorable storm development over SE Canada. Model forecast consensus through 120 hours (Monday) with preference towards ensemble means thereafter. Hit on targets of opportunity below. */ Discussion... Late Friday into Friday evening... Over-running precipitation event. Focus on thermal fields below H9, especially surface with 2m wet-bulb temperatures, beneath the warm nose up around H8. Surface high situated E, less N-funneling winds per ageostrophic motions. There`s the potential for a brief period of freezing rain. Dry air issues, above average heights, mid-upper level ridging, absence of deep layer lift, that with precipitable waters building towards 0.75 to 1.00 inch, light to moderate precip event is forecast with amounts upwards of around 0.25 inches, no higher than 0.50, with highest amounts along the W slopes of high terrain. A brief period though during the height of precip outcomes, ice accretion of 0.1 to 0.2 inches is possible over higher terrain. Highest confidence towards N/W MA especially Berkshires, Worcester Hills. High-res guidance colder than global. With the high situated E, return S flow, prefer to edge away from coldest guidance and not be pessimistic. Thus forecast ice accretion amounts up as high as around 0.10 inches over high terrain, especially Berkshires. Lean a non-GFS consensus with impacts to Friday PM commute. Saturday night into Sunday... Another over-running precipitation event associated with a classic warm occlusion into S Canada. Stronger synoptic forcing / lift that invokes a secondary surface low along our coast with a synoptically favorable cold surface high N/E towards a freezing rain event. More likely outcomes of colder temperatures funneling S, maintaining over the interior, notably occurring during an overnight period. Prefer EC/NAM/Canadian, higher confidence of freezing rain over Berkshires, Worcester Hills. Accretion amounts of 0.1 to 0.2 given less dry air intrusions, deeper lift / forcing. Precipitable waters once again in the 0.75 to 1.00 inch range. Early next week... Keeping with ensemble means, traffic build up across the N Atlantic, the strongly -NAO, the initial region of favorable storm development over SE Canada, will need to closely monitor for any retrogression as energy continues to eject out of a preferred H5 trof pattern over the W CONUS. Looking dry and near-seasonable till roughly Thursday into Friday, being W on the backside of favorable storm development. NW winds prevailing. Overall, a low confidence forecast. && .AVIATION /12Z THURSDAY THROUGH MONDAY/...
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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. 12z TAF update ... No major changes to the earlier forecast outlined below. Today... VFR in the morning, then lowering to MVFR/IFR in the afternoon with snow/sleet along and north of the Pike and rain/sleet to the south. Tonight... Precip moves offshore by early evening. MVFR/IFR improving to VFR north of the Pike, but MVFR cigs persisting to the south. KBOS Terminal...Moderate confidence TAF. Conditions lowering to MVFR then IFR in the afternoon with a period of rain, then snow and sleet. Any accum less than an inch. KBDL Terminal...Moderate confidence TAF. Conditions lowering to MVFR then IFR in the afternoon with a period of rain, sleet and snow. Any accum less than an inch. Outlook /Friday through Monday/...Moderate confidence. Thursday: Mainly VFR, with local IFR possible. Breezy. PL, RA likely, SN likely. Thursday Night: Mainly VFR, with local IFR possible. Friday: Mainly MVFR, with areas VFR possible. RA, FZRA. Friday Night: MVFR/IFR conditions possible. RA likely, FZRA likely. Saturday: VFR. Saturday Night: Mainly MVFR, with local IFR possible. Breezy. RA likely, FZRA likely. Sunday: Mainly MVFR, with areas IFR possible. Breezy. RA, FZRA. Sunday Night through Monday: 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 Tonight/...Moderate confidence. Today... Northerly winds turning NE with gusts to 20-25 kt at times. SCA issued for eastern waters due to seas. Vsbys reduced in rain and snow this afternoon. Tonight... Mainly NE winds with gusts to 20 kt at times. Seas mostly below SCA. Outlook /Friday through Monday/... Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain likely. Thursday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Friday: Winds less than 25 kt. Rain, slight chance of freezing rain. Friday Night: Winds less than 25 kt. Rain likely. Saturday: Winds less than 25 kt. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain. Sunday Night: Winds less than 25 kt. Areas of rough seas. Monday: Winds less than 25 kt. Seas up to 5 ft.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ250. Small Craft Advisory until 7 PM EST this evening for ANZ254. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell

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