Area Forecast Discussion
Issued by NWS Boston, MA

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231 FXUS61 KBOX 221803 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 103 PM 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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1 PM update... Not much change from previous forecast update. Periods of very light rain/sprinkles continue to overspread the area. Low level dry air continues to erode much of the precip as it enters the region. However when brief heavier bands enter the area sub freezing wet bulb temps are achieved resulting in ice pellets (sleet) mixing in a times. Column has cooled sufficiently across the Berkshires for all snow now at PSF and AQW, tho temps here remain above freezing. So the trend this afternoon is for steadier precip across NY state to overspread the region later this afternoon. 12z NAM soundings capturing this well with column not saturating until 2 pm to 5 pm from west to east. As this happens columns cools sufficiently for rain to change to sleet and snow, probably all the way into the HFD-PVD-BOS corridor and possibly to the south coast. Low level cold air is tumbling southward but subfreezing temps are slow to arrive, about 3/4pm interior and not until 7 pm or so remainder of the region. Best deep layer moisture and forcing for ascent occurs around 21z-00z west to east then rapidly moves offshore. Thus it`s a small window of opportunity for any accumulating snow. Best chance will be across the high terrain of western-central MA, west of I495 with a coating to two inches possible, especially secondary roads and non treated surfaces. Very low risk of isolated 3" amounts along the MA/NH/VT border. Elsewhere a trace to a coating for the HFD- PVD-BOS corridor and little if any toward the south coast. NAM soundings suggest 1-3 hr window of steadier/moderate snow possible 4 pm - 7 pm into the HFD-PVD-BOS corridor. However with storm total qpf amounts less than 0.25 inches (up to 0.50 inches western-central MA) and some of that falling as rain early this afternoon, duration of heavier precip will be very brief. Thus greatest impact will be higher terrain where subfreezing surface temps are observed 3 pm - 7 pm.
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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 /18Z THURSDAY THROUGH TUESDAY/...
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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. 18z TAF update ... Thru 00z... VFR trending toward MVFR 21z-00z in rain changing to PL/SN west to east. Snow accumulations of a coating to two inches possible western-central MA including KORH. Elsewhere trace to a coating possible. After 00z... MVFR in PL/SN ending 00z-03z from west to east then trending dry and VFR. Friday... VFR but trending MVFR late in rain with PL western- central MA. Friday night... low risk of LLWS with low level WSW jet. MVFR in evening rain improving to VFR and dry overnight. KBOS Terminal...Moderate confidence TAF. Low risk for period of steady SN/PL 22z-00z then improving. KBDL Terminal...Moderate confidence TAF. Low risk for a period of moderate SN/PL 21z-23z. Outlook /Saturday through Monday/...Moderate confidence. 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... 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. && .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...Nocera SHORT TERM...KJC LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...KJC/Sipprell

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