Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 150556 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1256 AM EST Fri Dec 15 2017 .SYNOPSIS... High pressure over the central U.S. builds E into New England into early Friday, providing dry but cold conditions. Two areas of low pressure, one tracking across southeast Canada and another well southeast of Nantucket may bring a period of light snow or flurries Friday evening to the area. Blustery and cold Saturday and Sunday. Potential for light freezing rain Sunday night into Monday, followed by additional disturbed weather through Tuesday. Cold and blustery Wednesday into Thursday. Monitoring a potential storm system for Friday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 9 PM update... Winds remain gusty, especially along the coast, but will gradually diminish after midnight as high pres builds in from the west. Lowered temps a bit in the colder outlying locations with expectation of good radiational cooling. Expect lows in the single numbers, except teens along the coast. Previous discussion... Cold advection continues to pour into southern New England behind departing ocean low with -18C air at 850 mb upstream across NY state per latest SPC mesoanalysis. Dry weather tonight in response to mid level confluent flow across the region. Beneath this confluent flow high pres builds across the area, resulting in gusty WNW winds this evening diminishing overnight. So cold/dry airmass combined with diminishing wind and a fresh snow cover from south-central CT-RI into southeast MA will yield a cold night ahead. Given the radiational cooling conditions derived mins from the colder MOS and bias corrected MOS datasets. This supports lows in the single digits and teens regionwide. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/... Friday... Cold start to the day but fortunately not much wind. Some dim morning sunshine possible behind increasing mid/high clouds. This combined with cold airmass remaining in place over the area with 850 mb temps around -12C, not expecting much of a temp recovery. Highs only 25-30 with warmest readings along the south coast and coldest across the high terrain. Friday evening and overnight... Then toward evening another vigorous northern stream short wave approaches southern New England from the west. Parent low tracks from Ontario to Quebec. However this short wave induces cyclogenesis off the Mid Atlc coast but races ENE south of the benchmark given the fast/progressive upper air pattern. Will have to watch and see if this short wave trends stronger and backs the mid level flow more along the eastern seaboard. This would result in deep moisture and lift tracking closer to the coast into southeast MA Friday evening. All model guid indicate trough amplification just east of Cape Cod with increasing mid/upper level Q-vector convergence. In addition mid level lapse rates begin to steepen too. If later model runs are 6 hrs quicker with trough amplification a few inches of snow would be possible over southeast MA, possibly eastern MA. However for now will follow a model blend which offers a period of light snow/flurries with snow accums less than an inch. Drying trend by midnight along with a reinforcing surge of cold air advection via blustery WNW winds on the backside of this departing wave. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... */ Highlights... - Cold and blustery Saturday into Sunday, snow showers possible - Sunday night into Monday, possible light mixed wintry precip - Monday night through Tuesday night, warmer first, then cold - Wednesday into Thursday, return cold and blustery - Friday onward, monitoring for potential mixed wintry precip events */ Overview... Transitional, ebb and flow pattern. Into the following week, strong onshore Pacific push of mild air as the H5 ridge breaks over the NW Pacific, warming over the CONUS, heights rise and bowled colder air over E Canada that has continually wrapped S over the NE CONUS this past week is shunted back N. H5 heights less pronounced, more flat. Impulses riding along the transitional zone, warmer air riding N/E along lingering cold air presents the opportunity of light mixed precip events, exiting quickly with absence of downstream traffic though wrapping up and deepening over offshore regions of better baroclinicity, colder air is drawn S, less pronounced given flatter flow. Again, ebb and flow. Then interpreting the pattern into the Christmas holiday. As noted yesterday, there`s considerable warming / height rises over N Canada withing the stratosphere as the polar low / night-time jet displace to the other side of the N Hemisphere. This concurrent with a strong push of mild air from the NW Pacific into the Arctic, stout H5 ridge into Alaska, pattern reloads as it buckles downstream over the CONUS into the N Atlantic. But the main polar low over E Canada seemingly pushes E, nudged by the aforementioned positive height anomalies to the W all the way up through the stratosphere. The N Atlantic wide open, looking at a SW-NE transitional zone towards regions favorable for storm development over the N Atlantic into NW Europe. Ejecting energy sheared S over the SW CONUS from the favorable H5 ridge over the NW Pacific, along the SW-NE transitional zone, H85 temperature and parent anomalies from ensemble means showing this well, looking at a pattern favorable over the E CONUS of mixed precip events. With storm development as noted over the N Atlantic, expecting colder air to remain bowled over E/NE Canada, held N up against southwesterlies and positive height / mean sea level pressure anomalies, presumably parent with the positive height anomalies / warmer air surging into the Arctic over Northern N America. Notably N Atlantic surface high builds back W towards the SE CONUS. Got awhile to make sense of it all. Low confidence forecast by the end of the week into the following weekend. Will hit upon threats / impacts in the discussion below, noting any particular details and touching upon any specifics. */ Discussion... Saturday into Sunday... Cold, blustery, snow shower activity off the Lakes. Storm system departing, deepening downstream Friday night into Saturday, with additional mid-level energy, moisture off the Lakes / over ocean waters, expect blustery winds and cold air advection to contribute to snow shower, potential snow squall activity. Highest confidence of impacts over the Berkshires and across the Outer Cape / Islands. Light accumulations anticipated along with reductions in visibility through Saturday prior to high pressure and colder air into Sunday. Wind gusts Saturday up around 30 to 35 mph forecast, holding below WIND ADVISORY criteria. Still cold wind chills going into Sunday morning with lows in the single digits to teens. Sunday night into Monday... Mixed precipitation event. Lifting warm front along which moisture ascends above lingering cold air, albeit weak lift. Not a clear cut scenario. Cold air damming signal via ageostrophic flow light with absence of stronger parent low S/W. Ascent / lift along the warm front being weak and quasi-parallel along isentropes. Ensemble means continue to signal decent 0.01 inch probabilities but now more N/W closer to the parent low collocated with strong SW ascending branch of warm, moist air. Aside from signals of light outcomes and mixed precipitation (snow to sleet to freezing rain), only takes a trace of ice to require WINTER WEATHER ADVISORIES. Operational guidance continues to poorly handle the weak wave event yet EC has remained consistent. GFS drier and flatter, little indication of a weak wave disturbance / H85 trof axis. Monday night into Tuesday night... Seasonable, dry, SW flow at first prior to a sweeping cold front, perhaps anafrontal, during the overnight period. A flat mid-level feature to accompany, looking at a lack of impacts associated with a potential rain to snow transition. However, operational guidance having difficulties in handling S-stream energy being swept up ahead of the N-stream trof. Separated flow regime or a combination of the sub-tropical with the polar jet. The 14.12z GFS more robust over the concurrent EC. Most GEFS members clustered lower than deterministic guidance. EC ensemble probabilities low with respect to 0.01 inches of liquid. Wednesday into Thursday... Lean cold, blustery and dry for Wednesday behind the departing storm system, deepening over SE Canada, drawing S cold air. High pressure for Thursday, a rebound of S flow during the evening ahead of an approaching warm front. Friday onward... Signals of a strong synoptic system sweeping the region. Following closely to EC ensemble means, and inside runner, could be dealing with mixed precipitation types out ahead of a lifting warm front, the follow-up sweeping cold front overnight putting us back into colder air as the storm system deepens over SE Canada. From there, the Atlantic high build back W, the amplified pattern emerging over the W CONUS, we get to a SW-NE transitional zone pattern welcoming mixed precipitation / ice events over the E CONUS if ensemble means are correct, gone over in detail in the OVERVIEW section above. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... Today...High confidence. VFR most of the day with lowering cigs. Some late day MVFR cigs developing near the south coast late in the day with chc -SN. Tonight... High confidence in a period of snow with IFR impacts developing across RI and SE MA in the evening. Localized LIFR in moderate snow possible near the south coast. Accums 1-3 inches. Northern and western extent of the snow still uncertain but it will likely reach BDL-ORH-BOS for a short time with brief MVFR possible and a coating to less than an inch. Snow exiting SE New Eng coast around 06z with improving conditions to VFR thereafter. Increasing NW winds along the coast 08-12z with gusts to 25 kt. Saturday...High confidence. VFR with sct-bkn 050-080 developing. WNW gusts to 25 kt. KBOS Terminal...Moderate confidence. Period of light snow with MVFR conditions this evening. Brief IFR vsbys possible. KBDL Terminal...Moderate confidence. Period of light snow with MVFR conditions this evening. Low risk for brief IFR vsbys Outlook /Saturday Night through Tuesday/... Saturday Night: VFR. Windy with local gusts to 30 kt. Sunday: VFR. Slight chance FZRA, slight chance SN. Sunday Night: VFR. Chance SN, slight chance FZRA. Monday: VFR. Chance SN, slight chance RA, slight chance FZRA. Monday Night: Mainly VFR, with local MVFR possible. Slight chance RA, slight chance SN. Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance RA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Tonight... Departing low over eastern Georges Bank yields WNW winds 15-20 kt with gusts up to 25 kt this evening across the MA/RI waters, then decreasing overnight as high pres builds in from the west. Dry weather and good vsby. Friday... Light winds with high pressure over the area. Dry weather and good vsby. Friday night... Light winds during the evening, then low pres over Ontario into Quebec along with low pres tracking well southeast of the 40N/70W benchmark, both move east and intensify. This yields increasing WNW winds after midnight with speeds of 15-20 kt, gusts possibly up to 25 kt toward daybreak Sat. Vsby may lower in snow across the eastern MA outer waters, east and southeast of Cape Cod. Outlook /Saturday Night through Tuesday/... Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain, slight chance of snow. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Sunday: Winds less than 25 kt. Slight chance of rain. Sunday Night through Monday: Winds less than 25 kt. Chance of rain, chance of snow. Monday Night: Winds less than 25 kt. Slight chance of rain. Tuesday: Winds less than 25 kt. Chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 AM EST early this morning for ANZ232>234. Small Craft Advisory until 3 AM EST early this morning for ANZ230. Small Craft Advisory until 4 AM EST early this morning for ANZ231-235-237-251. Small Craft Advisory until 7 AM EST this morning for ANZ250- 254>256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...KJC/Nocera SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...Nocera/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.