Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 150824 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 324 AM EST Fri Dec 15 2017 .SYNOPSIS...
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High pressure will move offshore today but bring cold and dry weather for most of the day. Low pressure off the mid Atlc coast will track well southeast of Nantucket but close enough to bring a period of accumulating snow to the coastal plain this evening, ending overnight. 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.
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&& .NEAR TERM /UNTIL 4 PM THIS AFTERNOON/...
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High pres moves offshore this afternoon. Morning sunshine will fade behind increasing clouds this afternoon. Developing low pres emerging off the mid Atlc coast this afternoon but it should remain dry through sunset as deeper moisture still to the south. Just a low risk for a few flurries or light snow developing near the south coast around sunset. 925 mb temps -8 to -9C so another cold day with highs mostly mid/upper 20s with some lower 30s outer Cape/Islands.
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&& .SHORT TERM /4 PM THIS AFTERNOON THROUGH 6 PM SATURDAY/...
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*** A period of accumulating snow likely across RI and southeast MA this evening *** Late today and tonight... Developing low pres off the mid atlc coast will be tracking NE late today and this evening. Meanwhile, another digging northern stream shortwave will be moving through the Gt Lakes and into the northeast. How these two shortwaves interact will determine how far north and west accumulating snow gets. Models have trended stronger and more amplified with this northern stream shortwave as it is becoming better sampled within the upper air network. This allows the mid level flow to back enough for deeper moisture to move further north and west into SNE. While the sfc low is tracking southeast of the benchmark, inverted trough extends back to the NW into SNE assocd with approaching northern stream trough. Models showing large scale synoptic forcing ahead of the shortwave along with favorable upper jet dynamics within the left exit region of 170 kt upper jet. Cross sections indicate a period of modest omega within the DGZ this evening, especially across SE New Eng. Given the deep moisture in place, expect a period of light to moderate snow across much of RI and SE MA this evening where snowfall of 1-3 inches is expected with a low risk for a few 4 inch amounts near the south coast and Cape Cod if trough is more amplified. Onset of the snow will be 5-8 pm so part of the evening commute may be impacted. Will let next forecast team examine the 12z model suite to decide whether an advisory is necessary. Expect light snow to extend north across much of SNE but amounts will be limited to less than an inch in the distant interior. This will be a progressive system and snow will be exiting SE New Eng coast after midnight. Saturday... Another northern stream shortwave will bring a reinforcing shot of cold air and blustery WNW winds to SNE. 850 mb temps -12C support highs from upper 20s to mid 30s with a mix of sun/clouds and wind gusts to 30 mph expected. Mainly dry weather is expected other than a few flurries in western MA as lake effect snow showers try to spill over the Berkshires.
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&& .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 /07Z 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...
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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 Saturday/...High confidence. Today... Gusty winds diminishing early this morning with W/NW winds shifting to SW this afternoon. Seas below SCA. Vsbys may lower in light snow south coastal waters late in the day. Tonight... Increasing NW winds developing after midnight as coastal low moves offshore. Gusts to 25-30 kt expected late tonight. Vsbys reduced in developing snow this evening, especially south coastal waters, improving after midnight. Saturday... SCA conditions with WNW gusts to 30 kt with low risk for marginal gale force gusts over southern waters. 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 4 AM EST early this morning for ANZ231-251. Small Craft Advisory until 7 AM EST this morning for ANZ250- 254>256.
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&& $$ SYNOPSIS...KJC NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell

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