Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 132032 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 332 PM EST Wed Dec 13 2017 .SYNOPSIS... * Long-term update only ... short/near term out shortly A gale center will move into the Maritimes today with windy and very cold conditions in its wake. A fast moving low pressure will track south of New England late tonight and early Thursday bringing a period of light snow to the south coast and especially the Islands. Cold persists into the weekend with a chance of snow showers Friday night into Saturday. Blustery and cold for Sunday but the beginning of a milder trend Monday ahead of which there`s the possibility of a mixed wintry precipitation event. Quiet and dry with an ebb and flow pattern during the week up till late Thursday into Friday during which time there`s the possibility of another mixed wintry precipitation event. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 115 PM Update... Peak wind gust this hour is 37kt/43mph at Worcester. Most locations gusting to 30-35 kt/35-40 mph. These wind speeds combined with air temps only in the low to mid 20s yielding wind chills in the single digits and teens. The combination of some low level moisture from the Great Lakes and cyclonic flow aloft producing some light scattered snow showers/flurries across the entire region. Ocean effect snow showers continue to clip Nantucket but the bulk of the activity remaining south of the islands. Previous forecast continues to verify nicely so no major changes planned with this update. Earlier discussion below. Previous Discussion... Mid level low moves across New Eng this morning before lifting NE this afternoon. Cold pool aloft with 500 mb temps around -32C combined with some low and mid level moisture moving through will lead to a period of bkn cu and can`t rule out a few flurries, especially higher terrain. Clearing later afternoon. The main weather story for today will be wind and cold in the wake of the departing low pres moving into the Maritimes. 850 mb temps bottom out around -15C to -16C today so only expect minor recovery from morning lows. Highs will range through the 20s to near 30 outer Cape/Islands, with some upper teens in the Berkshires. Wind chills will be in the single numbers and teens. Well mixed boundary layer with steep low level lapse rates will promote excellent mixing with soundings suggesting peak gusts 40-50 mph, strongest mid/late afternoon especially over higher elevations. We expanded the wind advisory to include all of MA/RI. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... A period of clear skies expected this evening with gusty winds gradually diminishing overnight. However, clouds will quickly return as a rather robust mid level shortwave moves across the Ohio Valley to south of New Eng late tonight and early Thu. Low pres tracks south of the coast and attempts to spread some light snow north into portions of SNE late tonight into early Thu. Plenty of dry air initially to overcome which will limit northward extent of snowfall. Best chance for a period of light snow will be near the south coast and especially the Islands where better moisture and lift. Guidance indicating a brief period of modest omega in the DGZ over the Islands 09z-12z with sharp decrease in omega to the north. Minor accum of a coating to an inch is possible for the south coast and especially the Islands with low prob for 2 inches BID-ACK. However, a sharp gradient is expected on the northern edge of the snow shield so if it shifts slightly south, little or no accum will be the result. Light snow may linger into Thu morning immediate south coast and especially Cape/Islands as low pres passes to the south. Otherwise, drier air and clearing skies behind the departing low pres will gradually move in from NW to SE but it may take until mid afternoon for clearing to reach far SE New Eng. A period of gusty NW winds will develop Thu afternoon with gusts 20-30 mph. Another unseasonably cold day with highs mid 20s to lower 30s. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... */ Highlights... - Light snow showers possible Friday night into Saturday - Cold and blustery Saturday night into Sunday - Possible mixed precipitation Sunday night through Monday night - Perhaps quiet, cold until Thursday night into Friday with another mixed precipitation event */ Overview... Interpreting through late December. Considerable forecast warming in the stratosphere, polar low / night-time jet displaced to the other side of the N hemisphere subsequent of noteworthy anomalous ridging across Northern N America. Beneath which warm air continues to surge into the Arctic from the NW Pacific (with perhaps MJO contributions) and the NW Atlantic displacing colder air S where it becomes bowled up across NE Asia / NE N America. Ensemble means continue to signal an active weather pattern with an amplified H5 ridge-trof pattern over the CONUS, anomalous H85 T warming over the N Pacific with cold anomalies continually reloaded over Central/E N America, however a trend towards higher heights / surface mean sea level pressure with stratospheric cross-polar flow shifting into NW Europe. Making sense of it all, NW Pacific disturbances translating E across the CONUS seemingly look to flatten the H5 pattern, lock colder air N for a time, the NE CONUS falling into the transitional zone of air- masses before colder air reloads and surges S again. An ebb and flow setup however not barred from N shots of colder air, the lack of S- stream dominance. Yet a trend of higher heights as signaled from the ensemble means, cold shots may not be as deep, storm development may not be so pronounced long-term. Perhaps La Nina signals are starting to emerge across N America with a lesser pronounced H5 trof pattern over Central/E N America downstream of stout ridging / warmth over the NW Pacific. Tough to get down on the details and specifics, taking a broad view approach with the discussion above. Will hit on any threats/impacts in the details below. With perhaps a flatter pattern evolving and the lack of downstream traffic over the N Atlantic, the move into a zone of transition between the airmasses as noted above, as forecast guidance suggests, could end up in a regime of quick-moving systems with mixed precipitation type outcomes that later deepen downstream and yield that reloading shot of colder air, yet that colder air not as deep, perhaps simply swiping the NE CONUS more to the N. */ Discussion... Friday through Saturday... Cold, blustery at times, chance of snow showers Friday night into Saturday. A nod to ensemble means for 24-hour rainfall probabilities in excess of 0.01 inches as N-stream clipper energy translates to a dominant S-stream disturbance across the offshore baroclinic front. Yet leaning very light outcomes. Lack of ascent within moist snow growth regions parent with the positively-tilted H5 trof undergoing neutral transition along the surface transition boundary. It isn`t till better mid-level curvature and ejecting vortmax above the S- stream can get things going, in this instance further downstream. Leaning snow accumulations along the N/W slopes of high terrain given orographic support, and along the SE coast as energy / ascent begins to come together before ejecting out to sea. Highest snow accumulations possibly over an inch for S/SE coast, especially over the Islands and Outer Cape. Visibility impacts, perhaps some slick road conditions. Behind the disturbance, winds kick up, colder air drives back in, wind chills drop back down into the single digits Sunday morning, lows in the teens. Sunday into Monday... Potential mixed precipitation event. Ascent overall light, seemingly quasi-parallel flow along the lifting warm front. Above a cold air damming signature with an indication of northerly ageostrophic flow, the surface high to the N however a weaker low over the Great Lakes, could see a period of freezing rain / drizzle centered around late Sunday night into early Monday morning, especially in the interior sheltered valleys where cold will be more difficult to dislodge. A nod again to ensemble mean for 24-hour rainfall probabilities in excess of 0.01 inches which is roughly 70 percent or greater. Again light outcomes but enough to potentially make for hazardous travel and could yield WINTER WEATHER ADVISORIES for a trace of ice. Some question as to how quickly the warm front will lift N and how long threats / impacts may linger. Could hold through Monday evening. A low confidence forecast given poor handling via operational guidance on individual upstream waves. Tuesday onward... Leaning a flatter, progressive flow regime with transitioning air- masses. With any additional waves will be looking for potential mixed precipitation events with the ebb and flow of 2m temperatures. Weaker energy ejecting E and deepening, expecting behind each system the return of colder conditions ushered by breezy N/W winds. Lack of confidence in the forecast beyond 96 hours (which does include the Sunday into Monday timeframe). In leaning with ensemble means, after the early week disturbance, our next event may not occur until late Thursday into Friday, an initial cold blustery pattern transitioning warmer, more seasonable in-between. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/... 1815z update... Thru 00z...high confidence. VFR but occasionally marginal MVFR in scattered light snow showers/flurries. Strong WNW winds with gusts up to 35-40 kt continue. After 00z...high confidence. VFR and winds slowly subside. Then after 06z and especially after 09z MVFR in light snow overspreads CT to the south coast of RI/MA. Elsewhere VFR cigs with just very light snow or flurries. Light west winds. Thursday...moderate confidence with some uncertainty how far north accumulating snows track. MVFR along the south coast of RI/MA in snow from 12z-15z. Low prob of brief IFR from BID-MVY-ACK. Snowfall of 1-3" along immediate south coast with highest totals on the islands. Conditions improving to VFR in the afternoon along with dry weather. Elsewhere, MVFR in very light snow or flurries improving to VFR around midday and turning dry. Thursday night...high confidence. VFR and dry with west winds becoming gusty over Cape Cod and islands. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Thursday Night through Monday/... Thursday Night: VFR. Breezy. Friday: VFR. Friday Night: Mainly VFR, with areas MVFR possible. Breezy. Chance SHSN. Saturday: VFR. Windy with gusts to 30 kt. Saturday Night: VFR. Windy with areas gusts to 30 kt. Sunday: Mainly VFR, with local MVFR possible. Slight chance SN. Sunday Night: Mainly VFR, with areas MVFR possible. Chance SN, chance RA, slight chance FZRA. Monday: Mainly VFR, with local MVFR possible. Breezy. Chance RA, chance SN, slight chance FZRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Gale force W-NW winds peaking this afternoon into early this evening with gusts 40-45 kt, then diminishing through tonight. Another pulse of W-NW gusts to 25-30 kt developing Thu afternoon. Rough seas. Vsbys lowering in developing snow late tonight and early Thu over south coastal waters, improving in the afternoon. Outlook /Thursday Night through Monday/... Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of seas approaching 5 ft. Chance of rain showers, chance of snow showers. Saturday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of rain showers, slight chance of snow showers. Saturday Night: Low risk for gale force winds with gusts up to 35 kt. Local rough seas. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night through Monday: Winds less than 25 kt. Chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory until 10 PM EST this evening for MAZ007-019- 022>024. Wind Advisory until 8 PM EST this evening for MAZ002>006- 008>018-020-021-026. RI...Wind Advisory until 10 PM EST this evening for RIZ008. Wind Advisory until 8 PM EST this evening for RIZ001>007. MARINE...Gale Warning until 1 AM EST Thursday for ANZ230>235-237-251. Gale Warning until 9 PM EST this evening for ANZ236. Gale Warning until 3 AM EST Thursday for ANZ250-254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Nocera SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Nocera/Sipprell MARINE...KJC/Sipprell

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