Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 082011 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 311 PM EST Fri Dec 8 2017 .SYNOPSIS...
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- Long term updates only... Mostly cloudy and seasonably chilly weather will continue today, however dry conditions prevail. A winter weather advisory is in effect as a low pressure will bring the potential for moderate snow accumulations Saturday and Saturday night for much of S New England. Still anticipate a mix of rain and snow confined to the Cape and Islands. Cold and breezy Sunday into Monday. Another potential storm system with wintry weather Tuesday into Wednesday. Colder and blustery Thursday into Friday.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 1 PM Update... Mid and high clouds continue to stream across the region as the WAA pattern continues to develop. Seasonably cold airmass over the region however surface temps running a bit colder than normal (only 35-40) given dim sunshine thru mid/high clouds. Previous forecast continues to verify nicely so no major changes planned with this forecast update. Earlier discussion below. Today... Chilly start to the day as upper level shortwave begins to dive into the northern Great Lakes. This will result in a deepening trough in the Southeast with weak ridging over southern New England. Zonal flow in the mid-levels will turn more meridional which will help pull in moisture from the south. This will yield to an increase in cloud cover through the day. High temps will be similar to yesterday or just a few degrees cooler as cloud cover will limit heating during the afternoon. Overall expect mainly dry weather for today. However towards the evening, precip chances will begin to increase as approaching system from the Carolinas will start to push some moisture towards the Cape and the Islands. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Tonight into Saturday night... *** Confidence is increasing in widespread 3-6 inches of snow for much of southern New England on Saturday *** Overview: Anomalous ridging over the West Coast with a broad upper level trough over the eastern two-thirds of the country. Several vort maxes moving through the trough will be the focus for our weather system tonight into Saturday night. The first is the southern stream wave which has produced snow down in south Texas. This wave will move over the Southeast and help develop a low off the coast of the Carolinas. The other piece of energy is still up in northern Canada and will quickly dive southward into the northern Great Lakes. By Saturday, these two systems will phase and deepen the offshore low pass the 40N/70W benchmark before moving towards the Maritimes by Sunday morning. Details: First significant snowfall for the region will occur late tonight and into early Sunday morning. Latest guidance members appear to come more inline than 24 hours ago on thermals and precipitation amounts leading to an increasing confidence on snow potential. Still some timing issues with the Hi-res guidance a bit on the slow side and the EC on the faster side of the envelope. NAM is a bit more robust on the QPF compared to the GFS/EC. Aside from some of these differences continued to trend the forecast towards a blend with more emphasis on the EC and GEFS/EPS. One thing to continue to watch is the axis of heavier QPF. Models are similar to previous run and keeps about an inch of QPF over Nantucket with 0.4 inches across Western MA and over 0.5 inches near the I-95 corridor. Feel that this is a good blend with all the guidance. However, with the northern stream wave getting closer to the region, feel that it will be sampled a bit better and thus cannot rule out a slight eastward or westward jog. Even if there is some wiggle, overall feel that this is a solid advisory event. Snowfall will begin to overspread the region late tonight and quickly take hold of the region Saturday morning. Expect p-type to stay all snow at the onset as temperatures will start out well below freezing. As the coastal low approaches the Benchmark during the day, thermals will become a bit more complicated as a warm layer begins to sneak into 925mb and surface temps warm along the coastline thanks to a developing coastal front. As the low pulls away cold air will wrap behind the system turning any liquid back to frozen by Saturday night. Forecast snowfall amounts is a general 3-6 inches with the potential for higher amounts across the Central Hills of Massachusetts and Rhode Islands. This region may have just enough QPF and higher snow ratios to push them above the 6 inch amounts. Confidence wasn`t high enough to issue a watch, but cannot rule out that this region could be upgraded to a warning. Locations across the Cape, Islands and will result in some mixing as surface temps warm and ocean waters are still in the upper 40s. More of a northerly flow in the later half of the day on Saturday will help keep some locations along the MA east coast as snow. did try to show the tight gradient in the snow grids. Continue to use a wet-bulb trend in the forecast by using high-res guidance to show lower temps within heavier rates. Intensity of the snowfall still a bit in question as stronger omega in snow growth region remains offshore, still some mid-level forcing which could help some places overperform. Overall this is a solid advisory event with a few spots having the potential to overperform. There is still the potential for this system to move farther west or trend more eastward. So please stay tune to the latest forecast. Just a reminder, this is the first snowfall of the season so motorist should use caution if traveling on Saturday. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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*/ Highlights... - Cold and blustery Sunday into Monday, snow showers possible - Potential coastal storm Tuesday into Wednesday, wintry mix - Colder and again blustery Thursday into Friday */ Overview... Cooler, active weather pattern persists. Continually rotating vort- maxes shear S out of the Arctic downwind of strong ridging across W N America, reloading colder air subtly further N/E as the long-wave pattern shifts ever so slightly E. The thermal gradient tightening across the NW Atlantic, generating an environment favorable towards storm development, potential snowy outcomes into the following week, with dependence on strength / amplitude / tilt of mid to upper level features in association with both N/S streams. With any energy that can ride along regions of greater baroclinicity in association with N-stream clippers, how effectively can they latch into S-stream sub- tropical moisture. That, in brevity, with consideration of conveyor belt motions / thermal wind response, spatially and temporally, is the main theme in this long-range forecast. Forecast confidence only as far out as 72-96 hours with respect to operational guidance. Thereafter preference to ensemble means. The associated N-stream clipper low energy well up in the Arctic where it is limited in sampling, we can only discern clues and signals as to potential impacts and outcomes from the breadth of all available forecast guidance, limited in getting into specifics concerning the timing, strength, track of any possible storms given both the spread and inconsistencies within, lending to erroneous results. Patience, simply preparing for the unknown, knowing how to respond when bad weather strikes. Welcome to Winter. UKmet / ECMWF / EPS / GEFS preferred, cold and blustery behind the Saturday storm system. Follow-up energy latching available moisture off the Lakes, snow showers persist prior to high pressure and cold air for the early week period. Then all eyes are on another clipper low contorting the H5 trof, potentially yielding an environment that is favorable for coastal storm development and snowy outcomes over the NE CONUS midweek. Lingering uncertainties as to its morphology, latching into the sub-tropics along a lingering offshore baroclinic zone. Back into the cold, blustery conditions behind, a repeat with snow showers persisting. Further details in the discussion below. */ Discussion... Sunday through Monday... Cold and blustery with scattered snow showers. Behind the departing storm system, strong gradient / isallobaric response. Both ensemble probabilistics and CIPS Analogs indicating the potential of upwards of gale force winds. Potential for headlines on the waters. As to snow showers, greatest threat Monday. Ascent with additional clipper energy with decent moist depth within the column in addition to fetch off the Lakes, can`t rule out isolated impacts with reduced visibility and light accumulations especially over the high terrain and also along the S/SE maritimes as mid-level energy exits. Notably though, forecast guidance PoPs signaling no outcomes. Lean towards slight chance PoPs with greater weighting over S/SE coastal waters. Tuesday through Wednesday... Impressive ensemble mean signals regarding coastal storm development of off S New England. Pivoting impulses through the broader H5 trof rotating H85 temperature anomalies through the Mid-Atlantic and NE CONUS. The baroclinic zone sharpening offshore as mid-level energy is sheared S through the base of the H5 trof, digging cyclonically with more time given the amplified, slower flow, the trof undergoes a negative tilt with H5 heights down -2 to -3 standard deviations. Thermal wind response and conveyor belt motions, a potential latch into the sub-tropics if the N-stream feature(s) doesn`t flatten / push out S-stream energy. Aside, seemingly a Miller-B setup. Clipper low diving S, emerging to the lee of the Appalachians, riding N/E, deepening and retrograding back beneath H5-7 negatively tilting features, the surface low bombs along the coast, a Nor`Easter storm setup sweeping delivering snowy outcomes. But still uncertainty giving the timing and speed of features, also strength and amplitude spatially. Initial cold air damming, question as to thermal profile with precipitation onset / over-running. Then how quickly do features evolve with respect to conveyor belt motions and closing off the H85-5 low, the morphology of the system and how effectively the comma-head wraps around the deepening low. Parent lift / deformation rearward within the dendritic growth zone, depth and quality of moisture mixing back around. Colder, Arctic air will undoubtedly dig into the backside of the system per the gradient and isallobaric wind response. Considerable spread within ensemble members. Certain of a wintry mix but largely unknown are specifics, both at the surface and up above within the mid to upper levels when it comes to strength, amplitude, and timing of features. As alluded to in the OVERVIEW above, simply need to be patient, be prepared, and be ready to respond in the face of adverse weather. Thursday through Friday... Similar to Sunday through Monday. Colder and blustery with scattered snow showers. Colder, Arctic air, reloaded, reinforced, it is likely during this timeframe we`ll be dealing with some of the coldest air of the season and there will likely be greater concern with respect to wind chills.
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&& .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 18z update... Thru 00z...high confidence VFR, dry weather and light winds. After 00z...high confidence. VFR and dry most terminals except Cape Cod and Islands where MVFR conditions overspread this area with a mix of rain and snow transitioning to mostly snow toward daybreak Sat. Light accumulations possible here. Saturday and Saturday night...high confidence on trends but lower and precise details. Trending MVFR in the morning with snow overspread the area, then trending toward IFR in periods of moderate snow during the afternoon. Snow changes to rain during the morning along the south coast including Cape Cod and Islands. The rain-snow then moves inland possibly very close to BOS-PVD but likely just to the southeast of this line. Preliminary Snow Totals are below... KBOS...4-7" KBED...4-7" KPVD...4-7" KORH...4-7" KBDL...3-6" KHYA...1-2" KACK...1" KBOS Terminal...High confidence in TAF. Snow begins approximately 16z-18z Sat, then a steady snow thru at least 00z and possibly until 03z-06z. Rain-snow approaches Logan from south and east but likely remains mostly snow. KBDL Terminal...High confidence in TAF. Outlook /Sunday through Wednesday/... Sunday: VFR. Windy with areas gusts to 30 kt. Slight chance SHSN. Sunday Night: VFR. Breezy. Slight chance SHSN. Monday: VFR. Windy with local gusts to 30 kt. Monday Night: Mainly VFR, with areas IFR possible. Chance SN. Tuesday: MVFR/IFR conditions possible. Windy with areas gusts to 30 kt. Chance RA, chance SN. Tuesday Night: VFR. Windy with gusts to 30 kt. Chance SN. Wednesday: Mainly VFR, with local MVFR possible. Windy with gusts to 30 kt. Slight chance SN.
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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 Saturday Night/...High confidence. Overnight into tomorrow... W winds will increase slightly this evening, with gusts to around25 kt. Seas respond, increasing to 5-6 ft on the ocean waters by early AM. Therefore, will continue small craft advisories through about mid-morning to mid day tomorrow, until seas gradually dissipate. Tomorrow night... Light flow shifts SW. Mainly quiet boating weather except for some SHRA on the SE waters. Outlook /Sunday through Wednesday/... Sunday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Rough seas up to 11 ft. Chance of rain showers. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of snow showers. Monday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain, chance of snow. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of rain. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain, chance of snow. Wednesday: Strong winds with gusts up to 30 kt. Areas of rough seas. Chance of snow.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory from 4 AM Saturday to 7 AM EST Sunday for CTZ002>004. MA...Winter Weather Advisory from 4 AM Saturday to 7 AM EST Sunday for MAZ002>021-026. RI...Winter Weather Advisory from 4 AM Saturday to 7 AM EST Sunday for RIZ001>007. MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ254>256.
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&& $$ SYNOPSIS...Frank/Dunten NEAR TERM...Nocera/Dunten SHORT TERM...Dunten LONG TERM...Sipprell AVIATION...Nocera/Dunten/Sipprell MARINE...Dunten/Sipprell

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