Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 090320 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1020 PM EST Fri Dec 8 2017 .SYNOPSIS... Low pressure in the Gulf of Mexico will track NE to a position southeast of Nantucket Saturday evening then into the Maritimes Sunday. This system will bring a moderate snowfall to much of the region with 4-7 inches along and northeast of I-95. Less snow southeast of I-95 as snow mixes and/or changes to rain. Mainly dry but cold and blustery weather follows Sun and Mon. Another coastal storm may bring more rain and/or accumulating snow Tuesday into early Wednesday. This storm will be followed by unseasonably cold and windy weather late Wednesday into Thursday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 1030 PM Update... Overall trend in the forecast remains on track for this evening. Potent northern stream shortwave is beginning to dig into the northern Great Lakes as seen in latest water vapor imagery. Across the southeast and up towards the Mid-Atlantic, moisture plume riding along a 700 mb front will continue to stream north and eastward overnight. Dewpoints at the surface are already slowly falling an thus overnight temps will begin to dip as well esp in the interior. However large cloud shield will keep temps from truly radiating out. As the northern stream digs, moisture southeast of the region will begin to back into southern New England being the focus for precip early tomorrow morning. Aside from some showers along the Nantucket expect a dry forecast for much of the night. Have noticed some trends in speeding up precip for the immediate south coast, so will continue to monitor. Otherwise warnings and advisories remain the same. Previous Discussion... Mid level trough amplification occurs overnight as northern and southern stream energy diving into the OH and MS/TN valleys back the mid level flow along the eastern seaboard. This captures deep layer moisture offshore and pivots onto the south coast as the night progresses. All Hi Res guidance brings qpf onto Nantucket 7 pm to 10 pm then 1 am to 7 am across Marthas Vineyard and immediate south coast of RI/MA including Cape Cod. For Nantucket, model soundings suggest precip begins as rain but then changes over to snow briefly toward midnight before changing back to rain. As for BID/MVY, Cape Cod and immediate south coast of RI/MA precip should begin as light rain and then changeover to snow toward morning as precip intensity increases and wet bulb temps are reached thru the column. Minor accumulations by daybreak Sat. As precip intensity increases late and especially toward daybreak wetbulb temps were utilized to determine ptype. Elsewhere...dry weather tonight and seasonably cold with lows in the 20s, around 30 in Boston and 30-35 Cape Cod and the Islands. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... *** Winter Storm Warning Issued for Eastern CT/northwest RI and much of central and northeastern MA including city of Boston *** *** Widespread 4-7 inch snowfall for much of southern New England Saturday & Saturday evening *** Overview: 12z models continued the theme of a more amplified northern stream short wave diving southward, backing the mid level flow sufficiently along the eastern seaboard to capture southern stream system in the Gulf of Mexico and track an amplifying surface wave over the 40N/70W benchmark Sat evening. This has resulted in 12z models trending a bit farther westward with heavy qpf. Steady snow tapers off Sat evening but developing comma-head may result in light snow persisting much of Sat night. Details: QPF... All global guidance including 12z UKMET/EC/GFS and NAM have trended westward with 0.50 inch qpf contour now into CT River Valley of western CT/MA with up to 0.7 inches into RI and eastern MA. ARW and NMM are less but given qpf falls beyond 24 hrs mesoscale guid skill typically diminishes after 24 hrs and especially after 36 hrs. Also given this is a southern stream system with origins in the Gulf of Mexico prefer the trends from the Global guidance. Ptype... Initially airmass over southern New England is cold enough to support all snow however as the low approaches light winds become NNE and this will change snow to rain across Cape Cod and the Islands Saturday morning. Farther inland have to watch a warm nose at 950 mb on model soundings that will likely push the rain-snow into southeast MA and coastal RI. Some uncertainty how far inland this boundary traverses but could enter the I-95 corridor of RI and southeast MA, thus getting close to Providence and Boston especially the southern suburbs of Boston. Not expecting heavy snow intensity to overcome this so followed model thermal fields and soundings closely to derive ptype. Thinking the city of Boston to the north shore and much of Cape Ann remain mostly snow given NNE traj and pres falls perhaps developing a weak coastal front to keep rain-snow line just south and east to the city. Will have to watch this closely with each model run. Snow Accumulations... Given 12z models trended a bit heavier with qpf new snow totals are an inch or two higher from previous forecast. This yields a stripe of 4-7 inches of snow across eastern CT, northwest RI into central and northeast MA including the city of Boston. Elsewhere 3-6" with an inch or two across Cape Cod and the islands given a shorter duration of snow before a changeover to rain. Think model qpf is reasonable and not expecting this event to overperform given only modest lift (10 ubar/s) in the snow growth region yielding snow to liquid ratios (SLR) of 10-1 to 12-1. In addition tomorrow`s snow event will feature multiple mesoscale snow bands, first across southeast MA/RI in the morning and then pivoting into the interior during the afternoon. The bands don`t appear to be collocated with much instability or pivot over the same area (very transitory). So there could be a few hours where snowfall rates may approach an inch per hour for a 1-3 hr period, but overall looking at a period of mainly light to moderate snow tomorrow. So in a nutshell not expecting this event to overperform model qpf by much if at all. Regarding ground temps, have had several nights now with subfreezing temps and today highs stayed mainly in the 30s. Temps tonight dip into the 20s for much of the region. So the ground will be cold enough for snow to accumulate tomorrow and especially now that we are in Dec sun angle is very low. If this was March there would be some concern on less accumulation on paved surfaces. Headlines... Given new forecast snow totals are now in the 4-7" range have decided to upgrade the Winter Weather Advisory to a Winter Storm Warning where forecast confidence is highest. This includes eastern CT/northwest RI into central and northeast MA including the city of Boston. Next forecast shift may need to expand the warning southeast deeper into the I-95 corridor of RI and southeast MA if 00z runs trend colder and/or heavier qpf. && .LONG TERM /SUNDAY THROUGH FRIDAY/... */ 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. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Tonight... VFR and dry most terminals except Cape Cod and Islands where MVFR conditions overspread this area with a mix of RA and SN transitioning to mostly SN toward daybreak Saturday. Light accumulations possible on grassy surfaces but expect runways to remain mostly wet. Saturday and Saturday night...high confidence on trends but lower and precise details. Trending MVFR in the morning with -SN overspread the area, then trending toward IFR in periods of SN during the afternoon. SN to RA during the morning along the S coast including Cape Cod and Islands. The RA/SN then moves inland possibly very close to BOS- PVD but likely just to the SE of this line. Thinking runways are going to remain mostly wet along the coast, with temperatures fluttering in and around freezing, rain mixing in. Although, for those along the coastal-N, towards later after- noon into evening, expecting snow accumulation up through around midnight when snow tapers off. Preliminary Forecast Terminal Snow Totals... 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. Roughly 16z-18z Saturday -SN begins, then SN thru at least 00z and possibly until 03z-06z. SNRA approaches Logan from S/E but likely remains mostly SN. Runways may remain wet through early afternoon, accumulating later into evening with forecast SN that tapers off overnight. KBDL Terminal...High confidence in TAF. Roughly 15-17z Saturday -SN begins, becoming SN around 18z. With temperatures forecast in the upper 20s, no issue with snow accum on all surfaces. Snow tapering during the late evening into the overnight hours. 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. && .MARINE... 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. Tonight... Weak ridge of high pres moves over the coastal waters so light winds tonight. Light rain and snow enter the southern RI/MA waters tonight lowering vsby. Saturday... Low pres off the NC coast in the morning tracks over or near 40N/70W by 7 pm. NE winds increase to 20-25 kt by days end. Rain and snow reduce vsby. Saturday night... Low pres near the 40N/70W benchmark races northeast into the Maritimes. NE winds 20-25 kt early becoming NW by daybreak Sun morning. Rain and snow change over to all snow then ending overnight. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory from 4 AM Saturday to 7 AM EST Sunday for CTZ002-003. Winter Storm Warning from 4 AM Saturday to 7 AM EST Sunday for CTZ004. MA...Winter Weather Advisory from 4 AM Saturday to 7 AM EST Sunday for MAZ002-003-008>011-016>021. Winter Storm Warning from 4 AM Saturday to 7 AM EST Sunday for MAZ004>007-012>015-026. RI...Winter Weather Advisory from 4 AM Saturday to 7 AM EST Sunday for RIZ002-004>007. Winter Storm Warning from 4 AM Saturday to 7 AM EST Sunday for RIZ001-003. MARINE...None. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera/Dunten SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.