Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 171359 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 959 AM EDT Fri Mar 17 2017 .SYNOPSIS... High pressure approaches from the west today providing dry, quiet weather conditions. It will remain colder than normal but less wind than yesterday will make it feel more pleasant. A clipper low tracks south of New England this weekend bringing the potential for accumulating snow Saturday night into Sunday, with the highest risk across RI and southeast MA including Cape Cod and the Islands. Dry weather with seasonable temperatures temperatures return later Monday, as high pressure builds into our region. Milder weather Tuesday ahead of an arctic front with temperatures possibly into the 50s! However behind the front a frigid airmass with near record cold overspreads the area middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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1000 AM Update... Overall the forecast remains on track for today. Clearing skies and dry weather will prevail. Only change was to increase the wind gusts for late this morning into the afternoon hours. Model soundings are indicating mixing up to 900 mb bringing down gusts closer to 20-25 mph. Otherwise no major changes at this time. ================================================================= Quiet weather. High pressure, dry conditions. Eroding low to mid layer cloud decks, lingering longest along the E coast of MA beneath cyclonic, moist flow with some ocean effect influence. Moderation of the airmass aloft at the top of the mixed layer, around -8C at H9. Given scattered cloud decks eroding through the day with the mid-March sun, and temperatures yesterday warming a degree or two above guidance, leaning mild with highs topping close to 40 degrees for S/SE CT, RI, and MA, around the mid 30s elsewhere. Coldest conditions N/W with deepest snowpack.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight... Cold, dry, quiet weather. High pressure with prevailing subsidence. Light and variable winds. Opportunistic time for radiational cooling. Possible mid to upper cloud decks into morning but not expecting much limitation. Expect mostly clear conditions. Lows getting down into the single digits for the N/W with deeper snowpack, teens to low 20s towards the coast. Saturday... Increasing clouds along with chances of light snow. Upstream ridge enhancement promotes equatorward flow downstream thereby promoting cyclonic enhancement and subtle cutoff of Pacific-origin energy into a H5 low into the Mid-Atlantic. Increasing pressure falls emerge off the Delmarva Peninsula as low center begins to undergo maturation. SW isentropic upslope strengthens in response along and ahead of the surface warm front developing roughly W-E off the S NJ coast, with an inverted trough extending back towards Lake Erie from the surface low center emerging off the Delmarva. This against low level cold air damming associated with the retreating high pressure N/E over the NE. Clouds lowering and thickening towards late with strengthening easterlies in response to pressure falls. Temperatures remain tricky. The airmass aloft moderates and there is the possibility that temperatures warm into the low 40s given east winds and potential sunshine before clouds thicken. Aside, given the magnitude of column moistening with the overruning setup, could see light rain / snow develop over S/W forecast areas, CT especially. Have chance PoPs over S/W portions of MA, RI and CT, with likelies into Hartford County by evening. If any accumulations, mainly of a coating with a greater likelihood of sticking onto surfaces at or below freezing. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Accumulating snow remains possible Sat night/Sunday with highest risk across RI/southeast MA esp Cape Cod & Islands * Temperatures more seasonable early next week * Arctic blast with near record cold returns middle of next week Saturday night and Sunday... Complex forecast as clipper low over the eastern Great Lakes Saturday redevelops south of the 40N/70W benchmark Saturday night. 00z models in very good agreement thru 48hrs but then diverge on how far south mid level low digs off the mid Atlc coast and also differ on amplitude and how progressive system will be thereafter. To further complicate things models differ on how quickly mid level low captures secondary surface circulation and how far north this surface low will track. Thus lots of uncertainty. The 00z NAM/UKMET/EC are most robust with modest qpf into south coastal RI and MA with heaviest qpf over Cape Cod and Islands. NAM over an inch of qpf Cape Cod/Islands with UKMET up to 0.50 inch, GFS 0.25 to 0.50 inches over this region and EC up to 1.0 inches over Cape Cod and Nantucket. However EC warms the column with time as the low occludes with snow mixing with or changing to rain over the Cape. The combination of a tight qpf gradient across the area along with potential ptype issues later Sun/Sun evening, results in big forecast bust potential. Basically comes down to how far north and west mid level low be able to wrap warm conveyor belt moisture back into the forecast area, including duration. As for ensembles, 00z GEFS has pops greater than 50% of 0.50 inches of qpf confined to Nantucket and Cape Cod. 00z EC ensembles have mean qpf ranging from 0.50 to 0.80 inches focused over southeast MA including Cape Cod and the islands. PTYPE...column plenty cold initially to support all snow across the entire region. Forecast becomes more complicated later Sun/Sun evening as offshore low becomes occluded and may eventually advect warmer air into southeast MA. Other forecast issue becomes precip/snow intensity. Now that it is mid March with increasing sun angle, if snow intensity is too light during the daylight hours Sunday snow will struggle to accumulate on paved surfaces. This would limit storm impacts. Other potential impacts from this storm would be wind. Noticeable low level jet forms with secondary cyclogenesis Sunday/Sunday night. Good model agreement low level (925 mb) jet 0f 50-60 kt moves across Nantucket and possibly over Marthas Vineyard and Cape Cod pending exact storm track. Thus could see NE winds gusting up to 50 mph or so. Will likely need a wind advisory for this region as the event nears. Monday and Tuesday... Monday`s forecast hinges on how progressive or slow moving ocean storm becomes. New 00z ECMWF has become less progressive with clouds, precip and gusty NE winds lingering across southeast MA much of the day Monday. Forecast a bit more certain farther inland where dry weather should prevail and temps near seasonable. Tuesday could be the pick of next week as forecast area is in between systems, departing ocean storm to the east and arctic front approaching from the northwest. Could see highs in the 50s! Given this blended the warmer EC mos to derive max temps Tue. Wednesday and Thursday... Arctic airmass overspreads the region with GFS and EC ensembles temp anomalies at 925 mb and 850 mb (-17C!) as low as -2 standard deviations colder than climo this period! Deterministic guidance even colder. This supports highs only in the upper 20s to lower 30s with single digits and teens at night. After checking the records these temp values will be near record cold for daytime highs and overnight lows! Thus sided with the coldest guidance for this period given ensemble && .AVIATION /14Z FRIDAY THROUGH TUESDAY/...
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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... VFR. NW winds with a few gusts near 20 kts during the afternoon. SCT- BKN low- end VFR cigs over E MA. Low risk MVFR. Tonight... VFR. Light and VRB winds. Low risk MVFR lingers across the Outer Cape. Otherwise possible mid to upper level cigs from the S/W late. Saturday... Increasing cigs lowering to MVFR with increasing chances of a RA/SN mix over S/W portions of CT, MA and RI. If any SN accumulation, amounts on the order of a coating. Increasing E winds. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Saturday Night through Tuesday/... Saturday night and Sunday... MVFR and/or IFR in snow along the south coast of RI and MA including Cape Cod and islands. VFR likely elsewhere. Strong NE winds develop along the coast especially Cape Cod and the islands. Monday... MVFR/IFR possible Cape Cod and islands especially early. Elsewhere VFR likely and dry. Tuesday... VFR likely along with a modest WNW wind.
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&& .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. High pressure settles across the waters into tonight. NW winds gusting up to 20 kts today diminish, becoming light while turning out of the E. Expect the conclusion of small craft advisories late today. Will see increasing easterlies along increasing wave action of the S waters as an area of low pressure emerges off the Delmarva Peninsula and begins to strengthen. Small craft headlines will likely be needed. Outlook /Saturday Night through Tuesday/... Saturday night... Clipper low dives south of New England with winds becoming ENE. Snow ovespreads the waters. Sunday/Sunday night... NE gales likely as low pres intensifies into a gale center southeast of the benchmark. Snow and rain limit vsby. Monday... NE winds may remain strong over the eastern waters with a diminishing trend western waters. Vsby may lower in rain and snow eastern waters early. Tuesday... Arctic front likely holds off until late in the day or at night. Thus pleasant weather at least into the afternoon. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ250. Small Craft Advisory until 5 PM EDT this afternoon for ANZ254. Small Craft Advisory until 8 PM EDT this evening for ANZ255- 256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Dunten/Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell

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