Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152042 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 342 PM EST Mon Jan 15 2018 .SYNOPSIS... Another very cold day is on tap with some accumulating snow across portions of eastern MA into this evening. Low pressure drives east from the Great Lakes Tuesday, then redevelops off the Mid Atlantic coast. This will bring the potential for several inches of snow to portions of the region mainly late Tuesday into Wednesday afternoon...but precipitation type is uncertain southeast of the Boston to Providence corridor. Dry weather likely follows Thursday into Friday with unseasonably mild temperatures expected for the weekend. && .NEAR TERM /THROUGH TUESDAY/... * Mesoscale snow event across portions of eastern MA into the evening * Highest snowfall totals expected south of Boston in Plymouth County 10 AM Update... Trends in the forecast so far have been on tracking. Northeasterly flow has trigger snow flurries all the way back into Connecticut thanks to some weak upslope from the Central Hills. You can see the cloud cover from the snow showers on the latest GOES-16 satellite meso sector 1. Therefore continued to mention some flurries across RI and Connecticut over the next several hours. Otherwise the focus is on the ongoing snow showers late this morning into the evening. Winds across essex county and even at BOS has begun to switch to a more northerly direction. This will push the coastal front and convergence region south of the city closer to the south shore and into Plymouth county. GHG is still out of the northeast where as OWD is more northerly. Latest TBOS trends continue to show reflectivity weakening over the past 30 minutes. This may be in part to the lost of the stout convergence region. However guidance does indicate that snow showers will pick back up with adv levels snow during the evening commute. No changes to the headlines at this time. Previous discussion follows... Very strong 1048 mb high pressure system across Quebec will combined with a weak wave of low pressure passing well southeast of the Benchmark. This was already resulting in winds shifting to the northeast along the coast and low clouds advancing westward. The main issue today will be a mesoscale snow event that will impact portions of eastern MA. Light snow showers and flurries were already developing across eastern MA and even into RI early this morning. We expect this activity to increase near daybreak and especially later this morning as the 925 easterly LLJ increase to between 25 and 30 knots. The other main component will be a coastal front that is currently setting up over Plymouth County. Temps to the northwest of this boundary will be in the upper teens/lower 20s, while areas southeast of the boundary will see readings in the upper 20s to lower 30s and some locales across the Cape/ACK will get above freezing. Lastly, the land/sea interface will also aid in forcing for this mesoscale snow event. The coastal front across Plymouth county along with a 25 to 30 knot easterly LLJ will result in the mesoscale snow event mainly across portions of eastern MA...but some lighter snow showers/flurries will impact RI. We have to rely on the high resolution models and climatology on these type of events as heavier snow bands tend to be quite narrow and localized. Based on the above factors...we favor Boston/s south shore into Plymouth county to see the heaviest amounts of snow but some of this may also clip Cape Ann. The heavily traveled Route 3 connecting Boston to the south shore will be impacted by these heavier bands of snow. We think 2 to 4 inches of snow seems reasonable in this region...but can not rule out very localized 5-6 inch amounts. The snow will likely be very banded with amounts varying considerably in a matter miles. Will continue the winter weather advisory for Plymouth county and eastern Norfolk county. As we get outside of these favored bands...generally thinking an inch or two of snow from Boston to Taunton. It is important to remember though that the mesoscale models being off by just 10 miles will make the difference of where the localized heavier snow develops. Meanwhile...just a bit northwest of the Boston to Providence corridor nothing more than some flurries are expected. We still do expect low clouds to push towards the CT River Valley given the onshore flow. High temps will mainly be in the lower to middle 20s. Ocean effect snow showers will continue along the eastern MA coast into part of the evening. However...this snow should be coming to an end after midnight or so as the easterly LLJ weakens and coastal front shifts offshore. Otherwise...dry weather expected elsewhere for much of the night. We may see a bit of light snow develop across western MA towards 12z as weak warm advection commences aloft. Lots of clouds tonight will probably keep low temperatures in the teens...although a bit milder along the southeast New England coast. PLEASE NOTE: Because of the more widespread snowfall potential Tue into Wed (See Long Term below), our snowfall maps on the web (including probabilities) will focus on that since a much larger area will be impacted. We will post the expected snowfall map for today into this evening on social media. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... Tuesday night through Wednesday night... Upper trough over the eastern USA with jet core 130-145 knots. The trough axis maintains a progressive orientation through midweek. The right entrance region of the jet moves near the Carolina coast Tuesday evening, which will generate cyclogenesis along the coast. The resulting coastal low will then move up the coast overnight and pass Cape Cod and Islands Wednesday afternoon/evening. Cross sections show some initial elevated lift early Tuesday night, but the best and deepest lift over Southern New England runs from after midnight until midday Wednesday. This will be the best time for accumulating snow. There are differences among the models regarding the track of the storm center. The NAM and GFS are slower and pass about 70 miles inside 40N/70W, while the GGEM and ECMWF are about 6 hours faster and pass directly over Cape Cod. We favored the offshore track. QPF continues to be expected in the 0.40 to 0.75 inch range, with snow to liquid ratios of between 12 and 15 to 1 in the interior and 10 to 12 to 1 in much of RI and Southeast MA. This supports 6 inches or higher over much of the region, with diminishing amounts south of a Boston-Hartford line due to temperatures near freezing along with a mix or change to rain. This brings the heaviest snow over interior Southern New England, affecting an area broader than the current watch. We have expanded the existing Winter Storm Watch to include Northern CT, NW RI, and Norfolk/Suffolk Counties in MA. The upper trough sweeps overhead Wednesday night. Expect diminishing snow in advance of this Wednesday afternoon and night. Thursday... Model trends are sufficiently progressive to push any new development well offshore and away from New England. The outlier solution comes from the GGEM, which maintains a closed upper low over the VA-Carolina coast Wednesday night/Thursday morning with surface low pressure just outside the benchmark and an upper flow guiding its moisture toward Cape Cod. We are discounting the GGEM solution and maintaining a dry forecast for Thursday with partly to mostly sunny skies and northwest wind. Temps aloft equiv to 850 temps of -15 to -17, so surface max temps should be roughly 25 to 30. Thursday night dewpoints in the teens and west wind 5 to 10 knots suggest min temps 15 to 25. && .LONG TERM /FRIDAY THROUGH MONDAY/... Overview and model preferences... The arctic PV lobe that we have been discussing for some time now, after settling near the Great Lakes through Mon night is expected to gradually open, as it responds to influence of the remnants of an upstream Rex Block. Even though the parent wave itself gradually dissipates (and the clipper associated with it), the breakdown of the cutoff into an open trof actually shifts the mean jet such that its poleward exit region orients offshore, developing a secondary weak low pres. It is this that will generate an enhanced precip shield within modest low-mid lvl f-gen, leading to widespread snowfall later Tue into Wed. However, this breakdown will also allow the remnants of the block to influence sensible wx by late week, leading to rising heights and moderating temperatures into the weekend. Regarding model preferences, noting enough agreement between GFS/ECMWF at the synoptic lvl to introduce more weight to these with respect to the ensembles. While there are still thermal differences to work out, overall lift and QPF forecast are close enough to use the full consensus as a baseline. Details... Fri through the weekend.. The remnants of the ridge to the W gradually build across New England along with high pres oriented to the S. This suggests continued moderation of the airmass and daytime snow melt as H92 temps warm from an average around -5C Fri to +6C by Sun. This would favor temps mainly above normal, and likely into the 50s to near 60 in spots by Sun. Overall dry wx is likely given soundings show influence of high pres in the form of a broad subsidence inversion. Early next week... Although early indications suggest another trof impacting the region, currently cutoff in the Pacific vortex, and that the moderated airmass suggests mainly a warm setup, there is enough uncertainty that any POPs will remain low with this update. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Today...High confidence overall. Areas of MVFR CIGS will spread mainly south and east of a line from PSF-BVY, or most of southern New England. VFR CIGS N. Otherwise, -SN will spill across mainly SE MA and RI with VSBY bouncing between MVFR and IFR mainly at terminals along and southeast of the Boston to Providence corridor. The snow dissipates late this evening. Otherwise winds NNE, 10-15 kt with gusts to around 20-25 kt at times. Tonight...Moderate to high confidence in TAF. Snow showers across eastern MA should wind down by midnight or so. Otherwise...low end VFR to MVFR Cigs expected in a broken deck of low clouds. Tuesday night-Wednesday... Mainly IFR/LIFR cigs/vsbys in snow, with areas of heavy snow north of a BOS-HFD line. Snow may mix with or change to rain south of a Marshfield-Providence-Westerly line. Conditions improve to VFR in CT and Western Mass Wednesday afternoon...and in RI and Eastern/Central Mass Wednesday night. Wind gusts 25 to 30 knots around Nantucket and parts of Cape Cod. Thursday...VFR. Northwest winds gust to 25 knots Cape Cod and Islands, with gusts to 20 knots elsewhere. KBOS Terminal...Moderate confidence in TAF. Heaviest snow likely remains just south of the terminal...but probably will see conditions vary from MVFR to IFR in snow showers much of the day. An inch or two of snow possible at the terminal today. KBDL Terminal...High confidence in TAF. Outlook /Friday through Saturday/... Friday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... High confidence. Today...High confidence. Increasing gradient between strong high pressure across Quebec and weak wave of low pressure passing well southeast of the Benchmark will result in NE wind gusts of 20 to 30 knots. SCA headlines posted for all waters except Boston Harbor and Narragansett Bay. Ocean enhanced snow will reduce visibility at times, particularly near the eastern MA coast. Tonight...High confidence. Northeast 25 knot wind gusts will diminish from west to east overnight as the pressure gradient weakens. Left over swell though will keep seas above small craft thresholds through daybreak across all outer waters. Tuesday night...Easterly winds with gusts to 25 knots overnight. Seas 5 feet on the outer waters. Mixed snow and rain developing. Visibility 4 to 6 nm. Small Craft Advisory may be needed. Wednesday...Easterly winds with gusts 20 to 30 knots. A few gusts to 35 knots possible southeast of Nantucket. Seas 5 to 7 feet on the outer waters and possibly on the RI waters. Rain and snow will diminish in the afternoon. Visibility 2 to 4 miles. Small Craft Advisory may be needed. Low chance of a gale warning southeast of Nantucket. Wednesday night...Winds turning from northwest at 20 to 25 knots. Seas remain at 5 to 7 feet on the outer waters. Snow tapers off at this time. Small Craft Advisory may be needed. Thursday...Northwest winds gusting to 25 knots. Seas remain 5 to 7 feet on the outer waters. Small Craft Advisory may be needed. Outlook /Friday through Saturday/... Friday: Winds less than 25 kt. Seas up to 5 ft. && .HYDROLOGY... Minor flooding continues along Taunton and Pawcatuck where Flood Warnings remain posted. Flood warnings were also issued last evening for the CT River at Hartford and Middle Haddam...where very minor flooding is occurring with a lot of ice causing river fluctuations. Colder weather will persist through midweek which will limit additional runoff, although we should see some building of snow pack from bands of ocean effect snow in eastern MA Monday and for much of region Tuesday into Wednesday. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service /AHPS/ graphs on our website. && .EQUIPMENT... The Taunton, MA WSR-88D (KBOX) has been returned to service. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Weather Advisory until 10 PM EST this evening for MAZ016-018-019. Winter Storm Watch from Tuesday afternoon through Wednesday afternoon for MAZ005>007-014-026. Winter Storm Watch from Tuesday afternoon through Wednesday afternoon for MAZ002>004-008>012. RI...None. MARINE...Small Craft Advisory until 1 AM EST Tuesday for ANZ231>235-237. Small Craft Advisory until 7 AM EST Tuesday for ANZ250-251- 254>256. && $$ SYNOPSIS...WTB/Frank/Doody NEAR TERM...Frank/Dunten SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Frank/Doody MARINE...WTB/Frank/Doody HYDROLOGY... EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.