Area Forecast Discussion
Issued by NWS Boston, MA

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024 FXUS61 KBOX 151200 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 700 AM 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 along and southeast of the Boston to Providence corridor. Dry weather likely follows Thursday into Friday with unseasonably mild temperatures expected for the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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* Mesoscale snow event across portions of eastern MA into the evening * Highest snowfall totals expected south of Boston in Plymouth County 7AM update... Did not want to make too many wholesale changes to the forecast this morning, and really did not need to. The biggest change will be to expand the POPs slightly as the TDWR in Weymouth is showing a decently widespread band of -SN and --SN from Essex CO down to near the canal. Not that this will ultimately change expected snowfall amounts, but it will expand areas that may see a coating and some flurries. Otherwise, will be adjusting temps based on current trends and lower lvl cloud cover this morning. 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. 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.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... 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. && .LONG TERM /TUESDAY THROUGH SUNDAY/... *** Risk For Some Warning Level Snow Late Tue into Wed *** 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... Tue into Wed... As mentioned above, looking at the synoptic scale, there is now generally decent agreement that as the initial clipper attendant to the opening longwave trof upstream weakens, a secondary low offshore of the Delmarva, will develop and track near or along the 40/70 benchmark. However, several members of both the ECENS and GEFS, as well as the deterministic ECMWF itself is well inside (00Z ECMWF actually tracks the low over the Cape). This will impact coastal snow totals, as it introduces air at H92 near or above +2C, so portions from Boston to Providence and points SE remain the area with most uncertainty given the introduction of warm air. Elsewhere, soundings are cold enough for all SN. SLRs are unlikely to be maximized, as the best f-gen forcing/weak deformation NW of the low will be below the DGZ, even though it fully moistens Tue night into early Wed. Agree with previous forecaster that better than 10 to 1, maybe in the 12 to 15 range is probably a good starting point right now given weak lift where it should be maximized. The PWATs, given the influence of Atlantic moisture with the secondary low Tue night into Wed, now average closer to 0.50 inches, which is close to 1 std deviation above climatological normals. Final QPF totals now range from about 0.40 to as much as 0.75 inches with this slight tick up in moisture. Using the applied ratios, this is advisory territory, with the potential for isolated low end warnings, with widespread 4-6 inches and a few spots higher. Timing of earliest snowfall would be Tue across the W with the initial clipper, with the heaviest (although limited to 1in/hr or less) mainly Tue night through Wed morning, including the Wed morning commute. Given the upward trends in both the strength of the secondary low and moisture. Stay tuned as this will evolve over the next 24-36 hours. Ensemble probs, WPC and CIPS analogs are all suggesting decent probs of approaching the warning threshold. Given all of these factors will issue a Winter Storm Watch for the interior or MA, where confidence is highest in this occurrence. Regarding temps, expecting mainly 20s and low 30s for the interior, while the introduction of warm air around or above H92 could allow temps above freezing SE of the BOS-PVD corridor. Thu... With the trend of opening the wave more quickly now than runs earlier this past week, the pattern is overall more progressive especially as the upstream Rex breaks down. This supports secondary coastal development further offshore for Thu, something even the ECMWF, which was the most amplified now falls in line with, including the majority of its ensembles. Therefore, will continue to trend grids toward a drier solution for Thu. However, the influence of of the moderating arctic air is likely to be felt. H92 temps drop following the exit of low pres, reaching about -10C by early Thu morning. This should keep overnight lows following the storm and highs on Thu near or just below normal. Mainly in the teens at night, to upper 20s and low 30s by day. 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 /11Z MONDAY THROUGH FRIDAY/...
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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 Tonight/... 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. 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 /Tuesday through Friday/... Monday: Mainly VFR, with areas MVFR possible. Breezy. SN. Monday Night: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHSN. Tuesday: MVFR/IFR conditions possible. Slight chance SN. Tuesday Night: Mainly IFR, with areas MVFR possible. Breezy. SN. Wednesday: Mainly IFR, with areas MVFR possible. Windy with local gusts to 30 kt. SN. Wednesday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SHSN. Thursday through Friday: VFR. Breezy.
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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 Tonight/... 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. Outlook /Tuesday through Friday/... Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft. Snow. Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of snow, chance of snow showers. Tuesday: Winds less than 25 kt. Areas of rough seas. Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Snow likely, rain likely. Visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. Areas of rough seas. Rain, snow. Visibility 1 to 3 nm. Wednesday Night: Winds less than 25 kt. Areas of rough seas. Chance of snow showers. Visibility 1 to 3 nm. Thursday through Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Friday: Winds less than 25 kt. Seas up to 5 ft.
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&& .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) is out of service due to a part that needs replacing. We hope to have the radar back in service sometime today. && .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...Frank/Doody NEAR TERM...Frank/Doody SHORT TERM...Frank LONG TERM...Doody AVIATION...Frank/Doody MARINE...Frank/Doody HYDROLOGY... EQUIPMENT...

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