Area Forecast Discussion
Issued by NWS Boston, MA

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013 FXUS61 KBOX 172002 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 302 PM EST Tue Jan 17 2017 .SYNOPSIS... A mixed bag of wintry weather will impact much of S New England from late this afternoon into Wednesday morning. Could see leftover spotty showers lingering Wednesday. High pressure builds over the region for Thursday through the weekend. Much of the period looks dry and seasonably mild. Low pressure developing off the Carolinas Friday could bring some light rain or snow Saturday, but at this time most of that should remain offshore. A more substantial weather system may develop over the Plains this weekend, and has a chance of bringing precipitation to our area Monday. && .NEAR TERM /UNTIL 7 AM Wednesday/... 300 PM Update... First short wave trough moving across the eastern Great Lakes early this afternoon with parent surface low of 1008 mb over Lake Erie. Warm advection precip associated with this feature was across much of NY state into northern NJ/NYC and Long Island. Latest meso analysis has pressure falls up to 4 mb in the past 3 hrs over the DC area. This will support secondary low pressure developing off DE/NJ coast later this evening and overnight. This combined with 1030+ mb high pressure over eastern Quebec will help advect dry/cold air into southern New England. Precipitation... Leading edge of precip continues to erode as it encounters very dry air across the region. This is apparent on the 18z observed sounding from ALY which reveals a very dry layer below 850 mb with a -20C dew pt at 925 mb! This yields subzero wet bulb temps thru the column except around zero just above 850 mb. This is a snow/sleet sounding with lots of evaporative cooling potential given magnitude of low level dry air. Timing... Onset of precip looks to be 3pm-5pm across CT and western-central MA...then moving east into RI and eastern MA 6-8 pm but possibly not until 9 pm for Cape Cod and the Islands given stubborn dry air below 850 mb. Impacts... Models in good agreement on burst of moderate to heavy precipitation around 22Z-02Z. Good snow growth signature at this time with strong lift in the DGZ. Where precip remains all snow could see snowfall rates of up to an inch per hour. However mid level warming as seen on the observed 18z ALY sounding just about 850 mb supports a mix of sleet and rain south of the MA Pike into northern CT and eventually freezing rain once surface wet bulb temps are realized. Eastward into RI and southeast MA probably a period of sleet given mid level warming. Farther northward into eastern MA including Boston a mix of rain/snow and sleet is likely with mainly snow north of the MA Pike and west of I495. This is where the worst travel conditions will be experienced. The heaviest precip will be this evening with somewhat lighter intensity overnight. Headlines... Not much change from previous forecast guidance as well as latest trends from radar and satellite imagery. Thus previous headlines and configuration of advisory still looks valid. Thus will continue will current headlines. Snow Potential... Looks like a swath of 2-4" north of the MA Pike and west of I495 with possibly up to 5 or 6" across the higher terrain in this region...northern Worcester county and northwest Franklin. Farther south an inch or two is possible to the MA/CT border and eastward into the I495 corridor. Elsewhere...less than an inch of snow and sleet across northern CT/RI into Metro-West Boston. Southeast MA and southern RI mainly rain with a bit of sleet this evening. Freezing Rain Potential... Mid level warming combined with shallow cold air will setup across the CT River Valley especially high terrain of Hartford county into southwest MA. A trace to a tenth of an inch of ice is possible with a tenth to a 0.25" possible across the high terrain of this region. This may result in isolated power outages. && .SHORT TERM /7 AM WEDNESDAY THROUGH 6 PM WEDNESDAY/... 3 pm Update... Nasty day for much of the region with a a cold wind swept rain during the morning for much of eastern MA and RI as secondary 1009 mb low well established near the 40N/70W benchmark. This combined with 1028 mb high over eastern Quebec will provide a gusty northeast winds into the region. Still could have snow falling across northern MA including interior northeast MA away from the coastline such as Lowell and Lawrence MA to the NH line. Best lift moves offshore after 12z so lighter precip the remainder of the day with any snow confined to the higher terrain. By the late day commute just looking at light spotty nuisance type precip. Chilly day with highs 35 to 40 but feeling colder along the coast given the gusty onshore winds. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Big Picture... Flat upper flow with embedded shortwaves midweek evolves by late week into a trough west/ridge east pattern. But this pattern shows serial low pressure areas ejecting from the western trough into the Eastern USA. The first wave moves to the Plains Wednesday night and jumps to the East Coast Saturday, then moves off. The second wave moves ashore from the Pacific by Friday, and sweeps to the Southeast USA by Monday. Long range models show similar mass fields through Friday, then some differences emerge over the weekend. Even with the differences, the fields show a general agreement on storm occurrence even at day 7. On the other hand, the features that may affect us next Sunday-Monday will not reach land until late this week, so plenty of time for shifting model solutions. Contour heights at 500 mb spend the forecast period in the 550s or higher, rather than the normal for January 540s. Meanwhile 925 mb heights are near normal during late week and above normal over the weekend. Expect temps on the mild side of seasonable. Details... Wednesday night and Thursday... Moderate confidence. The surface low and its associated upper reflection will shift off to the east. Low level flow remains N-NE Wednesday night and then turns from NW Thursday. Forecast moisture fields show lingering moisture below 850 mb. Thus a potential for clouds much of the time, with a clearing trend from the west or southwest Thursday afternoon. Also lingering sprinkles/flurries Wednesday night, then dry Thursday. Forecast dew points suggest min temps will mostly be in the 30s, possibly a couple degrees cooler in the northwest. Temps aloft Thursday will support max temps in the low to mid 40s. Friday through Sunday... Moderate confidence. High pressure builds over the region surface and aloft and should provide dry weather most of the time. Upper trough, supported by the left exit region of a southern stream jet, slips through the ridge Friday night and passes south of New England. The left exit region is close enough to us to leave open an overnight chance of precipitation, mainly rain, in areas south of the Mass Pike. This should quickly move out to sea. The high will then reassert itself with partly cloudy skies the rest of the weekend. Temps in the mixed layer remain similar Friday and Sunday, and a little milder on the cloudier Saturday. Continue to expect highs in the upper 30s to mid 40s, lows in the upper 20s to mid 30s. Monday... Low confidence. The main long term models generally agree on the next weather system coming onshore from the Pacific Ocean about Friday and generating a closed low over the Southern Plains over the weekend, then trending northeast toward New England. Currently, all suggest precipitation moving into our area on Monday, and all hint at cold air lingering in the interior. The agreement and the potential weather are impressive, but the system is still feet-wet over the Pacific and with 7 days for the pattern to evolve. We will indicate chance pops for Monday and watch developments. && .AVIATION /19Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 20z Update... Short Term /through Wednesday/...Moderate Confidence. Thru 00z... VFR to start but trending to MVFR as precip slowly overspreads the region from west to east. Burst of moderate to heavy precip toward 00z from west to east. May see snowfall rates of up to 1 inch per hour during this time...mainly western-central MA. After 00z... MVFR to start but lowering to IFR with LIFR likely across the high terrain. Moderate to heavy precipitation 00z-02z across RI and eastern MA mainly in the form of rain and sleet but snow westward into the Worcester Hills with snowfall rates up to an inch per hour. Freezing rain likely vicinity of BDL and BAF...possibly ORH too. ENE winds picking up by midnight and thereafter. Wednesday... MVFR with LIFR/IFR across the high terrain. Rain most terminals but rain and/or snow high terrain. Gusty NE winds eastern MA coast. Specific Terminals... KBOS TAF...bulk of precip will be rain however sleet and snow may mix in from 23z-01z. Gusty NE winds late tonight into Wed. KBDL TAF...Initial onset of RA transitioning to a wintry mix of SN/IP/FZRA. Light snow / ice accumulations are forecast with snow on the order of several tenths and several hundreds of an inch of icing. Outlook /Wednesday Night through Saturday/... Wednesday night-Thursday...Moderate confidence. MVFR-IFR cigs Wednesday night and early Thursday. Brief MVFR vsbys in drizzle possible near the eastern MA coast. Conditions improve to VFR during Thursday. Friday-Saturday...Moderate confidence. VFR. An area of MVFR cigs/vsbys in light rain may move up into CT/RI/SE Mass Friday night, then shift south offshore Saturday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 3 pm Update... Short Term /through Wednesday/...Moderate Confidence. Tonight... ENE winds gradual increase tonight to 30 kt or so by morning...strongest wind vicinity of Cape Cod and Nantucket waters. Vsby lowering in rain with some sleet possible this evening. Wednesday... ENE winds up to 30 kt with strongest winds across the Cape Cod and Nantucket waters. Low risk for a few gusts up to gale force...35 kt. Vsby reduced at times in rain and fog. Outlook /Wednesday Night through Saturday/... Moderate confidence. Wednesday night... Northeast winds with frequent gusts to 25 knots early in the night. Winds then diminish overnight. Seas 5 to 10 feet on the exposed waters will start to subside toward morning. Small Craft Advisories will continue for most waters. Thursday... Seas subside to less than 5 feet by Thursday night. NW winds will be 15 knots or less. Small Craft Advisory may be needed on the outer waters. Friday-Saturday... Northeast winds 15 knots or less. Seas 4 feet or less. An area of rain may move across the southern waters Friday night with brief reduction of visibility. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory until 7 AM EST Wednesday for CTZ002>004. MA...Winter Weather Advisory until 7 AM EST Wednesday for MAZ002>006-008>012-026. RI...None. MARINE...Small Craft Advisory from 11 PM this evening to 7 PM EST Wednesday for ANZ230>237-250-251-254>256. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...WTB AVIATION...WTB/Nocera MARINE...WTB/Nocera

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