Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170858 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 358 AM EST Tue Jan 17 2017 .SYNOPSIS...
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A mixed bag of wintry weather will impact much of S New England from later today 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.
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&& .NEAR TERM /UNTIL NOON TODAY/...
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Quiet this morning this increasing and thickening cloud decks as E onshore flow warms temperatures into the upper 30s to low 40s. The blanket has kept temperatures from dropping considerably overnight. Lows this morning observed in the 20s for the most part with a few spot locations in the teens. Low 30s along the immediate S-coast and over the Islands.
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&& .SHORT TERM /NOON TODAY THROUGH 6 PM WEDNESDAY/...
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*/ Highlights... - Winter Weather Advisories continue... - Threats developing towards the PM commute - Wintry mix of snow, sleet, freezing rain - Snow accumulations generally 2 to 4 inches across MA - Ice accums of a trace to a tenth mostly over the Berkshires */ Discussion... Overview: An initial over-running setup ahead of low pressure emerging out of the C CONUS against a 1030+ high over SE Canada. Focus upon a front- end thump of an isentropically ascending warm-moist conveyor belt over a shallow surface cold dome. Subsequent mixed-bag of precip beginning during the later-half of Tuesday into Tuesday evening, sweeping W to E. Transition to an immediate offshore low overnight ahead of a mid-level positively-tilted trough across regions of favorable baroclinicity offshore, colder and drier air is drawn S across New England. Mixed-bag of precipitation transitions to mainly rain and/or snow as storm exits through Wednesday. Timing: Focus on the later half of Tuesday into Tuesday night with height of the outcomes Tuesday evening. Lingering light showery weather into Wednesday. Trends: With onset will see low-levels saturate towards their respective wet- bulb, this as warmer air and rising thicknesses encroach from the SW. Will see onset of rain change over to a mix of snow, sleet and/or freezing rain, depending on location. As low pressure develops S of New England overnight into Wednesday will see a combination of cold air being drawn S from the 1030 high over SE Canada with an E flux of milder onshore winds. Precipitation types vary. Dependency on the temperature at a particular location and the thermal profile aloft, whether there is ice present as well. Will discuss the uncertainty below. Drier air working in from the W towards Wednesday and through the day, low-levels still saturated with a measure of lift, some light showery weather lingers. Ice in the column in question, but also will see temperatures warming. Some places, particularly the high terrain, may still be dealing with a wintry mix of light precip. Uncertainty: Thicknesses and low level thermal profiles in doubt with respect to the depth and magnitude of both intruding warmer air around H8 and colder air immediately below to the surface. This in addition to surface 2m temperatures and whether ice is present in the column. Subsequently there is a whole mixed bag of precipitation outcomes that make it entirely challenging to get the forecast exactly right. There are going to be some tight gradients with respect to outcomes, especially across interior S New England, more specifically Essex and Middlesex Counties in MA, and likely some angry customers that we didn`t get the forecast right. Several concerns and unknowns: 1) Duration of precipitation-type. For example, whether we get more sleet and/or freezing rain than snow which yielding lower snowfall totals. Forecast guidance has trended slightly colder limiting the intrusion of warmer air aloft, more than likely with the stronger and prolonged presence of low pressure developing S of New England overnight. Plus greater indications of ice being present within the column throughout the event, more than likely with mid-level ascent per follow-up positively tilted H5 trough. Still challenges in the interpretation of bufkit profiles as to the magnitude and strength of respective warm intrusion around H8 and colder air immediately below possibly down to the surface (think Bergeron principles). Consensus of probabilistics have increased in probability of a 6- inch snowfall as have deterministic forecast snow algorithms. Am inclined to believe we`ll see greater snowfall outcomes over N MA especially over the high terrain before mixing with or transitioning over to sleet and/or freezing rain. This in agreement with the consensus of forecast guidance and WPC forecast and probabilities. 2) The exact evolution of 2m wet-bulb temperatures, where they will be at or below freezing during the event. Confident N flow as low pressure develops and deepens S of New England during the overnight period. Ageostrophic / isallobaric response drawing colder air S from high pressure over SE Canada with H925 temperatures around -20C should maintain a level of colder air across the region, especially the high terrain. Leaning with high-res WRF / NAM solutions with forecasting 2m temperatures, keeping sub-freezing conditions in longer especially as low pressure develops immediately S overnight. Toughest part to determine is the sharp coastal front boundary likely over Essex / Middlesex Co MA. Going to be a tight gradient in temperatures complicating the forecast. 3) What happens into Wednesday. Drier air working in aloft as low levels remain saturated. This subsequent of the low pressure still deepening S of New England as it moves out to sea. A strong onshore surface flow while a mid-level trough sweeps the region invoking ascent aloft. Light showery weather / drizzle forecast but uncertain is the presence of ice in the column. Some indications, leaning a mix of drizzle, freezing drizzle, light snow / flurries that either taper off or change to liquid through Wednesday as temperatures warm at the surface. Will add to snow total amounts early, complicated by not knowing exactly where the aforementioned coastal front over NE MA will setup. Will keep with high-res forecast guidance 2m temps. Model Preference: Holding close to the suite of high-res guidance inclusive of the NAM, WRF model solutions, and Canadian. Trending less with global model solutions, however evaluating trends from the UKMET with respect to 2m and H85 temperatures. Forecast Outcomes: Snow...A general 2 to 4 inches with locally higher amounts, mainly over the high terrain, for areas N of the I-90 corridor extending N/E somewhere in-between the NE corridor of the I-495 / I-95 belt- ways (where the aforementioned coastal front is expected to reside). Lighter amounts for areas S into the N-tier of CT / RI. Ice..Greatest icing expected over the Berkshires, perhaps even the Worcester and Tolland Hills at the highest elevations, with ice accretion amounts ranging from a trace up to a tenth of an inch. Headlines: No adjustment to headlines both spatially or temporally. Areas such as Middlesex and Essex County MA are areas of dispute given their location with respect to an anticipated coastal front. Definitely going to see a gradient across the region of outcomes but not exactly sure where. Continued confidence in tandem with ensemble probabilistics that the higher elevations as well as N MA will see the greatest impact with respect to wintry weather. Areas of greatest concern with regards to impact are across the high terrain, namely elevations in excess of 500 feet, definitely at or above 1000 feet. Impacts: Focusing on the Tuesday PM and Wednesday AM commute. Expected wintry precipitation will likely result in hazardous travel conditions, especially on untreated roads. Bridges and overpasses are most susceptible. Echoing the previous forecaster, it only takes a thin coating of ice to make conditions dangerous. Best advice is to remain aware of the oncoming weather, stay tuned to the latest forecasts, and if you have to travel at any time Tuesday into Wednesday, plan accordingly and give yourself extra time as necessary.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
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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.
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&& .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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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 Wednesday/...Moderate Confidence. 9z update... Today... VFR much of today. Lowering towards IFR-LIFR by late afternoon into evening. Precipitation spreading across the terminals W to E either as RA and/or a wintry mix of SN/IP/FZRA. Low visibility with SN. Increasing E onshore flow over much of S New England with a more N flow in the CT River Valley. Gusts up to 20 kts along the E shore of MA. Tonight... IFR-LIFR over much of the region. Either RA or a continued wintry mix of SN/IP/FZRA potentially changing to RA. Increasing E onshore flow becoming N towards morning. N flow prevailing in the CT River Valley. Gusts up to 30 kts for the E MA shore. Wednesday... Precipitation diminishing but IFR-LIFR conditions remain. Threats of either DZ or a wintry mix of FZDZ/SN. N winds persist with gusts up to 30 kts across the E/SE shore of MA. Specific Terminals... KBOS TAF...With the increasing E onshore flow, believe the bulk of precipitation for the terminal will be in the form of RA. Gusts up to 30 kts tonight into Wednesday. Lowering down to LIFR with cigs around 500 feet. KBDL TAF...Will see a N flow prevail through the CT River Valley. Initial onset of RA transitioning to a wintry mix of SN/IP/FZRA. Late today into tonight. Light snow / ice accumulations are forecast with snow on the order of several tenths and icing roughly a trace. 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.
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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 Wednesday/...Moderate Confidence. Increasing E winds beginning late today and continuing overnight into Wednesday as an an area of low pressure develops and deepens S of New England pushing E out to sea late. Gusts up to gale force are possible, especially for the SE outer waters. Waves building into Wednesday up to 6 to 9 feet especially across the E/SE waters. Rain expected with the developing low pressure. Could see brief reductions in visibility along with mist. Small Craft Advisories are in effect. 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory from 1 PM this afternoon to 7 AM EST Wednesday for CTZ002>004. MA...Winter Weather Advisory from 1 PM this afternoon to 7 AM EST Wednesday for MAZ003>006-010>012-026. Winter Weather Advisory from 10 AM this morning to 7 AM EST Wednesday for MAZ002-008-009. RI...None. MARINE...Small Craft Advisory from 11 PM this evening to 7 PM EST Wednesday for ANZ230>237-250-251-254>256.
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&& $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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