Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152345 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 645 PM EST Sun Jan 15 2017 .SYNOPSIS... High pressure maintains dry and seasonably cold conditions into Tuesday morning. Low pressure will push east out of the Ohio Valley toward the region, with the potential of snow and ice impacting much of the interior late Tuesday afternoon into Wednesday morning. The low moves offshore by Thursday morning, but could see leftover spotty showers along the shore late this week. High pressure will build along the eastern seaboard with dry, milder conditions into next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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635 PM Update... Mainly clear skies across the region this evening, except for a few high thin cirrus clouds at a few locations at 23Z. Area of clouds E of Cape Cod shifting offshore as seen on latest IR satellite imagery. Also noting some lower clouds across central NY state, though those are tending to shrink rather than shift E, while more high clouds across N VT/NH shift E-SE. Have kept the previous clear forecast intact. Have updated near term grids to bring conditions current. Previous Discussion... Tranquil weather overnight (at least by mid Jan standards) as confluent flow aloft promotes surface high pressure to advect into New England. As 1030 mb surface high crest over the region expecting winds to decouple and combine with mostly clear skies and dew pts in the teens to support radiational cooling and temps falling quickly with sunset. Thus have sided with the colder MAV guid for mins tonight...which supports lows in the teens for most areas...single digits across the CT river valley and possibly interior eastern MA...with mins in the low 20s for the city of Boston...outer Cape Cod and Nantucket.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Monday... Dry northwest flow aloft will promote another day or abundant sunshine and temperatures at or slightly above normal with highs 35 to 40. Light winds in the morning given 1030 mb surface high overhead. However as next northern stream trough zips across Ontario and Quebec pressure gradient increases and results in high a southwest wind of 10 to 15 mph to develop. Overall a real nice day for mid Jan. Monday night... Northern stream short wave trough exits southeast Quebec and enters Atlantic Canada. Its associated cold front sweeps across our region without much fanfare, just a wind shift from WSW to NW. However this sets the stage for colder/drier air to bleed southward into southern New England as 1032 mb high begins to advect into northern New England. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Possible significant icing event across portions of interior southern New England late Tuesday into Wednesday * Onshore winds will keep lingering clouds across the region into Friday * Dry and milder conditions should set up along the eastern seaboard by next weekend Overview... 12Z model suite and ensembles continue to signal slow but steady transitioning upper level steering pattern across the northern hemisphere through this timeframe. The fast, progressive flow, which has been persistent over the last few weeks, looks to feature more H5 amplification and the possibility of omega blocking setting up from the central U.S. to the western Atlantic by next weekend. Depending upon how this sets up, could see period of dry and milder conditions along the eastern seaboard. Prior to this, however, will see potential of significant icing and light snow accumulations across portions of interior southern New England Tue through Wed night as cold air gets trapped, which could allow for a wintry mix including sleet and/or freezing rain. As the low passes south of New England, should see winds become onshore during Wednesday, which should allow the mixed precipitation to change over to all rain as temps rise. Details... Tuesday through Wednesday night...Low to moderate confidence. Low pressure will push eastward out of the Great Lakes Tuesday morning, while high pressure extends from Quebec into northern New England. Noting a good cold air damming signature setting up, especially on the 12Z GGEM/NAM and ECMWF. Appears that the 12Z GFS is much faster than the remainder of the model suite in its exit of the high across the Maritimes, which brings the precip in much faster. Used a blend of remaining guidance for this portion of the forecast, which backed off the onset of the precip until later Tue across the CT valley and into Tue night further east. There is still some solution spread, but much better without including the GFS for this forecast. Big questions for this forecast will be how deep the cold air damming sets up, as well as how quickly the partial thicknesses rise overnight. Looks like the H85 to H7 thicknesses rise to critical 1540-1549m overnight while the H100 to H85 thicknesses remain at or below 1300m, which will allow IP/FZRA to develop across the higher inland terrain into the CT valley and N central Mass overnight. This, along with potential of 0.2 to 0.3" of QPF, where the best shot for freezing precip occurs, could see significant icing that may warrant winter weather headlines across portions of the interior. Will continue to monitor this very closely. May also see some light snow accumulations as well, and may even see some mixed snow/sleet as close as the eastern coastal plain for a time overnight Wed night. This will all depend upon the eventual temp profile. Winds will veer from N to easterly overnight into Wednesday, so this should bring milder air onshore far enough inland to bring a change to rain inland. Could still see some icing across the E slopes of the Berkshire and the northern Worcester hills into mid morning, but temps should rise above freezing then. As the ridge continues to shift E, the low should push off near or S of Cape Cod during Wed. Back edge of precip should work into western areas Wed afternoon, but will be slower to clear the east Wed night. Winds will back to N again, which will bring colder air back in. Precip could end as a mix of rain and snow showers even down to the coast. A lot to consider during this portion of the forecast. Thursday and Thursday night...Low to moderate confidence. As the low slowly pushes off the coast, it appears to stall over the western Atlantic with H5 omega blocking looks to set up. However, models and ensembles have some solution spread with its position of the low over the Atlantic as well as across the central U.S. Friday through Sunday...Low confidence. At this point, it looks like ridge at the surface and aloft sets up along the eastern seaboard. So, should see dry conditions during this timeframe. Should see mainly N-NE winds, which will keep temps milder than seasonal normals. However, this timing will be dependent upon movement of systems earlier in the week as well as the eventual set up of the blocking pattern across the central and eastern U.S., so lower than average confidence for this part of the forecast. && .AVIATION /00Z 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 Monday Night/...High Confidence. 23Z Update...Little if any changes to this forecast needed. Remains mainly clear and dry. VFR. Dry weather and mainly light winds thru Monday night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday through Wednesday night...VFR to start early Tuesday, then lowering to MVFR-IFR from W-E as area of precip moves across Tue afternoon. Will see mixed RN/SN/FZRA/IP by Tue afternoon across the interior, with ice accretion forecast mainly across the higher inland terrain and portions of the CT valley and N central Mass. Sleet could reach as far E as east coastal areas Tue night, though temps will slowly rise as winds become onshore overnight. Precip should change over to RA with lingering patchy fog Wed. Expect E winds gusting to around 25 kt along east coastal areas Wed, then back to N Wed night. Precip should taper off to showers Wed night, but could see mix back to IP/FZRA across N central and W Mass before ending. Patchy fog lingers through the night with MVFR-IFR VSBYS. Thursday-Friday...Will see MVFR to local IFR CIGS/VSBYS linger Thu morning as northerly winds keep low clouds/fog mainly across eastern areas. Should improve during Thu afternoon. Wind gusts up to 25 kt possible through Thu, then diminishing. Should see mainly VFR conditions Fri.
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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 Monday Night/...High Confidence. 23Z Update...Few if any changes needed. Updated to bring wind and seas conditions current. Tonight... Modest N-NW winds slacken and become more N tonight, then light and variable toward daybreak Mon as 1030 mb high pres settles over New England. Monday... High pressure moves south of New England resulting in light/variable winds in the morning becoming W-SW by late morning/midday and then increasing to 15 to 20 kt late Mon afternoon. Monday night... W-SW winds 15 to 20 kt Mon evening become NW and slacken. Dry weather and good vsby prevail. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday-Wednesday...Winds veer from N to E during Tuesday, with gusts increasing to 25-30 kt Tue night and Wed across the eastern waters as low pressure moves across the southern waters. Seas will build up to 5-8 ft on the eastern waters Wed. Even with rather low astronomical tides, tide heights may reach 9-11 ft on both the Tue night and Wed tides along the east facing beaches as well as in Cape Cod Bay. Winds will back to N-NE but remain gusty Wed night across most of the waters as the low exits. Reduced visibilities in rain and patchy fog through Wed, then gradually improving. Thursday-Friday...N winds will continue to gust to 25-30 kt Thu, then will diminish Thu night. Seas remain very high, up to 8-11 ft on the eastern open waters through Thu night, then will slowly subside but remain well above 5 ft on most of the outer waters as well as the eastern near shores Fri.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera/EVT SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT

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