Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152158 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 458 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/... 245 pm update... Tranquil weather this evening and 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. && .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 late Tuesday into Wednesday - Disturbed area of weather lingers into Friday - High pressure and more seasonable weather possible for the weekend */ Discussion... Without going into too much detail, fairly apparent we`ll see an airmass transition throughout next week as forecast guidance has continued to signal the past several days. But it is during this transition that we may see considerable impacts with the onset of inclement weather, specifically Tuesday into Wednesday. An over-running setup up ahead of low pressure emerging out of the the C CONUS, sweeping N/E across the Great Lakes region, against a nearly 1030 high over SE Canada into N/E New England. Anticipating a likely wintry mix mainly of sleet and freezing rain transition over the region to rain with shallow surface cold air damming beneath an over-riding warm-nose around H8. A significant icing event is forecast for interior portions of S New England, especially N/W MA into CT. Some aspects to monitor Tuesday into Wednesday: 1) Magnitude of cold air damming / wet-bulb temperatures / surface winds. GFS / NAM both suggesting modest ageostrophic / isallobaric flow up to H925. A more NE onshore signal along the coast with a more N component across the interior. This as the inside-runner low over the Great Lakes transitions towards the coast into regions of better baroclinicity beneath a potent mid-level shortwave. MET guidance has the best handle on the surface winds. Forecast modified to prevail a more N funnel of flow in the CT River Valley, NE along the coast. Trended closer to forecast wet-bulb temperatures with time given onset of precipitation to which there is good agreement towards late Tuesday (more on that below). Thinking it will take some time to dislodge colder air through late Wednesday when energy aloft sweeps across the region invoking cyclogenesis / low pressure development offshore. Agreeing with WPC, could be talking 0.1-0.2 ice accretion for the high terrain in N/W MA, possibly into CT. Want to highlight one last point and that is ground temperatures. It`ll be quite cold in the days preceding the event. Concern that during transition periods when air temperatures rise above freezing that the ground still may remain cold allowing for continued ice accretion, especially in areas of N/W MA where there remains snow cover. Would consider a buffer of time in possible hazards for a short period after temperatures have risen a degree or two above freezing. 2) Dry air. Stout 1030 high and dry air will likely delay the onset of precipitation towards late Tuesday to which forecast guidance consensus including ensembles are in good agreement on. Slowed the approach of PoPs. As to the presence of ice aloft, good agreement of a saturated dendritic zone with onset so snow is not out of the question at first. But some indications of column drying above -10C as we go Tuesday night into Wednesday. Thus preference to a more sleet / freezing rain event. 3) Lift / moisture availability. Rather weak forcing during onset, thus thinking it will take some time for low levels to saturate and surface temperatures to fall towards their respective dewpoint, thus the trending to the wet-bulb temperature with time. With minimal forcing per isentropic upslope of maritime-tropical moisture mostly confined within the low to mid levels, up to late Wednesday into Wednesday night when the potent shortwave sweeps the region, expect a light precipitation event. Per CIPS analogs showing a strong signal of prolonged freezing rain beyond 6 hours, SREF probabilities, GEFS plumes, and ensemble mean precipitation timing, confidence growing of a prolonged freezing rain event possible beginning late Tuesday and continuing into Wednesday with a gradual transition N/W of the freezing line. More than likely the high terrain will see freezing rain outcomes the longest. Likely leaning too much towards one side of the goal post, perhaps with too much freezing rain over snow and/or sleet. Thermal profiles of consideration along with dry air implications, a whole slew of potential outcomes for the area. But given aforementioned agreement of probabilistics and ensemble guidance, along with analogs, the signal of a potential icing event can not be ignored. Will highlight interior forecast areas for the potential ice threat late Tuesday into Wednesday, leaving out coastal zones. Thursday and beyond... Potential inverted trough behind the departing system Wednesday into Wednesday night, additional wintry precipitation is possible if the 15.0z GFS is correct. Low confidence. Remainder of the forecast period appears dry and quiet. Most of the synoptics exiting the region Thursday night into Friday, there is only the issue of possible ocean-effect clouds lingering across the E coast of MA. N winds prevailing, temperatures look to remain near seasonable. && .AVIATION /22Z SUNDAY THROUGH FRIDAY/... 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. 18z Update... 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. && .MARINE... 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. 245 pm update... Tonight... Modest NNW winds slacken and become more north tonight and 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT

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