954 FXUS61 KBOX 072026 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 326 PM EST Mon Jan 7 2019 .SYNOPSIS... High pressure builds into New England today with diminishing wind and below normal temperatures. A frontal system will bring a period of light snow late tonight, changing to rain on Tuesday. Rain changing back to snow Tuesday night into Wednesday across the interior as an associated storm center deepens offshore. Could make for some impacts during the Wednesday AM commute. Cold and dry conditions Thursday through Saturday. Another storm may threaten Southern New England Saturday night and Sunday, but forecast confidence is low at this time. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Still appears that a fairly minor snow event will impact southern New England late tonight. Progressive high pressure continues to move offshore while a low pressure moves across southern Canada. This will eventually turn winds to be from a southerly direction, causing temperatures to remain nearly steady or slow rise late tonight. In addition, the window for accumulating snow should be rather small. Lots of low level dry air will need to be overcome before precipitation will even likely reach the ground. This is most likely to not happen until well after midnight. Have continued the forecast of up to 2 inches of snow, mainly across the higher terrain toward the NH/MA border west of Fitchburg. Elsewhere, little if any snow is expected, especially within the I-495 corridor and south of the Mass Pike. Increasing clouds will prevent temperatures from getting as low as last night, in addition to the aforementioned light southerly wind. expecting near to slightly above normal low temperatures. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Any accumulation snow Tuesday morning will likely be shut off from west to east towards mid to late morning. Drier air surges into the snow growth region, and temperatures also rise. Expecting just about all of southern New England to be above freezing by about noon. Thus, expecting a transition to all rain for a precipitation type for the afternoon. Until this happens though, there remains the possibility the drier air in the snow growth region coincides for a time with subfreezing temperatures near the ground. This sets the stage for a period of freezing rain or freezing drizzle. Do not have a lot of confidence in a window for this to occur. Winter Weather Advisories will need to be considered for the threat of light icing. The timing is not the best, as this window might be during the Tuesday morning commute, especially across the interior. Another developing mid level low pressure will cross New England Tuesday night. There are enough signals this low will likely slow down as it reaches the Gulf of Maine. This will keep the threat of some precipitation lingering through Tuesday night. Temperatures will be mainly above freezing, meaning rain would be the predominant type. However, colder air arrives late Tuesday night from the NW, forcing a transition to a rain/snow mix or just plain snow across the highest terrain. Little additional snow accumulation is expected Tuesday night. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Big Picture... Large scale pattern shows ridge building over the Western USA while a trough digs over the Northeast USA during the long term period. Shortwave scale shows a closed low over New England midweek ejecting through the Maritimes. Over the weekend we get a pair of shortwaves, one in the northern stream with New England in the right entrance of the jet and a second in the southern stream with the left exit of the upper jet passing south of New England Sunday. Contours are in a colder than normal pattern most of the period except for Saturday. But surface high pressure in Canada brings a north to northeast low level flow that should keep temps cold in Southern New England. Upper contours are in good agreement through Friday and possibly Saturday, then show differences Sunday and Monday. At the surface the GFS has trended a little toward the ECMWF, but remains closest to our shores. The ECMWF maintains a progressive tilt to the trough and favors a storm track farther to sea. Moderate to high confidence in the forecast Wednesday through Saturday. Disagreement among models leads to low confidence for Sunday and Monday. Concerns... Wednesday through Saturday... Closed upper low moves overhead Wednesday, and is forecast to generate 20-30 mb/hr lift. Best lift would be north of the Mass Pike. A broader cyclonic flow lingers Thursday. The upper cold pool lingers both days with lots of moisture for clouds. The lift on Wednesday, though relatively modest, is enough to generate a few mixed rain/snow showers. The cold pool instability should also be able to mix 35-45 kt wind gusts to the surface Wednesday, and somewhat less wind on Thursday. Mixing to 900 mb each day will tap temperatures around -2C Wednesday and -7C to -8C Thursday, supporting max temps in the 40s Wednesday and 30s Thursday. Min temps far upstream were in the upper teens and 20s this morning; these may moderate a few degrees as they move to our lower elevation, but still suggest Wednesday night min temps in the mid 20s to lower 30s. High pressure approaches Friday and moves overhead Saturday. Overall a dry period due to subsidence, but with increasing clouds Saturday. Saturday night through Monday... GFS maintains a trough axis phased between northern and southern streams...drawing the Gulf of Mexico storm up the coast, bringing snow to our area starting Saturday night through Sunday. Meanwhile the 12z ECMWF maintains seperate streams, with some jet-induced lift over our area but also a progressive jet axis that guides the southern stream out to sea. High pressure to our north keeps a cold airmass over us. Either scenario is possible, but the ECMWF has been more consistant run-to-run and so we favor it over the GFS. We will use pops near 20 pct north and near 30 pct south at this time. Temperatures through the column support mainly snow. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... Tonight...High confidence in trends. VFR cigs, lowering to MVFR in the west after 08/08Z as light snow overspreads the region. Light snow or mix rain/snow reaching SE New Eng toward 08/12Z. Tuesday...Moderate confidence. MVFR/IFR conditions. Light rain near the south coast, with snow changing to rain further inland from south to north. South wind gusts to 25 kt developing Cape/Islands in the afternoon. Tuesday Night...Moderate confidence. Mainly VFR, with areas IFR possible. Becoming gusty after midnight. Chance of RA/SN. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Wednesday through Saturday/... Wednesday: Mainly VFR, with areas MVFR possible. Windy with gusts to 35 kt. Chance RA, chance SN. Wednesday Night: Mainly VFR, with local MVFR possible. Windy with gusts to 35 kt. Slight chance SHSN. Thursday through Friday Night: VFR. Breezy. Saturday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High confidence. Tonight...Light north winds this evening, then increasing south winds late tonight but gusts below Small Craft Advisories. Light rain developing late. Tuesday...Southerly wind gusts to 30 kt developing over the open waters. Vsbys reduced at times in light rain and fog. Tuesday Night...West winds diminish during the evening, then increase once more after midnight. Gusts up to 30 kt possible late across the southern coastal waters. Outlook /Wednesday through Saturday/... Wednesday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 11 ft. Chance of rain. Wednesday Night: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Slight chance of rain showers. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 4 PM EST this afternoon for ANZ231>234-256. Small Craft Advisory until 10 PM EST this evening for ANZ250- 254. Small Craft Advisory until 7 PM EST this evening for ANZ255. && $$ SYNOPSIS...WTB/Belk NEAR TERM...Belk SHORT TERM...Belk LONG TERM...WTB AVIATION...WTB/Belk MARINE...WTB/Belk