178 FXUS61 KBOX 292011 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 311 PM EST Tue Dec 29 2020 .SYNOPSIS... Dry and seasonably chilly weather are on tap today into Wednesday. A cold front will bring a period of mainly scattered rain showers to the region late Wednesday night into Thursday morning. High pressure builds in from the west later Thursday into Friday morning, bringing dry and chillier weather. A low pressure passing to our west will bring more rain later Friday into Friday night, but a period of icing is possible at the onset for portions of the interior. We will dry out on Saturday and become unseasonably mild. Somewhat chillier weather arrives early next week, but it still will be above normal for this time of year. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... The core of a high pressure will approach the Mid Atlantic coast late tonight. This will maintain a flow of colder air into southern New England with mainly clear skies. The pressure gradient will also relax as this high pressure gets closer to southern New England. Definitely a chilly night by most standards. Thinking mostly teens for low temperatures across the interior, but could get lower if winds diminish quicker than current expectation. Mainly clear skies to start, with some mid and high clouds arriving late tonight. These will also work to prevent really bottoming out temperature-wise. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... This high pressure moves offshore Wednesday, turning the flow across our region to be more southwesterly behind a warm front. Despite the increased clouds and very chilly start, still expecting temperatures to rebound to near to slightly above normal levels. The warmer we get, the more low level mixing will take place. Gusty winds should develop during the afternoon hours. Our next chance for some precipitation looks to be Wednesday night. A low pressure moving across southeast Canada will swing a cold front our way, increasing the southerly flow. Temperatures should hold nearly steady or slowly rise Wednesday night as a result. The question is how much cold air gets locked in at the surface across the interior. Thinking the door is still open for some freezing rain or snow Wednesday evening across the higher terrain of interior southern New England. This should become less of a risk late Wednesday night. Will continue to monitor this potential for possible winter weather headlines with later forecasts. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Cold front moves through New Years Eve with a period of light rains, clearing late-aftn to early evening. * Turning cold New Years Eve night under strong high pres. * Monitoring developments regarding a late-Fri into Sat frontal system that could bring wintry weather to a portion of Southern New England. Potential for accumulations of snow/ice mainly north of the Mass Pike. * Brief dry weather Sat night. * Some hints/potential for a coastal storm Sunday into Monday - confidence low on details. Details... Thursday/New Years Eve: 12z guidance now is showing better agreement on a Clipper low which moves in very late Wed night into mainly the day portion of New Years Eve. A mid-levelshortwave trough in the northern stream begins to deamplify/shear out on Thursday as it spreads its related cold front through the remainder of Southern New England. Minor timing differences with the frontal passage, with the 12z NAM and GFS a bit faster than the international guidance, the latter would be more late-day/early night. Temperatures should be marginally warm enough for mainly a plain rain. Fairly weak forcing and PWATs mainly 0.8" or less should keep QPF limited to up to a tenth of an inch. Decent pressure gradient should support some southwest breezes 10-15 mph, flipping with frontal passage. Highs upper 30s to mid/upper 40s. New Years Eve Night: A brief period of easing NW winds early New Years Eve night gives way to a period of strong radiational cooling as a 1030+ mb high pres settles over New England. Dry wx with lows in the upper teens to the 20s. New Years Day into Saturday: Overall no significant change noted from current and prior model runs and trends regarding a frontal system that looks to spread a period of wintry weather mainly across areas near and north of the Mass Pike. Much of New Years Day should be mainly dry with high pressure from the overnight shifting eastward to a position off the New England seacoast late New Years Day. Precip onset may be delayed by initially dry air in place, and this dry air may permit a short period of dynamic wet-bulb cooling through evaporative/phase stage change effects. However dewpoints will be on the rise so this process may be somewhat brief. Warm frontal precip likely to begin as a plain rain south of the MA/CT/RI border late Friday and into Friday night, but near and north of here where colder air is more entrenched, snow should start. As warm layer aloft comes in this will shift p-types to more of a wintry mix (FZRA/IP) mainly across northern and western MA into part of northern CT through the nighttime hrs. Biggest uncertainties are specific to p-type transitions: those being how quickly can shallow sub-freezing surface air recover and on the thermal profile for top-down p-type approaches. In general, we should see a transition over to plain rain across all of Southern New England by Sat, but it may take some time for sub-freezing temps to fully scour out. We may not know the answer to that question until we get into the range of the higher-res models that can handle those small scale details somewhat better than models can in the medium range. Potential exists for winter headlines especially for parts of the interior, either due to snow accumulation and/or ice accretions, but confidence on these accumulations is still too low to consider at this point. Period of rain showers should continue into Sat, but cold front and dryslot comes in later Sat to permit drier wx for the second half of the day. Sunday into Monday: Low predictability in this period with models diverging on potential outcomes and details regarding how the next potent southern-stream shortwave comes out of the southern Plains. GFS brings this trough mainly eastward with coastal low development which mainly progresses east, with largely dry and northerly flow. The 12z ECMWF remains the most bullish in bringing a coastal low up to and just inside of the 40N/70W benchmark, with a plain rain to interior accumulating snow for the interior. Has some support from the 12z EPS mean, but spread in the EPS member lows remains large with little spatial clustering. The 12z Canadian GEM now also has a similar depiction to the ECMWF. Certainly a lot that could change between now and then, especially with 2-3 storm systems that could alter outcomes, but this will bear some watch as we move forward. Temps in this period trend near to slightly above average. Tuesday: Ridging and drier wx returns to the region, though another disturbance moving through the Great Lakes could bring some light precip later Tues night. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...High confidence. VFR with gusty NW winds during the evening. Gusty 15-20 kt winds diminishing and winds shift to the W. Wednesday...High confidence. VFR with light winds shifting to the SW/S. Increasing cloud cover with ceilings of 4-7 kft. Lower risk for some MVFR cigs towards the Cape and islands late in the day. May become a bit breezy late in the afternoon with gusts of 15-20 kts. Wednesday Night...Moderate confidence. VFR to start for most. MVFR cigs across the higher terrain towards the Berkshires and monadnocks in the evening. Conditions lowering to MVFR/IFR in showers and fog. Low risk for some -FZRA across the interior higher terrain. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Thursday through Sunday/... Thursday: Mainly VFR, with local MVFR possible. Windy with local gusts up to 30 kt. Chance SHRA. Thursday Night: VFR. New Years Day: VFR. Chance RA, chance FZRA, chance SN. Friday Night: Mainly MVFR, with areas IFR possible. Breezy. RA likely, SN, chance FZRA. Saturday: Mainly VFR, with areas IFR possible. Breezy. Chance RA, slight chance FZRA. Saturday Night: VFR. Breezy. Sunday: Mainly VFR, with local MVFR possible. Breezy. Slight chance RA, slight chance SN. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Minor tweaks to the marine headlines through this evening based on latest observations. Downgraded Boston Harbor to a strong Small Craft Advisory. Winds diminish this evening, but seas will take a little longer to subside. High pressure moves south of the waters Wednesday, with a cold front approaching Wednesday night. Looking like SW gales are a possibility during that time. Outlook /Thursday through Sunday/... Thursday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain showers. Thursday Night: Winds less than 25 kt. Local rough seas. New Years Day: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Rain. Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain. Saturday Night: Winds less than 25 kt. Areas of rough seas. Sunday: Winds less than 25 kt. Areas of rough seas. Chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ230- 232>235-237. Gale Warning until 9 PM EST this evening for ANZ231-250-251- 254. Small Craft Advisory until 7 PM EST this evening for ANZ236. Small Craft Advisory until 7 AM EST Wednesday for ANZ255. Small Craft Advisory until 1 AM EST Wednesday for ANZ256. && $$ SYNOPSIS...Belk/Loconto NEAR TERM...Belk SHORT TERM...Belk LONG TERM...Loconto AVIATION...Belk/Loconto MARINE...Belk/Loconto