628 FXUS61 KBOX 220024 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 724 PM EST Mon Dec 21 2020 .SYNOPSIS... An upper level disturbance tracks south of New England tonight followed by dry and blustery conditions Tuesday. High pressure builds into the region Wednesday. A strong storm system approaches from the west Christmas Eve, bringing heavy rain, strong winds, and well above normal temperatures into Christmas Day. Then, dry and much colder conditions arrive this weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 730 PM update... Abundant low level moisture across the region in the form fog and freezing fog across portions of CT, RI and MA. This is due to radiational cooling with mostly clear conditions and light winds, promoting temps falling AOB freezing. However as mid level trough approaches from the west, cyclonic mid level flow promoting mid deck of clouds overspreading SNE. This should allow temps to rise a deg or two overnight, resulting in fog and freezing fog to become more isolated. Also, as mid level trough approaches, secondary low pressure tries to form South of New England, this will increase NW winds overnight. Thus black ice and freezing fog should be most widespread and impactful this evening, with some slow improvement overnight. Otherwise, previous forecast remains on track. Earlier discussion below. ================================================================= Coastal low pres moves into the Maritimes tonight. Low level moisture remains plentiful this evening with light winds so expect stratus and patchy fog to expand across SNE. HREF indicating high probs of low cigs across a large area tonight. HREF also has a signal for fog which may be locally dense in spots so areas of black ice expected to develop again tonight as temps fall back below freezing with small T/Td depressions. Fairly robust mid level trough approaches from the west with strong shortwave reaching the mid Atlc coast toward daybreak. Best moisture and forcing for ascent remains to the south so expect mainly dry conditions although can't rule out a rain/snow shower along the south coast and Islands. Lows will range through the 20s, except low to mid 30s over the outer Cape/Islands. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday... Strong shortwave tracks off the mid Atlc coast and passes well south of New Eng. The passage of this shortwave will be followed by increasing NW flow which will bring in drier air. Any early morning stratus and patchy fog will erode giving way to increasing sunshine by midday. Seasonable temps expected with highs ranging from mid 30s to lower 40s. It will become blustery in the afternoon with 20-30 mph gusts developing. Tuesday night... Blustery NW winds will continue into the evening in cold advection pattern, then winds diminishing overnight as high pres approaches from the west. Gusty winds will persist however along the eastern MA coast and Cape/Islands. Mainly clear skies with lows mainly in the 20s, except lower 30s outer Cape/Islands. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Highlights... * Dry and seasonably cold on Wed. Likely the pick of the week. * Robust storm system likely to bring soaking rain, strong winds, and well above normal temperatures Christmas Eve into Christmas Day. * Turning sharply colder but dry this weekend. Wednesday... * Likely pick of the week with seasonably cold temperatures and plentiful sunshine. With a 564 Dm ridge building into New England and a strong surface high pressure passing south of the region, it will be a seasonably cold day with highs in the mid 30s to low 40s and with plentiful sunshine to boot. Christmas Eve into Christmas Day... * Potentially high impact storm system likely to bring soaking rain and strong to damaging winds Christmas Eve into Christmas Day. * Localized flooding concerns with heavy rainfall and rapid snowmelt possible, especially for the western zones. Areas of dense fog possible with a mild and moist air mass moving over snow cover. * Rapid freeze up possible with sharp cold frontal passage. Confidence is increasing on a high impact storm system affecting the region late Christmas Eve into Christmas Day. This forecast used a blend of model solutions, but leaned towards the more amplified and slower ECMWF. A high amplitude 500mb trough digs from the Great Lakes to the Gulf Coast states and then becomes negatively tilted over the Mid-Atlantic states on Christmas Day. Southern New England looks to be in the favorable right rear quadrant of a 150 kt 250mb jet for deep lifting throughout the column. Closer to the surface, models are unanimous in showing a screaming 80 to 90 kt southerly low-level jet between 925 and 850mb late Christmas Eve into Christmas Day. There will also be a sharp thermal gradient with 850mb temperatures ranging from +12C to -12C across a few hundred miles! This will allow for strong warm air advection, which further increases the likelihood of eroding the low-level inversion and mixing down damaging winds to the surface. Even applying a conservative 60 percent mixing of 925mb winds mean 50 to 60 mph winds are possible, which could lead to scattered power outages. The areas that could see the strongest winds include the MA coastal plains and locations away from the immediate south coast. SSTs in the 40s could lead to slightly weaker winds on the immediate south coast as well as the Cape and Islands. Overall, this is likely going to be a highly anomalous event. Both the EC Ensemble and NAEFS show up to 5 standard deviation anomaly of the v winds and PWATs. And speaking about rainfall, thinking a widespread 1-2 inches of soaking wind-driven rain with up to 3 inches possible. This combined with melting snowpack, could lead to localized river and street flooding, especially in the western zones where a deeper snowpack exists. Unfortunately for snow lovers and those dreaming of a White Christmas, this is unlikely especially away from the interior high terrain because an air mass with 50 degree plus dew points are prolific 'snow eaters'. Also have introduced isolated thunder in the forecast due to the sheer dynamics of the system and the possibility of seeing a fine line of low-topped convection. Temperatures wise, we have gone with a non-diurnal temperature trend, with temperatures holding steady or even rising slightly into the 50s Christmas Eve night and not falling until mid afternoon on Christmas Day. With dewpoints also climbing into the lower 50s, this will likely produce areas of dense fog as it moves over the remaining snow cover. Cold front moves through late Friday afternoon, bringing an end to the rain, perhaps ending as a few snow showers. There could be a rapid freeze up as temperatures fall, but the wind direction (west to northwest) is climatologically unfavorable for a flash freeze. Nonetheless, would need to watch for slick spots. Lows in the upper teens to upper 20s. Saturday through Monday... Dry and cold weekend in store with highs slightly below seasonal norms in the upper 20s to mid 30s. Slightly warmer on Monday. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 00z update...VFR/MVFR with areas of IFR/LIFR with areas of freezing fog will slowly improve overnight, with IFR/LIFR becoming less widespread. Earlier discussion below. ================================================================= Tonight...Moderate confidence. Trapped low-level moisture again leads to IFR/LIFR conditions in stratus and patchy fog. May see some improvement overnight as W/NW flow becomes established. Tuesday...High confidence in trends. Any IFR stratus/fog will improve to VFR through the morning. Increasing NW winds with gusts 20-25 kt developing. Tuesday night...High confidence. VFR. NW gusts 20-25 kt along immediate coast, Cape/Islands. KBOS TAF...Moderate confidence in the TAF. KBDL TAF...Moderate confidence in the TAF. IFR/LIFR stratus and fog may redevelop by 00z. Outlook /Wednesday through Saturday/... Wednesday: VFR. Breezy. Wednesday Night: VFR. Breezy. Slight chance FZRA. Thursday: Mainly MVFR, with local IFR possible. Strong winds with local gusts up to 40 kt. Chance RA, patchy BR. Thursday Night: Mainly MVFR, with local IFR possible. Strong winds with gusts up to 55 kt. RA, chance TSRA, patchy BR. Christmas Day: Mainly VFR, with local IFR possible. Strong winds with gusts up to 55 kt. RA, FZRA, chance TSRA. Friday Night: Mainly VFR, with areas IFR possible. Windy with local gusts up to 30 kt. Chance SN, patchy BR. Saturday: VFR. Breezy. && .MARINE... Combination of SE swell from departing coastal low and increasing W/NW flow will bring 5-6 ft seas to outer waters tonight. Otherwise, increasing NW winds Tue through Tue night with gusts up to 30 kt developing. SCA issued for the waters, except Boston Harbor and Narragansett Bay but SCA will likely be needed in later forecasts. Outlook /Wednesday through Saturday/... Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Thursday: Low risk for gale force winds with gusts up to 40 kt. Rough seas up to 11 ft. Slight chance of rain. Thursday Night: Moderate risk for storm force winds with gusts up to 45 kt. Rough seas up to 15 ft. Rain, chance of thunderstorms, patchy fog. Christmas Day: Low risk for storm force winds with gusts up to 45 kt. Rough seas up to 15 ft. Rain, chance of thunderstorms. Friday Night: Moderate risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Rough seas up to 13 ft. Chance of rain, chance of snow, patchy fog. Visibility 1 to 3 nm. Saturday: Winds less than 25 kt. Rough seas up to 10 ft. && .CLIMATE... Christmas Eve... Boston Record High Minimum 1). 49F 2015 2). 44F 2014 3). 43F 1888 4). 42F 1964 5). 41F 1965 ============================ Providence Record High Minimum 1). 50F 2015 2). 46F 1979 3). 44F 1965/1964 5). 42F 1987 ============================ Worcester Record High Minimum 1). 47F 2015/1964 3). 43F 1979 4). 41F 1982 5). 38F 2014/1895 ============================ Hartford Record High Minimum 1). 43F 2014 2). 42F 2015/1987 4). 41F 1979 5). 38F 1965/1964 Christmas Day... Boston Record Highs 1). 65F 1889 2). 62F 2015 3). 60F 1964/1888 5). 59F 2014/1982/1915 Boston Record Precip. 1). 0.89" 1889 2). 0.82" 1886 3). 0.80" 1986 4). 0.79" 2002 5). 0.55" 1966 ============================ Providence Record Highs 1). 64F 2015 2). 63F 2014/1964 4). 62F 1940 5). 60F 1965 Providence Record Precip. 1). 1.68" 1978 2). 1.19" 1986 3). 0.74" 2002 4). 0.63" 1974 5). 0.60" 1945 ============================ Worcester Record Highs 1). 60F 2015/1964 3). 59F 2014/1940 5). 57F 1982 Worcester Record Precip. 1). 1.15" 1986 2). 1.05" 1965 3). 0.73" 1979 4). 0.58" 1979 5). 0.53" 1997 ============================ Hartford Record Highs 1). 64F 1964 2). 60F 2015 3). 58F 1994 4). 56F 2014/1940/1915 Hartford Record Precip. 1). 1.39" 1979 2). 1.10" 1978 3). 1.08" 1986 4). 0.80" 1945 5). 0.79" 2005 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from noon Tuesday to 7 AM EST Wednesday for ANZ231>235-237-251. Small Craft Advisory until 7 AM EST Wednesday for ANZ250-254- 255. Small Craft Advisory from midnight tonight to 7 AM EST Wednesday for ANZ256. && $$ SYNOPSIS...KJC/Chai NEAR TERM...KJC/Nocera SHORT TERM...KJC LONG TERM...Chai AVIATION...KJC/Nocera/Chai MARINE...KJC/Chai CLIMATE...