362 FXUS61 KBOX 270848 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 348 AM EST Wed Jan 27 2021 .SYNOPSIS... Light snow or a wintry mix will continue tonight into Wednesday, and may change to light rain near the coast Wednesday afternoon. The coldest air of the season arrives Thursday and lasts through Saturday with bands of ocean effect snow possible near Cape Cod. Temperatures should climb close to normal early next week, but a low pressure system tracking south of New England should bring snow and rain. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 120 AM Update... * Scattered snow showers continue early this morning. * Areas of patchy freezing drizzle and freezing fog could develop across parts of the interior overnight. While the bulk of the warm advection snow (aka the front end thump) has moved off the Eastern MA coast, there appears to be a weak mid-level deformation band along with an approaching weak surface low from Western NY. This has been enough to produce periods of light snow showers, with reduced vsby during times of brief moderate snowfall. As the snow growth region dries out, the snowflakes will become smaller so any additional accumulation will be in the order of a coating to a few tenths of an inch. In addition, with the lack of ice crystal in the DGZ, there could be patchy freezing drizzle to go with patchy freezing fog from the residual moisture. So have kept the existing Winter Weather Advisories going to account for the possibility of light icing. If you are out and about early this morning, be sure to take it easy on the roads. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Highlights * Winter Weather Advisory in effect through 1PM Wednesday has been expanded to include all of MA west of I-95 as well as northern CT and RI for light accumulating snowfall this evening into Wednesday morning. Moderate snowfall Tuesday night will transition to light ocean-effect snow Wednesday morning. * Ocean-effect snow-showers likely to see some rain mix in during the afternoon hours on Wednesday before transitioning back to snow Wednesday night into Thursday morning. A deamplifying wave propagates eastward Wednesday evening producing areas of moderate snowfall ahead of a surface warm front. Antecedent dry air has made it challenging for this system to reach it's full potential in terms of accumulating snowfall. Nonetheless, forecasting 3-5" of accumulating snow across the interior with 1-3" across the coastal plain. As this wave slides east, a secondary-low pressure system which has developed off the east coast will merge with the surface low to our west and result in a persistent northeasterly flow over southern New England through Thursday morning. This will allow for ocean- effect snow showers to occur intermittently through early Thursday afternoon. Residual warm air from the surface warm- front will result in periods of light rain mixing into this ocean-effect precipitation tomorrow afternoon. As temperatures drop overnight Wednesday into Thursday morning, expecting a transition back to light ocean-effect snow showers during this time period. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Big Picture... Closed upper low over Quebec sweeps south over New England Thursday night and Friday. A portion of the upper low will linger overhead of Maine and Eastern MA through Saturday before moving off. High pressure moves overhead Saturday night and part of Sunday. Southern stream shortwave and associated surface low will generate a coastal system early Monday. This coastal system will move up the coast past Srn New England early next week. Height contours with the upper low Thursday night/Friday will fall to 2-3 std deviations below normal. Expect much colder than normal temperatures during this time and remain below normal through Saturday night. Contours and expected temperatures climb back to near normal early next week. Although there are small differences in the mass fields, the models are similar in handling the upper low and cold air outbreak late this week and the high pressure area Saturday night and Sunday. Confidence in this part of the forecast remains high. Later in the forecast... the southern stream shortwave and surface low pressure are handled similar through Saturday, but the GFS trends faster and the ECMWF/GGEM trend slower Sunday and Monday. The ECMWF also generates the coastal secondary off the Carolinas while the GFS generates it off the Mid Atlantic coast. The result is a faster start of precip from the GFS while the slower/southerly models are about 18 hours slower. Confidence in this part of the forecast remains low. Details... Thursday night through Saturday... As noted above, cold upper low moves over the region bringing much below normal temps. The airmass will be cold and dry with not much moisture for clouds. So expect partly to mostly clear skies. Strong northwest winds are expected Thursday night and Friday, with sustained winds of 15 to 25 kt/20 to 30 mph and gusts of 25 to 35 kt/30 to 40 mph. The combination of this wind with temperature 2-3 std deviations below normal will mean wind chills of zero to 10 below at night and single numbers during the day. The strong winds moving this cold air over ocean water temps in the upper 30s and lower 40s will cause ocean-effect snow showers. This will cause clouds over Cape Cod. Northwest winds will maintain through Saturday. This fetch will favor snow showers either along or just east of Outer Cape Cod, especially off Provincetown and Truro. Mixing to 950-mb will tap temps of -12C to -17C at that level, supporting max temps Friday of 15F to 25F. Airmass temps will be a couple of degrees less cold Saturday, so max temps may reach the upper teens to mid 20s. Dew points in the single numbers and below zero suggest min temps to at least 0 to 15F Thursday night and Friday night, possible with spots below zero in Wrn Mass Friday night. Saturday night-Sunday... High pressure moves overhead, providing radiational cooling Saturday night and fair weather for much of Sunday. Dew points will be 0 to 10 below west of I-95, allowing for min temps below zero in the western interior and single numbers above zero farther east...and in the teens along the coast. Temperatures moderate a little aloft Sunday, so max temps should reach the mid 20s to lower 30s. Depending on how quickly moisture from the next system arrives, there may be increasing high clouds Sunday afternoon and night. Monday-Tuesday... There continues to be a roughly 18-hour difference between the fast GFS and slower ECMWF-GGEM regarding the arrival of the next weather system coming up the coast. It is notable that with this 00Z run the GFS is trending later with the arrival of precipitation than earlier runs. This and the digging trough/higher amplitude aloft suggest a trend toward the slower solutions. Will hold off the arrival of precip until Monday morning, which is a compromise between the two solutions. Used two blended grid sources that will better add the effects of the slower solutions. P-types look to be snow inland with some rain along the coast. && .AVIATION /09Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 06Z TAF Update: Moderate confidence, mainly due to timing. Widespread IFR to LIFR conditions early this morning with -SN resulting in reduced vsby at times. There could also be patchy -FZDZ and FZFG at times. Interior terminals should dry out by 18z Wed with the Eastern MA and RI terminals seeing ocean- effect showers at times through Thu 12z thanks to a northeast flow. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Thursday Night through Sunday/... Thursday Night: VFR. Windy with gusts up to 30 kt. Friday through Friday Night: VFR. Windy with gusts up to 35 kt. Saturday: VFR. Breezy. Saturday Night through Sunday: VFR. && .MARINE... Weak low-pressure system passes southeast of New England overnight tonight. This will bring with it periods of snow/rain showers over the coastal waters. Persistent northeasterly flow through Thursday will allow these showers to continue intermittently near the coast. Another system forms along the Carolina coast Thursday and moves out to sea south of the waters. This system will create gusty north- northwest winds that will draw much colder air across the waters. This may bring gales to the waters on Friday as well as moderate to heavy freezing spray. Outlook /Thursday Night through Sunday/... Thursday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. Chance of freezing spray, slight chance of snow showers. Areas of visibility 1 to 3 nm. Friday: Moderate risk for gale force winds with gusts up to 35 kt. Rough seas up to 14 ft. Chance of freezing spray, chance of snow showers. Areas of visibility 1 to 3 nm. Friday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Chance of freezing spray, slight chance of snow showers. Areas of visibility 1 to 3 nm. Saturday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of freezing spray. Local visibility 1 to 3 nm. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Watch from late Thursday night through Friday evening for ANZ230>237-250-251-254>256. && $$ SYNOPSIS...WTB/BL/RM NEAR TERM...Chai/JWD SHORT TERM...RM LONG TERM...WTB AVIATION...WTB/RM MARINE...WTB/RM