208 FXUS61 KBOX 062118 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 418 PM EST Sat Feb 6 2021 .SYNOPSIS... A quick hitting but significant winter storm will bring accumulating snow to much of Southern New England on Sunday, with mainly rain expected over the Outer Cape and Nantucket. Drier weather returns on Monday. Additional snow and rain storms are possible around Tuesday and Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... A quiet and seasonably cold night is in store for Southern New England as a ridge of high pressure passes to our south. Light winds along with mainly clear skies during the first half of night will allow for good radiational cooling, especially northwest of I-95 where a deep snowpack is in place. Low temps should bottom out in the teens to lower 20s and may even see a few high single digit readings in the normally coldest outlying areas of northwest MA. Trended the overnight lows towards the 10th percentile of guidance but also did blend in some of the high- res guidance to account for likely colder temps across the interior and milder temps near the coast with the approaching storm and associated warm air advection. Low pressure developing off the mid Atlantic states will result in a significant increase in mid/high level cloudiness after midnight. This will result in temps leveling off or even rising a few degrees toward daybreak. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... * A quick-hitting but significant winter storm Sunday with lesser amounts expected across western MA and CT. There is concern for 1-2 and perhaps localized 2-3 inches per hour snowfall rate where mixing is not an issue across SE MA and RI between 12-6 PM. * Expect rain to mix in along Eastern MA coast during onset due to strong onshore flow but eventually flip over to heavy, wet snow as winds turn northerly. * Greatest concern is across RI/SE MA where heavy wet snow may result in the risk of some power outages. Low pressure developing off the Carolina coast early Sunday morning will rapidly lift northeast and pass near the 70W/40N Benchmark Sunday afternoon/early evening. Snow will rapidly overspread the region from southwest to northeast Sunday morning and continue Sunday afternoon. Precipitation type with this system should be mainly snow away from the immediate coast. The Outer Cape and Nantucket will likely see mainly rain, while rain/sleet mixing should cut down the snow totals across Eastern MA coast, including parts of Boston. Further west towards the East Slopes and Hartford County, they will be on the northwest edge of the heaviest precipitation, so have issued a Winter Weather Advisory for these areas. Block Island is a tough call because they are west enough to be removed from the strongest warm advection (as opposed to Nantucket and Martha's Vineyard) so have put them in an Advisory as well. Dynamics couldn't be more perfect for heavy snow where mixing is not an issue. Omega is maximized in the -12 to -18C Dendritic Growth Zone (as low as -10C for the coastal area due to abundance of salt nuclei) along with 700-925mb speed/directional convergence will result in a 3 to 4 hour window of impressive frontogenesis. This is a strong signal for convective snow and where mixing is not an issue, could easily see 2 to maybe even 3 inches per hour snowfall rate. So even giving the progressive nature of the system, snow could easily pile up and accumulate 7 to 10 inches in the most persistent mesoscale bands. Key issue that we have been grappling with is where the rain/snow line would set up. With such a robust system, the thinking is that it will bring in quite a bit of warm air out ahead of it to preclude snow for most of the time for the Outer Cape and Nantucket. The 00z and 12z HREF have both been very bullish with intense snowfall rates over the Cape and Islands. The global ensemble guidance is also very bullish with the probability of 3 and even 6 inch plus snowfall over the Cape and Islands. However, wet bulb temperature bottoms out around 32-34F across the Cape and Islands even according to the NAM. So thinking at best high-end Advisory/very low-end Warning snowfall for the Inner Cape and possibly no more than a couple inches for the Outer Cape and the Islands. The fact that the snow is falling during the daytime hours also makes it harder to accumulate. In addition, I did lower the snow totals over the immediate Boston area and much of the Eastern MA coast. With the winds initially out of the E and NE during precip onset, snow ratios will be in the 5-7:1 range initially but gradually increase to around 10:1 as winds back to the N by 20-21z. As for the interior, higher SLRs are expected generally 12-15:1. QPF wise, expect generally 0.3 to 0.5 inches west of Worcester and 0.5 to 1 inch over SE MA and RI. Overall, highest snowfall totals should be from NE CT to Worcester to Metrowest Boston in the order of 6 to 9 inches where the mesoscale banding coincides with a thermal profile that minimizes mixing. Temperatures wise, have used Hi-Res Ensemble Forecast (HREF) to populate the hourly temperatures given that the most reasonable way to get around uncertainty with the thermal profile is using an ensemble approach. It should be noted here that the forecast temps by the HREF does not correspond with the most likely precip type (snow) across SE MA, which is a red flag against forecasting too much snow especially over the Outer Cape and the Islands. Sunday night... Snow will taper off during the evening as the low lifts out and high pressure builds in, allowing skies to clear from west to east. As northwest winds advect -10 to -15C air in, expect lows to bottom out in the teens across much of the region, except for low to mid 20s along the south coast. && .LONG TERM /MONDAY THROUGH SATURDAY/... Big Picture... Low pressure sits over Central Canada through much of the forecast period, with upper jet flow around the low and across the northern tier of the USA. Smaller shortwaves will likely be moving through this flow during the week, with notable wave passages currently pointing to Tuesday and Friday, although timing may vary as it usually does with shortwaves. Thermal fields start the week are a little colder than seasonable. After the Tuesday shortwave, temperatures at 850 mb are forecast to drop 10C, signaling a much colder period Wednesday-Thursday. Temperatures moderate Thursday night-Friday, before cooling off again behind the Friday shortwave. Mass fields are consistent through midweek. Solutions then start to differ Friday-Saturday, possibly later Thursday. Thermal fields show a similar consistency. Forecast confidence is moderate-high early to midweek, trending to low by next Saturday. Details... Monday-Tuesday... Weak high pressure brings dry weather Monday, but moisture fields show increasing high level values...probably increasing mid and high clouds...Monday. As the shortwave approaches Tuesday, expect surface low in the Midwest to jump to the coast with best upper jet support just off our South Coast. This will bring precipitation Tuesday, and with a southeast wind would expect mainly snow inland and rain or a rain/snow mix South Coast. Potential for several inches accumulation where there is no changeover. Precip then winds down Tuesday evening as surface low moves off to the east and surface winds over New England turn from the northwest. Temps aloft Monday support surface values in the mid to upper 20s, maybe around 30 on the South Coast. Dews of 5-15F and light wind would leave room for min sfc temps 10-20 with some cold spots in the high single numbers. Low level temps Tuesday support mid-upper 30s south coast and upper 20s-low 30s inland. Northwest wind and colder lower layer temps support Tuesday night min temps roughly 15-25F. Wednesday-Thursday... High pressure weakly builds over the region bringing dry weather Wednesday and Thursday, but with moisture again increasing in the mid and high level cloud levels. Surface winds turn from the east Thursday night, which would bring increasing lower clouds and a chance of precipitation after midnight. Temps at 950-mb each day are fcst in the range from minus 8C to minus 11C, supporting max sfc temps in the 20s. Wednesday could be a few degrees less cold along the coast with highs in the lower 30s. Friday-Saturday... Upper shortwave generates coastal low along the Carolinas Friday which moves up the coast and passes offshore of Srn New England Friday night, then moves to the Maritimes Saturday. GFS brings the track over the 40N/70W benchmark with a cold air dam signal over interior Srn New England. This would suggest a snow signal from early Friday morning through Friday night as the precip shield moves in. Easterly winds at the surface would suggest a chance to rain along the coasts. But as noted above, diverging model solutions bring lower confidence in details at this time. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent Through 00Z...High confidence VFR conditions expected today with gusty W to WSW winds of 25 to 35 kts. Expect the 35 kt gusts to be more likely over Cape Cod and the Islands. May see FEW to BKN cloud bases around 5 kft per the RAP/NAM BUFKIT soundings and per the latest satellite obs over Upstate NY into western MA. Tonight...High confidence VFR with an increase in mid to high clouds late. Winds becoming light and shifting from the W to the N and eventually the NE/E. Sunday...High confidence on trends, moderate on timing. Conditions deteriorating to IFR/LIFR from southwest to northeast during the morning and continue through the afternoon. This is in response to widespread snow that will fall heavy at times, especially along and southeast of a Hartford to Worcester to Boston line. Ptype should be snow for much of the region, except for the immediate Eastern MA coast as well as the Cape and Islands where rain and/or sleet is expected to mix in. E winds are expected through 18z, then turning NNE before becoming N between 21-23z. For BOS, expect -SN to move in by 16z, and may mix with rain through 18z before a quick burst of +SN between 19-22z. Winds wise, expect gusts between 15 and 25 knots along the I-95 corridor and 25 to 35 knots across the Cape/Islands. We may even see a few gusts around 40 knots across Nantucket. Sunday night...High confidence. Mainly VFR with localized MVFR. WNW winds 10 to 15 knots with gusts 25 to 30 knots possible across the Cape terminals. KBOS TAF...High confidence on trends, moderate on timing. KBDL TAF...High confidence on trends, moderate on timing. Outlook /Monday through Thursday/... Monday: VFR. Breezy. Monday Night: VFR. Tuesday: Mainly IFR, with local MVFR possible. Breezy. SN likely. Tuesday Night: Mainly VFR, with areas IFR possible. Breezy. SN likely. Wednesday: VFR. Breezy. && .MARINE... * Gale Warnings issued for all outer waters with Small Craft Advisory for Boston Harbor and Narragansett Bay effective Sunday into Sunday night. Tonight...High Confidence. Winds diminish to small craft advisory levels by early evening and below that by around midnight. This is response to a brief ridge of high pressure building to our south. However, seas will remain rather high given lingering southerly swell. Sunday and Sunday night...High Confidence. A fast moving low pressure system will lift northeast from the mid Atlantic and pass near the 70W/40N benchmark Sunday afternoon and early evening. NE small craft wind gusts of 25 to 35 knots expected across our northern waters with 35 to 45 knot gusts possible over our southern waters. Outlook /Monday through Thursday/... Monday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain, chance of snow. Areas of visibility 1 to 3 nm. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Rain likely, chance of snow. Visibility 1 to 3 nm. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory from 7 AM Sunday to 1 AM EST Monday for CTZ002. Winter Storm Warning from 7 AM Sunday to 1 AM EST Monday for CTZ003-004. MA...Winter Weather Advisory from 7 AM Sunday to 1 AM EST Monday for MAZ002-003-008>011-023-024. Winter Storm Warning from 7 AM Sunday to 1 AM EST Monday for MAZ004>007-012>022-026. RI...Winter Weather Advisory from 7 AM Sunday to 1 AM EST Monday for RIZ008. Winter Storm Warning from 7 AM Sunday to 1 AM EST Monday for RIZ001>007. MARINE...Gale Warning from 7 AM Sunday to 1 AM EST Monday for ANZ231>235-237-250-251-254>256. Small Craft Advisory from 1 PM Sunday to 1 AM EST Monday for ANZ230-236. && $$ SYNOPSIS...WTB/Chai NEAR TERM...Chai SHORT TERM...Chai LONG TERM...WTB AVIATION...WTB/Chai MARINE...WTB/Chai