000 FXUS61 KBOX 162057 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 357 PM EST Sat Feb 16 2019 .SYNOPSIS... High pressure will bring dry and seasonable weather tonight into Sunday. Confidence has increased in a light to moderate snowfall late Sunday night into Monday in association with a low pressure system. Drier and colder conditions follow Monday night into Tuesday. Another low approaches the region which should bring snow late Wednesday into Wednesday night, then possibly changing to a wintry mix inland, and over to rain along the coast before tapering off during Thursday. Dry conditions should return Friday into next weekend. && .NEAR TERM /THROUGH SUNDAY/... 320 pm update... Tonight... Quiet weather tonight as sprawling high pressure across Canada results in a dry cold night. Mainly clear skies and diminishing northwest winds will allow overnight lows to bottom out mainly in the teens to the lower 20s. Sunday... A ridge of high pressure overhead will result in a seasonable day for the middle of February. Plenty of sunshine will result in high temperatures reaching the middle to upper 30s. Winds will be rather light though, so pleasant weather for this time of year. && .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY/... * Accumulating snow late Sunday night into Monday Sunday night and Monday... Confidence has increased that accumulating snow will impact the region late Sunday night into Monday. However, specific snowfall totals and where the highest amounts occur remain uncertain. We will break things down a bit more below. A closed mid level low across the midwest will lift northeast and open up as it lifts towards the eastern Great Lakes. This in response to confluent flow to our north. It appears though that while differences remain, the trends of the guidance has been to push that confluent flow further north. This is likely in response to the upper level ridge off the southeast coast. So while confidence in accumulating snow has increased, uncertainty in amounts remains uncertain. The snow will be generated by two mechanisms which we will discuss below. Snow is expected to overspread the region from southwest to northeast roughly between 9 pm and 3 am Monday morning. This is a result of mid level warm advection overruning the cold dome at the surface. The NAM by far is most aggressive with this showing modest omega/forcing in the snow growth region all the way up to the NH border. Meanwhile...the ECMWF is on the south side of the envelope showing the bulk of the forcing confined to near the immediate south coast. The RGEM/UKMET/GFS are more in between showing the bulk of the forcing south of the MA turnpike but also clipping parts of western MA. Will have to see how things pan out, but this area has the best chance of receiving 2 to 4 inches of snow by daybreak Monday. Periods of snow are still expected during the day Monday with the focus shifting to eastern New England. This will be the second portion of the event as moist onshore low level flow combines with an inverted trough feature. It remains uncertain how far north this axis gets, but the east slopes of the Worcester Hills might be one favorable spot with possible upslope enhancement. The other area would be the eastern MA coast, where some land/sea interface and ocean enhancement may occur. This will again hinge on how far north this trough axis gets, but periods of snow will be possible into Monday afternoon with the focus shifting to eastern New England. Ptype looks to be mainly snow, although we can not rule out a period of sleet near the south coast. So in a nutshell...based on the latest data we are going with 2 to 4 inches of snow across southern New England. This will likely need to be refined with later forecasts. If the system trends further north, we may be dealing with a 3" to 7" type of event. A trend back to the south would likely result in mainly an inch or two for most of the region. No headlines will be issued with this package given the uncertainty and the event is still in the 3rd and 4th periods. However, they certainly may be needed by the next forecast package depending on model trends. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... * Frontal system exits Monday night * Large high pressure brings dry, cold conditions Tue and Wed * Another low approaches Wed night and Thu, snow to start then a wintry mix/possible change to rain across coastal plain * Another dry period late next week into next weekend Overview... Overall mid level steering pattern rather stagnant with blocking pattern across North America and beyond. Noting a high amplitude ridge across the eastern Pacific into British Columbia/NW Territories Mon night, with responding long wave trough sitting across the Rockies into the western Plains. With E-W elongated mid level high center extending from S FL across the central Atlc, this combination appears to keep a nearly zonal H5 flow from the north central Plains across the Great Lakes to New England for most of the medium range period. May see an H5 short wave eject from the western long wave trough, possibly riding across the region around the Wed night/Thu timeframe. Associated surface low pres may push out of the southern Mississippi/Ohio valleys in the W-SW flow aloft. Some question on track of this low, which lends to higher uncertainty as to timing and PTYPE regionwide. Details... Monday night... As the low exits to the central Atlc, last of light snow on the backside ends Monday evening across central and eastern areas. As colder air moves in on N-NW winds, may see gusts up to around 20 kt after midnight along the immediate E coast. Colder air works in across the interior after midnight as H85 temps fall to -7C to -10C, coldest across N central and W Mass. This will allow temps to bottom out in the upper single digits to mid teens there, ranging to 20-25 along the coast. Tuesday and Tuesday night... Large 1038 hPa high center across the northern Plains early Tuesday will slowly but steadily shift E, pushing across New England late Tuesday night. With the strong high to the W, noting a good pres gradient in place during the day. Also, 12Z GFS low level lapse rates rather strong Tuesday afternoon across the region from H925 to H95, so could see gusts mix down at around 20 kt or so. Still seeing rather chilly temperatures for mid to late February, running around 10 degrees below seasonal levels with highs only in the 20s inland to lower 30s along the coast. The pres gradient begins to relax Tue night as the center of the highs shifts into NY state, then into western areas overnight. Should see mostly clear skies early, then some high clouds may start to shift northward overnight. Expect another chilly night with lows again in the single digits and teens for most areas, except around 20 along the S coast. Wednesday and Thursday... As the large high slowly shifts E, expect light W winds that will start to back to SW during the day. With the continued W-SW wind flow aloft, will likely start to see mid and high clouds stream NE well ahead of low pressure across the southern Mississippi valley. It will remain chilly with highs similar to Wednesday, remaining near or below freezing. As the ridge builds offshore Wed afternoon and evening, 12Z models continue to signal deamplification of the approaching southern stream trough and associated surface system. Most of the deep moisture will pass offshore S of the region as seen on the 12Z GEFS PWAT SD values, but could still see QPF values up to 0.15 to 0.3 inches for most areas, possibly a bit higher across the inland high terrain with S-SE winds in place which could cause some decent orographic lift. Another issue with this system is PTYPE. Not much question that precip should start as light snow with the cold air in place Wed afternoon and evening. However, with the mid level short wave passing well N and W of the region, as well as surface winds shifting to SE-S, should see the milder air riding over the low level cold dome. At this point, should see a wintry mix of sleet and/or freezing rain push into southern areas by around midnight Wed night then steadily shift NE across most of the region from about Route 2 southward, especially across the interior, by the Thu morning commute. Still quite a few more model runs to see the overall evolution of this system. With the fast W-SW flow aloft, should see precip push E-NE during the morning and midday hours on Thursday. Temps should also rise close to or above freezing by midday or early afternoon just as most of the precip exits, expect for some lingering light rain and/or snow across N and E areas. Thursday night through Saturday... Clouds may linger early Thursday night, then will start to break around or after midnight. As another high slowly shifts E toward the region Friday into Saturday, current forecast suggests dry and seasonally cold conditions. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... Tonight...High confidence in VFR conditions. Light NW winds. Sunday...High confidence in VFR conditions. Light N winds becoming NE. Sunday night and Monday...Moderate confidence. Snow overspreads the region from southwest to northeast between 02z and 08z Monday. Periods of snow continue through Monday afternoon, especially across eastern New England. Ptype should mainly be snow, but a period of sleet is possible near the south coast. Overall, expect IFR conditions to dominate once snow begins but localized LIFR conditions are possible at times. 2 to 4 inches of snow seem reasonable for much of the region through Monday afternoon, but still uncertainty given track/location of the low pressure system. KBOS Terminal...High confidence in TAF. The Monday morning push will be impacted by light to perhaps moderate snow. KBDL Terminal...High confidence in TAF. The Monday morning push will be impacted by light to perhaps moderate snow. Outlook /Monday Night through Thursday/... Monday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SN. Tuesday through Tuesday Night: VFR. Breezy. Wednesday: Mainly VFR, with areas IFR possible. Chance SN. Wednesday Night: Mainly IFR, with local MVFR possible. Chance SN, chance FZRA, chance PL. Thursday: MVFR/IFR conditions possible. Breezy. Chance RA, chance SN, chance PL, chance FZRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Tonight...Northwest winds may occasionally gust to around 20 knots, but should generally remain below small craft criteria. However, will continue small craft headlines into this evening across the outer-waters for left over seas. These should diminish below criteria overnight. Sunday...High confidence. A ridge of high pressure will keep winds/seas below small craft advisory thresholds. Sunday night and Monday...Moderate to high confidence. A period of small craft wind gusts and seas may develop late Sunday night into Monday with a low pressure system. Highest risk will be across the southern waters. In addition, a mix of rain/snow and sleet will reduce vsbys for mariners at times late Sunday night into Monday. Outlook /Monday Night through Thursday/... Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow. Areas of visibility 1 to 3 nm. Tuesday: Winds less than 25 kt. Seas up to 5 ft. Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Wednesday: Winds less than 25 kt. Slight chance of snow. Areas of visibility 1 to 3 nm. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of snow, chance of freezing rain, chance of sleet. Visibility 1 to 3 nm. Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain, chance of snow, chance of freezing rain, chance of sleet. Areas of visibility 1 to 3 nm. && .TIDES/COASTAL FLOODING... High astronomical tides next week with several tides above 12 ft in Boston. Thus won't take much onshore flow to produce minor coastal flooding. Greatest risk comes late Wed into Thu as surface wave tracks south of New England providing onshore flow. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 11 PM EST this evening for ANZ250- 254>256. && $$ SYNOPSIS...Frank/EVT NEAR TERM...Frank SHORT TERM...Frank LONG TERM...EVT AVIATION...Frank/EVT MARINE...Frank/EVT TIDES/COASTAL FLOODING...Staff