921 FXUS61 KBOX 061516 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1016 AM EST Sun Jan 6 2019 .SYNOPSIS... Weak disturbance moves across New England this afternoon, turning winds from the northwest at all levels. Expect dry weather, but also expect increasing wind which will draw colder air over us during the afternoon and generate some clouds. Turning much colder tonight and Monday with clearing skies. A frontal system will bring a transition to snow to rain Monday night through Tuesday, with more rain or snow Tuesday night into Wednesday as another storm moves across the region. Cold conditions follow Thursday through Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Shortwave trough axis at the NY Border at 15Z should sweep across Southern New England by mid afternoon. Morning soundings showed strong west winds at GYX and Chatham, but strong northwest winds across NY State and Quebec. This will bring colder air to the region, with models showing CAA after 18Z. The cold advection will encourage mixing to at least 900 mb and likely to 850 mb this afternoon. Winds in this mixed layer this morning were showing 25-30 kt, with areas of 40 kt moving down from Canada. Expect wind gusts of 30-40 mph from mid afternoon through early tonight. Once the cold advection diminishes, this wind should also diminish. Dew point depressions this morning at Chatham were quite high, but to our northwest...where the wind will be coming from this afternoon...the depressions are less than 5 degrees with some close to 0 degrees. Mixing this near-saturated air should generate more clouds this afternoon. Once the mixing ends, the clouds should dissipate. Tempertures in the mixed layer were -3C to -4C at 900 mb and -6C to -7C at 850 mb. This suggests max temps in the upper 30s and 40s, coolest in NW Mass where the cold advection would start soonest. If mixing doesn't reach 850 mb then the max temps would be upper 30s to mid 40s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Tonight... Gusty NW winds will persist through the first half of the night given the strong cold advection. Gusts 30-40 mph possible into the evening before winds gradually diminish. However, it will remain blustery through the night in the east and especially near the coast. Clearing skies expected. Lows will be mostly in the teens, closer to 20 in the urban areas and lower 20s Cape/Islands. However, with the persistent wind most of the night, wind chills will drop into the single numbers late tonight and Mon morning. Monday... Strong high pres builds to the north with ridge axis moving into SNE. Winds still gusty along the coast in the morning, then becoming light in the afternoon. Lots of sunshine although high clouds will be increasing in the afternoon. Shallow mixing combined with a cold start will keep high temps in the upper 20s and lower 30s. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... */ Highlights ... - Generally snow to rain Monday night thru Tuesday, low risk fzra - Transition back to snow Tuesday night into Wednesday morning - Blustery NW winds Wednesday into Wednesday night - Turning fairly cold Thursday through Saturday with high pressure */ Overview ... Evaluating trends. Poleward sub-tropical contributions, especially over the N Atlantic, per potential temperature along the dynamic tropopause, putting pressure up against a persistent Africa through N Pacific H3 jet. Several gyres stretched across the N hemisphere, undergoing perturbations, amplification, and interactions yielding abundant potential outcomes difficult to nail down with certainty. Inspecting 5-day average H5 height ensemble means, NW Russia and N Pacific gyres putting an additional poleward squeeze. W CONUS split flow complicating matters further. Question going forward as to a continual forecast polar vortex disruption noted in 50-10 mb height anomalies, whether there are stratospheric - tropospheric coupling interactions, tied into a 5-day average ensemble mean forecast of a tighter, buckled H5 height pattern over our neck of the woods. In other words, whether there's greater opportunity for deeper storm development in vicinity of New England going further into Winter. A lot of uncertainty and unknowns, but worthy of noting as the 03.0z EC control noted strong -AO / -NAO teleconnections late January on into early February, while a equatorial Pacific modest MJO slides E per phase 8 forecasts with potential E propagation of sub-tropical Pacific low-level wind anomalies from a weak +ENSO to the CONUS. A greater weighting towards EC / ECens, pattern appears to be turning stronger, but as to exact outcomes is largely unknown. Break down details, forecast thinking in the discussion below. Low confidence with forecasts beyond day 5 given increasing chaos as noted above. */ Discussion ... Monday night through Tuesday ... Still expecting onset snow after midnight transitioning to rain into Tuesday morning with a low risk of freezing rain in-between. Front- end warm frontal thump brought by low-level convergently focused H92- 85 SW isentropic upglide flow beneath broad H3 diffluence aloft. Yet a flattening wave, evident in low-mid level theta-E fields, rearward drying quickly moving in as synoptics stretch. Over 2m shallow cold air weakly maintained by NE ageostrophic drainage flow, wet-bulbing processes considered, anticipating mainly snow to rain with mild air pushing in aloft, however parent dry air thru the dendritic growth zone with potential brief pockets of trapped sub-freezing air before slowly warming with latent heat release associated with falling rain can't rule out some freezing rain / drizzle. An eye on valleys of N MA. Potential advisory level snows over the Berkshires and Worcester Hills per ensemble probabilities before rain change-over. Continued concern for the Tuesday AM commute. Clearing out, warming up through Tuesday beneath the warm sector into the 40s. Tuesday night into Wednesday ... Rain transitions to snow with a Tuesday evening low risk of isolated pockets of freezing rain. Main vortlobe rotating through a deepening mid-upper level low across the Gulf of ME as upstream traffic slows the longwave pattern promoting vicinity storm maturation. Negative- tilting, strong ascent netted beneath the left-front-quadrant of a cyclonic curving H3 jet, QG-forcing noted. Inverted trof and NORLUN tendencies with trowaling moisture noted by prior forecaster, lift promoted along the deepening Gulf of Maine surface low, especially along high terrain. Initial rain, low risk isolated freezing rain. Column cools with presence of ice, change-over to snow. Increasing confidence as to advisory-level snows for the Berkshires when all is said and done. Wednesday night through Thursday night ... Drying out Wednesday night into Thursday with NW winds increasing per isallobaric / gradient flow. Brief lake fetch, scattered snow showers before drying out as a 1040 Canadian highs begins to build S. Think that winds may be underdone in deterministic guidance given a vicinity quick deepening low with 2-3 mb / hr pressure falls, Wednesday into Wednesday evening especially, cold air advection, steepening lapse rates. Continued NW breezy winds Thursday. Friday onward ... Will hold with high pressure and cold air given persistent signal of a 1040+ high building S out of Canada. Low confidence as to outcomes and still not sold on anything beyond day 5. Wiggles, perturbations, resultant amplitudes, etc., considerable forecast challenges as to our next potential storm system. Low confidence, stay tuned to later forecasts. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... This afternoon...High confidence. Lingering 2500 foot cigs along the East Slopes of the Berkshires, but mostly 4000-5000 foot bases in Western and Central Mass which should spread east this afternoon. So most places will be VFR. Strong winds from 5000 feet AGL will be brought to the surface in strong gusts 25 to 35 kt this afternoon and evening. Could be isolated light snow showers coming off the Berkshires early this afternoon, but little or no accumulation. Tonight...High confidence. VFR with clearing conditions. NW gusts to 30 kt in the evening, then gradually diminishing overnight. Monday...High confidence. VFR. NW gusts to 25 kt along the immediate coast early, otherwise winds becoming light. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/... Monday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SN, slight chance FZRA. Tuesday: Mainly VFR, with local IFR possible. Breezy. RA likely, FZRA likely, chance SN. Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Chance RA, chance SN, slight chance FZRA. Wednesday: Mainly VFR, with areas MVFR possible. Windy with gusts to 30 kt. Chance SN, slight chance RA. Wednesday Night: Mainly VFR, with local MVFR possible. Windy with gusts to 35 kt. Slight chance SN. Thursday: Mainly VFR, with local MVFR possible. Windy with gusts to 35 kt. && .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. This afternoon and tonight... Increasing NW winds with a surge of gale force gusts to 35 kt developing by mid afternoon and lingering into this evening. Winds slowly diminish overnight but gusts remain above SCA. Rough seas with freezing spray developing on the waters tonight. Monday... SCA gusts in the morning but winds diminishing through the day and becoming light later in the afternoon as high pres builds over the waters. Outlook /Monday Night through Thursday/... Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain, chance of snow, slight chance of freezing rain. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Rain likely, chance of freezing rain. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of rain. Wednesday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Chance of rain. Wednesday Night: Moderate risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of rain. Thursday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning from 1 PM this afternoon to 11 PM EST this evening for ANZ232>235-237-255-256. Small Craft Advisory from 1 PM this afternoon to 7 AM EST Monday for ANZ230. Gale Warning from 1 PM this afternoon to 1 AM EST Monday for ANZ231-250-251-254. Small Craft Advisory until 7 AM EST Monday for ANZ236. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...WTB/KJC/Sipprell SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell