603 FXUS61 KBOX 051159 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 659 AM EST Sat Jan 5 2019 .SYNOPSIS... A low pressure system tracking northeast from the mid Atlantic coast will bring rain today which will be heavy at times near the south coast. Pockets of freezing rain are expected across portions of interior MA this morning. Dry, blustery conditions will follow Sunday, then colder temperatures arrive for Sunday night and Monday. Another system will likely bring a transition to snow to rain Monday night through Tuesday. Potential bouts of snow or rain Tuesday night through Wednesday night. Cold and dry conditions Thursday through Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7 am update ... Warm rain falling out of a mild layer immediately above the shallow surface cold air contributing to rising 2m temperatures this morning allowing the WINTER WEATHER ADVISORY to expire northward. Expiration set for 10 am but it is entirely possible associated headlines will drop off earlier as there are only isolated spots at this hour that are at or below freezing in N/W MA. Forecast update incorporates a consensus of high-res forecast guidance, though the high-res NAM nest is doing the best with the HRRR guidance following with 2m temperatures and precipitation outcomes. A glaze of ice still possible. Trends going forward of the warm-frontal precip-shield slowing its northward progress, coalescing and wrapping around the mid-upper level low feature as seen via latest satellite before exiting out this evening. The threat of moderate to heavy rain, even a rumble of thunder as outlined in the prior forecaster discussion below. Previous discussion ... GOES 16 imagery shows well defined mid level low over SW VA and western NC with nice baroclinic leaf along the east coast with colder cloud tops lifting north into New Eng. There is good model agreement with low moving NE from the Delmarva and tracking between ACK and the benchmark late today and this evening. Deep moisture plume within the warm conveyor belt and deformation/fgen NW of the mid level low track will bring a period of heavy rain to SNE, mainly near the south coast. There will likely be a sharp cut off in QPF amounts north of the Pike given upper level confluent flow and drier to the north with the QPF gradient across SNE. The main concerns with this storm are freezing rain potential this morning across interior MA and heavy rainfall and possible t-storms near the coast. Rainfall... Decent PWAT plume moves int the SNE today with values exceeding 1 inch which is 2+ SD above normal. Wind anomalies are not that impressive as low level jet is a modest 30-40 kt. Rainfall this morning assocd with initial burst of warm advection, then closed mid level low track south of New Eng with mid level deformation and frontogenesis impacting SNE mid to late afternoon. This is when heaviest rainfall will likely occur, focused across RI and SE MA. In addition, instability parameters are favorable for isold thunder near the coast as steep mid level lapse rates north of the mid level low overspreads SNE with some elevated CAPE. This will enhance precip amounts. Used a blend of hi-res guidance for QPF which indicates rainfall 1-2 inches across southern RI and SE MA. HREF has fairly high probs of 2 inches near the south coast. Localized amounts exceeding 2 inches are possible near the south coast if convection plays a role. Do not expect river flooding, but some minor flooding of urban and poor drainage areas is possible. Confidence in rainfall amounts is lower further north given expected gradient. Current forecast has 0.10-0.20" near NH border to around 0.50" near the Mas Pike. Wind is not expected to be a big factor given modest low level jet. NE wind gusts to 30 mph expected this afternoon for the Cape and Islands. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight... Rain will be focused along and east of the I-95 corridor this evening, but will be quickly moving east with rain exiting the Cape/islands by midnight as low pres pulls away. Partial clearing is expected overnight. Gusty north winds Cape/Islands will diminish overnight. Sunday... Multiple shortwaves will rotate through SNE. First in the morning, then a secondary shortwave during the afternoon. Moisture is rather limited and the column is drying so not expecting much more than a few brief rain/snow showers over central/western MA. Expect broken diurnal cu developing which will limit sunshine at times. The secondary shortwave will bring a surge of gusty NW winds in the afternoon as gradient tightens. Well mixed boundary layer supports gusts to 30-40 mph. Highs mid 30s higher terrain to low/mid 40s coastal plain. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... */ Highlights ... - Potential coldest air of the season Monday morning - Generally snow to rain Monday night through Tuesday - Potential bouts of rain/snow Tuesday night into Wednesday night - Mainly cold and dry Thursday through Saturday */ Overview ... Interpreting signals further into Winter. Persistent Africa through N Pacific H3 jet concluding-split W of the CONUS. Continual energy, CONUS through N Atlantic, undergoing perturbations, amplification, and interactions, yielding abundant potential outcomes difficult to nail down with any certainty. Meanwhile sudden stratospheric warming and polar vortex disruption, potential stratospheric - tropospheric coupling to several gyres (i.e., N Canada) rotating round the N pole oncoming. Higher amplitude pattern, greater blocking? EC control is going considerably negative with AO / NAO trends into early February suggesting Arctic air release round the pole along with N Atlantic blocking. A weak +ENSO, MJO transitioning from phase 6 to 8 per the EC, monitoring for E propagation of sub-tropical Pacific low-level wind anomalies in tandem with long-range signals. Hopefully insight into trends into Winter. Break-down details, forecast thinking in the discussion below. Would put greatest confidence in EC / ECens given mid-range chaos brought about by NE Pacific split-flow and echoing perturbations downstream poorly handled among deterministic forecast model solutions. Not sold on day 5 forecasts onward. */ Discussion ... Sunday night through Monday ... Winds taper with 1030+ Canadian high building S. Higher confidence W CT / W MA of light winds, clearing conditions, yielding radiational cooling. Lean coldest of guidance. NW 20-30 mph gusts linger for E / SE coastal MA and RI. Single digits certainly possible along the Berkshires. Keep it dry Monday with increasing clouds late. Holding widespread near or sub-freezing temperatures for highs given light winds overall, coastal return S flow whereas can't rule out interior N drainage flow per E / NE ageostrophic flow in response to pressure falls in association with a Great Lakes low center and attendant frontal boundaries sweeping towards the NE CONUS by Monday evening. Monday night through Tuesday ... Snow transitions to rain, with a low risk of freezing rain. Less mid- Atlantic secondary surface low development given more of a flat wave progression aloft. Front-end thump, isentropic upslope, focus along a lifting warm frontal boundary. However less influence of cold air entrenched given E/NE ageostrophic flow. Cold air retreating SW - NE while watching ice presence within the column. Would think the warm front wake dry-punch would wipe out precipitation chances, but top- down drying above entrenched shallow cold air could yield a low risk of freezing rain / drizzle briefly before clearing out. Watching the Merrimack River Valley. Ensemble probabilities have pulled back on snow amount likelihood, CIPS Analogs as well. Advisory-level snows remain possible for Berkshires, Worcester Hills before the change- over to rain. Continued concern for the Tuesday AM commute. Clearing out, warming up through Tuesday beneath the warm sector. Could see highs top out close to 50 degrees. Tuesday night into Wednesday night ... Potential bouts of rain / snow. Downstream traffic allowing upstream NE CONUS storm maturation / troughing. Main mid-upper level low with attendant vortmax energy deepens as it matures and digs. Acting on a continental airmass, low-level lift beneath stout mid-level ascent through the dendritic growth zone, plus falling heights / cyclonic curvature, secondary punch associated with surface reflection low pressure center and follow-up sweeping cold front, comes down to 2m temperatures, saturating / wet-bulbing low-levels, with respect to precip-type. Messy, low confidence on outcomes. Earlier advertised as quiet, cold and dry, the game has changed. So hedge patience and later forecast model runs to iron out details. Consistency through majority of guidance on deeper troughing, rotating energy, ironing out details and potential impacts the challenge. ECens probabilities suggest a low-risk of advisory level snows N/W MA. Thursday through Saturday ... Scattered snow shower activity at onset, whether it be associated with lake fetch and / or rather continued ascent beneath lingering mid to upper level trof, energy rotating within acting upon low to mid level moisture. But decent signal clearing out, 1030+ Canadian high setting S across the NE CONUS, turning colder with coldest air set for Saturday morning, renewed risk of single digit lows N and W given mostly clear conditions and light winds. Sunday onward ... Not sold on anything. Wiggles, perturbations, resultant amplitudes, etc., considerable forecast challenges as to our next potential storm system. Low confidence, stay tuned to later forecasts. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... 12z update ... Through today...Moderate confidence. Cigs lowering to MVFR/IFR from south to north 10-15z as -RA/RA overspreads the region. Lingering pockets of -FZRA over N/W MA the next hour or two. Focus shifts towards RA/+RA with isolated TSRA possible across RI and SE MA this afternoon. NE wind gusts to 30 kt developing in the afternoon across Cape / Islands. Tonight...High confidence. LIFR eastern New Eng and MVFR/IFR elsewhere to start the evening, then improving to VFR from west to east late evening through overnight with partial clearing. Rain exits SE New Eng in the evening. N wind gusts to 25-30 kt Cape/Islands diminishing overnight. Sunday...High confidence. Mainly low end VFR cigs developing with patchy MVFR possible. NW gusts to 25-35 kt developing in the afternoon. KBOS Terminal...Moderate confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR. Windy with gusts to 35 kt. Monday: VFR. Breezy. Slight chance SN. Monday Night: Mainly VFR, with local IFR possible. Breezy. SN likely. Tuesday: Mainly VFR, with local IFR possible. Breezy. Chance RA, SN likely. Tuesday Night: Mainly VFR, with local MVFR possible. Chance RA, chance SN. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Chance RA, chance SN. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Today... Increasing NE winds this afternoon with gusts 20-30 kt developing, strongest over southern waters. SCA in effect. Vsbys reduced in rain and fog. Isold t-storm possible this afternoon south coastal waters. Tonight... Northerly winds diminishing below SCA overnight. Rain exits by midnight with improving vsbys. Sunday...Increasing NW winds with potential for gales with gusts 35 kt. Outlook /Sunday Night through Wednesday/... Sunday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. 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. Chance of snow, rain likely. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Rain likely. Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain. Wednesday: Winds less than 25 kt. Local rough seas. Chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Weather Advisory until 10 AM EST this morning for MAZ002>004-008-026. RI...None. MARINE...Small Craft Advisory from 11 AM this morning to 1 AM EST Sunday for ANZ232>235-237. Small Craft Advisory from 1 PM this afternoon to 1 AM EST Sunday for ANZ231-251. Small Craft Advisory from 1 PM this afternoon to 10 PM EST this evening for ANZ236. Small Craft Advisory from 1 PM this afternoon to 7 AM EST Sunday for ANZ250-254. Small Craft Advisory until 7 AM EST Sunday for ANZ255-256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell