Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 082104 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 404 PM EST Thu Dec 8 2016 .SYNOPSIS... A strong front crosses the region late today. Mainly dry, but unseasonably cold temperatures are on tap for our region Friday into Sunday. Some mixed wintry precipitation is looking more likely late Sunday evening into the day on Monday, but some uncertainty remains. Cooler conditions return by mid next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Rest of today... Scattered to broken cloud decks in association with the cold front sweeping offshore with lake effect fetch to the rear. Some of that lake effect is getting across the Berkshires and per local webcams seeing the associated snow shower activity push visibilities down to a mile or two. Have chance PoPs for the E slopes of the Berks. Monitoring winds upstream gusting 20 to 25 mph. Should see that momentum work into W New England late along with much colder air. Cold air advection proceeding with the winds, will see temperatures begin to plummet. Tonight... Turning colder. Cold air advection proceeding behind the initial cold frontal boundary with H85 temperatures down around -10C by Friday morning. Lapse rates becoming well-mixed nearly as high as H5, will see the mix down of faster momentum and drier air. Just some question as to the strength of the winds within and at the top of the mixed layer and how effectively they`ll mix down, whether there will be some stability right at the surface that`ll limit mixing. Considering this along with the trends upstream, given the magnitude of cold air advection, believe we`ll see westerly winds sustained around 10 mph with gusts 15 to 20 mph, strongest across the high terrain and over the waters with gusts up to 30 mph. Maybe perhaps shallow interior valleys will be spared strong winds over- night. With lows dropping down into the 20s looking at wind chills down into the teens in spots. Scattered to broken cloud decks subsequent of cyclonic flow and fetch off the warmer lakes. Decent signal of snow squalls possibly into the Berkshires. Depends on availability of moisture within the first 2 km above the surface. Will go ahead with chance PoPs for the E slopes of the Berkshires with slighter chances to nil elsewhere with downsloping wind off the high terrain. Light snow accumulations of around a tenth or two possible along with restrictions to visibility of a mile or two. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday into Friday night... Cold conditions continue. Cold air advection continuing aloft with lapse rates remaining well-mixed to at least H85. Will see weak ridging and drier air around H8-3 aloft move across the region limiting deep layer mixing, but with well-mixed low levels, expect scattered to broken cloud decks to continue especially with the reinforcing arctic front around the morning hours. Continued cyclonic flow and fetch off the warmer lakes, maybe a flurry over the Berks. Been a challenge to forecast the magnitude of the winds, what will effectively mix-down to the surface and what will the winds be aloft. NAM seems to be the most robust guidance. Overall anticipate NW blustery flow 10 to 15 kts sustained with gusts up to 35 kts immediately behind the arctic front, relaxing towards Saturday morning as high pressure builds in from the W and the pressure gradient begins to relax. Going to feel some 10 degrees colder than actual. Looking at wind chills overnight in the single digits. Looking at highs around the low to mid 30s during the day, lows dropping down into the teens. && .LONG TERM /SATURDAY THROUGH THURSDAY/... */ Highlights... * Cool and dry Sat into early Sun. * Mixed wintry precip with some impact late Sun into Mon. * Some uncertainty remains with p-types. * Cooler, but unsettled possible later next week. */ Overview and model preferences... The long term looks to favor very broad, un-amplified and nearly zonal W-E mean jet across the N tier of the CONUS through the long term. While this might suggest relatively persistent conditions, there are a couple of waves that will need to be watched as go through next week. While there remains fairly good agreement on the large scale, the smaller scale features and thermal profiles remain somewhat in flux, and these will have everything to do with the final sensible wx details. The first wave of note is currently wrapped within the vortex sitting of the British Columbian coastline at this time, it will weaken and open as it moves across the CONUS into Sun, but maintains enough energy to provide a focal point for a two phased mixed-precip event that should begin by late Sun and continue into Mon. The second wave is actually allowed access to the northeast via the height falls provided by the initial late weekend/early week wave. It is a stronger vortex associated with an arctic influenced airmass from Nunavut and the Northwest Territories. This could provide a focal point for a much more seasonably cold late week period. Given there are good agreements here in the broad term, and the uncertainty of small scale features and thermal profiles, a consensus blend of 08.12Z operational models will provide the baseline for this long term forecast update. */ Details... Sat into early Sun... Chilly conditions in spite of slight warming at H92. The issue at hand is the warming at this level is from -14C to -12C, still almost a full standard deviation below climatological normal for mid December. Full mixing will still only feature highs reaching the upper 20s and low 30s, as several locations are unlikely to even reach the freezing mark. By Sun night with weak high pres ridge at crest, the combination of clearing and weak flow should yield good radiational cooling in spite of the lack of an established snowpack. Looking at lows falling into the low teens and possibly even a few single digits. Highs on Sun before the loss of sunshine should reach once again near or even slightly above freezing as the H92 temps continue to increase. Sun night into Mon... A two phased event as inland low pres combines with weak wave development along a stalled frontal boundary to the S. Moisture increases to nearly 0.75 in PWATs at their peak on Mon, so moisture is generally not lacking. Initial overrunning setup under return flow N of the stalled front could yield an initial round of light snowfall, although noting that within the column, omega is rather light within the snow growth regime. Still increased LLJ near 40 kt could help this. Initial thermal profiles through the Sun evening hours are supportive of snow nearly universally across S New England, so the initial start could feature measurable snowfall even as thermal profiles warm the overnight hours. QPF totals are generally 0.1 inches or less, so the final totals should generally remain around an inch or less assuming this holds. The secondary and potentially higher impact feature arrives by early Mon AM in the form of a frontal wave developing near the Delmarva, ahead of the parent low pres in the Great Lakes. This increases deformation in the mid lvls N of the secondary wave, adds moisture (this is when the aforementioned PWATs peak). It also looks to increase omega within the snow growth region although the bulk of the moisture is actually below it per latest profiles (this is likely a result of the warming expected, raising the height of the snow growth regime). Therefore, final precip types and totals are a uncertain. Precipitation/Snowfall...The initial overrunning during the overnight is generally cold enough for all snow, but the dynamics (omega, deformation, moisture) are lacking in comparison to the Mon time-frame. The second wave for Mon, features better dynamics but much more marginal thermal profiles as H92 increases above 0C across CT/RI/SE MA by Mon afternoon on several ensembles and operational guidance. Even with more favorable high pres setup to the N yielding stronger Nly isallobaric flow, the warm advection supplied by the 40 kt LLJ will be able to counter. Therefore, S to start, but gradual transition to maybe light icing before a change to rain for some is certainly possible. Counting the initial overrunning snow potential, have moderate confidence that some portions of interior S New England could see plowable snowfall especially if the thermal profiles remain cold enough through the day given the Nly isallobaric flow mentioned early. Further S, totals could be less and feature more of a mix of P-types. In any case, the early AM commute is likely to be impacted with wintry precip across much of the region. For what it`s worth, ensemble 24 hour probabilities of 3 inches or more of snow are as high as 60-80 percent in NW MA, dropping to as low as 10-20 percent across RI/SE MA and CT due to uncertainty in the timing of warming in the lowest levels and amount of moisture/lift in the snow growth regime increasing SLRs. Timing...Lightest snowfall begins Sun evening from S-N overspreading into the region through midnight. Heaviest should begin through the early AM hours and continue into the commute. Some precip changing would begin mainly after sunrise and continue to attempt to lift N through the daylight hours. Note, more specific timing of the start/end and changeover will be coming over the next 48 hours. Tue... Transitional period with arctic frontal passage and drying behind the exiting low pres centers. Wind chills could be quite low during this period, but there are some differences in the pres gradient that will need to be resolved. In any case, colder wx is expected as H92 temps settle back to nearly -6C. Some ocean effect snows possible, if winds are more N oriented given nearly 18C SST- H85 delta-Ts. Unsettled conditions under cold advection could also allow for occasional flurries, but these would be hit or miss. Highs return to the mid 30s to near 40. Wed and Thu... In spite of the good agreement synoptically, uncertainty continues in the sfc pattern as cold vortex continues to shift E-SE. Weak energy from this vortex looks to create a second system developing near the OH valley and shifting offshore. Ranges in track between ensembles and operational models are from well to the S (far enough for a complete miss and much colder temperatures) to near or even within the 40/70 benchmark (potential for another round of wintry precipitation). Given the zonal flow, timing/track very uncertain and depends on how the early week system ultimately develops. In any case, looking at back to seasonably colder conditions. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. 21z update... Rest of today... VFR. SCT-BKN cigs. Winds turning W/NW increasing. Gusts 20 to 25 kts emerging out of Upstate NY. Should starting seeing this faster momentum work into the W New England terminals late. -SN possible for the Berkshires and points immediately E along with IFR visibilities. Tonight... VFR. Blustery W/NW winds especially along the immediate coast and high terrain with gusts up around 25 kts. Sustained around 5 to 10 kts. Interior low-lying terminals within the valleys are expected to see less wind. -SN possible for the Berkshires with accompanying IFR visibilities. Friday into Friday night... VFR. NW winds continuing. Sustained around 10 to 15 kts, gusts up to 35 kts, diminishing late towards Saturday morning. SCT-BKN low-end VFR cigs. KBOS TAF...W/NW flow on the increase overnight. Gusts up to 35 kts possible going into and through Friday. KBDL TAF...NW winds gusting up to 20 kts tonight, increasing on Friday before diminishing late. Outlook /Saturday through Tuesday/... Saturday into Sunday afternoon...High confidence. VFR. Winds will shift from the W Sat, to the S on Sun. Some light flurries possible during the day on Sat, but clearing and cold is expected Sat night into early Sun. Late Sunday through Monday...Moderate confidence. Mix of IFR/LIFR at times possible as light snow spreads over the region from S-N during the overnight and early Mon morning period. Some of this changes to a mix of light ice and rain, mainly along the CT/RI/SE MA borders and points S, while northern areas could remain all SN into the day on Mon. Ice/Snowfall amounts are somewhat uncertain, but some area runways may need to be plowed. Winds mainly NE. Tuesday...Moderate confidence. Expect improvement Mon night into Tue as winds shift from the NE to N to NNW. Some gusts on tuesday may range between 25-35 kt at times especially along coastal locations. More widespread VFR as mixed precipitation ends. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. W/NW winds on the increase with the likelihood of gales on the E waters but can not rule out gales on the S waters as well. May need to update headlines in the near-term. Looking at winds increasing into this evening, becoming sustained around 10 to 15 kts beginning tonight with gusts 30 to 35 kts. Wave heights building 5 to 8 feet on the outer waters. Will begin to see a diminishing trend into Saturday morning. Outlook /Saturday through Tuesday... Saturday into Sunday...High confidence. W winds will continue to gust to around 25 kt int Sat afternoon while seas also recede. Therefore, small craft advisories will continue much of the day, but be dropped by evening and overnight. On Sun, generally quiet boating weather is expected with winds shifting out of the S. Monday...Moderate confidence. Winds shift to the northeast through the day, but should generally remain between 10 and 20 kt. Seas will gradually build as low pres develops well to the S, but generally should remain at or below 5 ft. Snow/changing to rain is expected Sun night into the day on Mon, with some vsby restrictions likely. Tuesday...Moderate confidence. Rainfall will be ending and winds gradually shift from NE, to N, to NNW early Tue morning into the daylight hours. Wind gusts could approach gale force, between 35-40 kt with seas building to 7-9 ft. Small craft advisories are likely, but Gales may also be needed. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST Saturday for ANZ231>235- 237-255-256. Small Craft Advisory from 7 PM this evening to 1 AM EST Saturday for ANZ230. Small Craft Advisory from 7 PM this evening to 4 AM EST Saturday for ANZ236. Gale Warning from 10 AM to 10 PM EST Friday for ANZ250-251-254. && $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Doody AVIATION...Doody/Sipprell MARINE...Doody/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.