Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 090823 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 323 AM EST Fri Dec 9 2016 .SYNOPSIS... Mainly dry, but unseasonably cold temperatures are on tap for our region through 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 /THROUGH TODAY/... Colder air will continue to overspread southern New England. This will steepen low level lapse rates, and result in gusty west to northwest winds. The challenge has been trying to gauge how effective the momentum transfer will be. Winds early this morning were not that strong at the lower elevations, but gusting up to around 30 kt already at elevations about 1000 ft, or higher. Should the momentum transfer be maximized, we might come quite close to wind advisory criteria for gusts. At this time, am more confident in gusts just shy of needing advisories. This will need to be watched closely by the next shift. Other than the winds, expecting some diurnal clouds to develop across our region. Clouds will be more prevalent toward the Berkshires, where deeper moisture was pooling from the Great Lakes. Temperatures expected to be about 5-10 degrees lower than Thursday. The gusty winds however, will produce wind chill values in the teens and 20s. && .SHORT TERM /TONIGHT THROUGH 6 PM SATURDAY/... Our region should remain in between the major weather features tonight and Saturday. Mid level flow is fairly zonal, with a stronger shortwave arriving Saturday. At the surface, high pressure remains to our southwest, with a low pressure over the Maritimes. Expecting gusty northwest winds to continue, although not quite as strong as today. Not much chance for precipitation, due to lacking humidity. However, there should be enough moisture for some clouds as the shortwave arrives. Temperatures will be 10-15 degrees below normal. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... */ Highlights... * Cool and dry 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, unamplified, 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 off the coast of British Columbia 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 night into early Sun...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 snow to liquid ratios. 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 /07Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... Today into Tonight...High confidence. 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. Saturday...High confidence. VFR. NW winds diminishing. KBOS TAF...W/NW flow increases. Gusts up to 35 kts possible. KBDL TAF...W/NW flow increases. Gusts up to 30 kts possible. Outlook /Saturday Night 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 Saturday/...High confidence. W/NW winds on the increase with the likelihood of gales on most of the outer waters. Latest buoy reports show gusts already right around gale force. Will be expanding the gale warnings, soon. Rough seas also building through today. Winds and seas gradually diminish Saturday. Outlook /Saturday Night through Tuesday/... Saturday Night into Sunday...High confidence. W winds will continue to gust to around 25 kt int Sat afternoon while seas also recede. 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 until 1 AM EST Saturday for ANZ230. Small Craft Advisory until 4 AM EST Saturday for ANZ236. Gale Warning from 10 AM this morning to 10 PM EST this evening for ANZ250-251-254. && $$ SYNOPSIS...Belk/Doody NEAR TERM...Belk SHORT TERM...Belk LONG TERM...Doody AVIATION...Belk/Doody MARINE...Belk/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.