Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 060616 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 116 AM EST Tue Dec 6 2016 .SYNOPSIS... Low pressure off the mid Atlantic coast may bring some mixed light rain and/or snow into Southern New England Tuesday night into early Wednesday. A cold front crosses the region late Thursday or Thursday evening with another cold front Friday. This will be followed by blustery and unseasonably cold conditions Friday night into Saturday. Low pressure may bring some snow or rain Sunday night into Monday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1 AM Update... Prior forecast still looking good overall. Dropped overnight temperatures include forecast lows a little across north central MA given current observation trends. Mostly clear skies and fresh snow cover, even though only an inch or two, is helping to drop temperatures rather sharply in that region. There continues to be clouds from moisture picked up over the eastern Great Lakes spilling over the Berkshire crest, but these should diminish during the early morning hours as surface high pressure edges toward New England. 1015 PM Update... Some lake effect clouds have been spilling over the Berkshires this evening and are expected to continue to do so until early AM when surface high pressure and shift to more of a NW or even NNW flow cuts off the moisture feed. Made some modifications to sky grids and only very minor tweaks to temperatures as forecast remains generally on track for the overnight. Will freshen up the black ice SPS shortly. 630 PM update... Lake moisture extending into western New Eng has resulted in some lower clouds across western MA and northern CT. Otherwise, mostly clear skies prevailed. Expect lake effect clouds to persist into the early morning hours across western MA, then clouds should erode as high pres builds in and flow weakens. Adjusted short term sky grids, otherwise forecast on track. Previous discussion... Expect less radiational cooling tonight as pres gradient increases with the approaching high and weak low pres development associated with the trof. Most of this will be felt offshore, but winds may remain 5-10 mph and mitigate temperature drop through the early portion of the overnight. Still temps will fall back into the 20s by the early AM hours thanks to weak CAA. Will be issuing a special statement for black ice, mainly for the interior as area roadways are unlikely to dry completely before temps cool once again below freezing. This may need to be expanded for areas that received less snow or all rain should the roads be observed as remaining wet through the evening hours.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Tomorrow... Mainly clear day to start although some CAA induced SC is possible, lingering from the overnight hours. Temps should moderate given the sunshine, thanks mainly to H92 temps still between 0C and -2C. Expecting highs then to rebound into the low- mid 40s. Winds will be light as they begin to veer from the NW to the E by evening, in advance of another 2-wave system approaching from the W and SW. Tomorrow night... Similar setup to last night as double-barreled system approaches with dominant shortwave remaining in the N stream and to the NW of S New England. Meanwhile, S stream wave initiates strong cyclogenesis offshore, such that coastal low pres strengthens and gradually shifts E, well S of the area by Wed morning. Therefore, most of the impact comes from an inverted trof and modest convergence between these two systems, rather than either one themselves. PWATs increase to about 130-150 percent of normal and modest lift is noted within the snow growth region as the column moistens from the top down. Therefore we are monitoring for the development of light snowfall late Tue night and continuing into Wed AM, similar to this past event. More detail to follow. Temps... Went lower than most guidance with temps as even with increased upper lvl, then lower lvl clouds, some modest radiational cooling during the evening could allow mins to drop into the upper 20s and low 30s before precipitation onset. Afternoon dwpts are lower, suggesting some wetbulb effect cooling is likely as the precip shield approaches from SW to NE. Snowfall/Rainfall... With this combination of cool temps and wetbulb, combined with modest moisture and some lift within the snow-growth regime, could see the development of another round of light accumulating snowfall across the region particularly during the AM hours Wed morning. Lift/temps suggest snow ratios between 10-12 per inch liquid, but rates will once again be light enough that only a widespread coating to an inch can be expected by sunrise. This may once again impact commuting traffic as some un-treated roads could become slippery. Areas across W Ma and the higher terrain may relieve more along the line of 1-3 inches, mainly due to better moisture convergence under E flow. This flow also will keep areas near shore, likely all rain thanks to warm SSTs. More information will come, but stay tuned due to the possible impact this may have on the Wed morning commute. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Mixed snow/rain lingering into Wed morning * Blustery and unseasonably cold Fri night into Sat * Some snow/rain possible Sun night into Mon Wednesday... Southern stream shortwave and attending coastal low pass well south of New Eng. However, inverted trof extending to the north combined with deep moisture axis will result in some light snow/rain lingering into the morning. Much of any accum will occur before 12z Wed. Then good mid level drying moves in from the west during the afternoon resulting in dry conditions with partial clearing possible toward evening. Thursday... Decent mid level shortwave passage and attending cold front in the afternoon and there is some indication that a weak wave may develop just to the east. But model consensus and GEFS/EPS ensembles suggest any precip will remain mostly offshore so looking at just a few snow/rain showers with the fropa. Seasonable temps. Friday and Saturday... The unseasonably cold air that moves in Fri night into Sat will be the main story as mainly dry weather expected with exception of a few flurries for the Berkshires on Fri. Secondary fropa brings 850 mb temps down to -10 to -12C Fri with further cooling to -14/-15C Sat. Friday will be a transition day with highs mostly in the 30s, cooling to the mid 20s to lower 30s Sat. Gusty NW winds both days will add to the chill with wind chills single numbers and teens Fri night into Sat. Sat night lows tricky and will depend on extent of radiational cooling. There may be some clouds and leftover wind which would limit cooling but potential exists for temps to be several degrees colder than forecast if it ends up clear with winds decoupling. Sunday into Monday... Next shortwave approaches from the Gt Lakes and there are significant differences in timing/amplitude between GFS and ECMWF. Low confidence on details but appears there is an increased risk of some snow/rain, sometime Sun night into Mon. Deterministic GFS is more threatening than ECMWF but very low confidence given it`s a Day 7 forecast and this will change. Most of the guidance and ensembles keep it dry with moderating temps on Sun. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Overnight...High confidence. VFR most locations with a couple of exceptions. MVFR ceilings will persist near the Berkshire crest through 08Z or 09Z. Also, there will be some patchy dense fog in some of the interior valleys. Tuesday...High confidence. VFR. Winds NW in morning becoming N and then NE during the afternoon. Tuesday night...High confidence in trends, lower confidence in timing. Similar setup to last night/this morning. VFR early gives way to light snow/rain mix (mainly snow first with light accumulations generally an inch or less). This transitions to rain during the morning. With the arrival of precip, expect category to drop to widespread IFR/MVFR. Winds mainly E. KBOS TAF...High confidence. VFR. Winds becoming N or NNW by around daybreak. KBDL TAF...High confidence. VFR. Outlook /Wednesday Night through Saturday/... Wednesday...Moderate confidence. Areas of MVFR/IFR in lingering snow/rain Wed morning, then gradual improvement in the afternoon. Thursday...Moderate confidence. Mainly VFR cigs but areas of MVFR possible interior with a few snow/rain showers. Friday and Saturday...Moderate confidence. Mainly VFR cigs. Localized MVFR in isolated snow showers far interior MA Fri.
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&& .MARINE...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... 1 AM Update... Have opted to keep Small Craft Advisories in place. MAREP suggested that winds had eased across Vineyard Sound but later reports from MVY showed some increase in gustiness. Hence, have opted to keep the Small Craft Advisory hoisted there. Noting that we are still getting gusts to around 30 knots just east of Cape Cod. Prior Discussion... Overnight into early Tue...High confidence. W-NW winds increase overnight as low pres shifts E of the waters and high pres moves in to take it`s place. Gusts around 25 kt can be expected across much of the waters as a result. Otherwise, this will lead to a slight building of seas, reaching 5-6 ft on the ocean waters which will linger into the early AM hours. Current SCAs seemed good, but will extend outer waters as seas may take a bit longer to diminish. Rest of tomorrow and tomorrow night...High confidence. High pres will slowly build from W-E across the waters leading to diminished winds and seas below small craft advisory thresholds. Some early morning rain is possible, mainly across the S and SE waters. Outlook /Wednesday Night through Saturday/... Wednesday...High confidence. Light winds and seas. Thursday...High confidence. Increasing W winds in the afternoon with gusts to 25-30 kt developing Thu night. Friday into Saturday...High confidence. NW gusts to 30 kt likely Fri/Fri night and there is a moderate risk for gale force gusts. Winds gradually diminishing Sat.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST this morning for ANZ231>235-237-251. Small Craft Advisory until 10 AM EST this morning for ANZ250- 254>256.
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&& $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC/Doody/Thompson SHORT TERM...Doody LONG TERM...KJC AVIATION...KJC/Doody/Thompson MARINE...KJC/Doody/Thompson

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