Area Forecast Discussion
Issued by NWS Boston, MA

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041 FXUS61 KBOX 051500 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1000 AM EST Mon Dec 5 2016 .SYNOPSIS... Brief period of light to moderate snow during the morning commute will result in snow covered roads especially across CT...western and central MA. Steadiest snow occurs early this morning then mix and/or change over to rain toward midday from south to north. Then partial clearing late in the day. Dry Monday night into Tuesday beneath high pressure. Low pressure off the mid Atlantic coast may bring some mixed light rain and/or snow into Southern New England Tuesday night into Wednesday. A polar front crosses the region late Thursday or Thursday night, with the chance for rain and snow showers. Very windy and cold conditions are then expected for Friday and next Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1000 AM Update... *** Snow gradually shifts to rain and dissipates through afternoon. *** Radar telling the tale this morning. Drier air already begun to dissipate remnant snizzle through the CT valley, and this drier air will continue to push E through mid day allowing all precip to give way to clearing skies through the afternoon. Otherwise, warmer air aloft continues to push N from RI/SE MA through 10-20 mph N of KBOX per ZDR and this continues to push NNW. So gradual transition to rain then clearing is expected through mid day (although possible later Cape/Islands). The good news is the dry air push/subsidence inversion should lead to pretty rapid clearing of even the low clouds. Timing has been closely adjusted to latest HRRR. Previous Discussion... Snowfall... Warm advection snows entering NJ into eastern NY at 3 am. Column cold enough to support snow across all of southern New England. Bulk of the short wave energy tracks northwest of southern New England. Thus best forcing for ascent tracks across NY state into northern New England. Hence upward vertical motion is modest across our region. In addition system is very progressive limiting duration of precip. Furthermore deep layer moisture is lacking. This may be why bulk of guidance only has likely pops and not categorical. All of these parameters will limit snowfall potential. Both NAM and GFS show modest omega in the snow growth region from 09z to 12z across southern New England from west to east this morning. However during this time low level dry air hangs tough and by the time the column saturates dry air aloft is rapidly advecting eastward into the snow growth region. This will limit duration of steady/moderate snow to about only an hour or two (3 hrs at the most) this morning from 09z to 12z west to east. Given this leaned toward the lighter qpf from the 00z GFS/EC and RGEM. This yields snowfall potential of a coating to an inch across RI and eastern MA...before mixing or changing to rain later this morning before ending this afternoon. Farther inland (north and west of I-95)...given snow band is expected to weaken from west to east...slightly more snow inland with 1-2" likely. Low risk for a few spots receiving up to 3" across western CT/MA before snow band weakens eastward. Also low risk for a few interior spots receiving a brief period of freezing drizzle later this morning as dry air advects into the snow growth region...eroding ice crystals with supercooled droplets in warmer clouds below dominating. So in a nutshell some slippery travel for the morning commute along and northwest of I-95 corridor in RI and eastern MA. Southeast of this region duration of snow will be very brief. Temperatures... Cold this morning but once precip ends temps will rise into the upper 30s and lower 40s this afternoon. May even see a few breaks of sunshine mid to late afternoon CT...western-central MA and toward sunset RI and eastern MA. Winds will be fairly light today given weak pgrad, so not much of a wind chill to speak of. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... 345 am update... Tonight... Strong mid level subsidence behind departing short wave which yields deep layer drying thru the column across southern New England tonight. Thus dry weather with mostly clear skies. Modest cold air advection behind this wave with blustery NW winds developing along the coast including Cape Cod and the islands. Seasonably cold with lows in the 20s...low 30s in Boston, Providence and the remainder of the immediate coastline including Cape Cod and the islands. Tuesday... Short wave ridging over the area will yield sunshine much of the day. However progressive upper air pattern with southern stream system approaching New England late in day resulting in clouds on the increase from SW to NE. Temps slightly colder than normal with highs in the upper 30s and low 40s. However it will feel seasonable given lots of sunshine and light winds with weak high pressure overhead. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Mixed precipitation possible Tuesday night into Wednesday morning * Very strong front crosses the region Thu or Thu night * Windy and cold into next weekend, with strong gales over the waters Tuesday night through Wednesday night...Moderate confidence. Not much change from previous thinking. Southern New England should be impacted by a mid level shortwave. Still some question how far north a low pressure will track from the Mid Atlantic coast. At this time, thinking the low track will remain too far south to pose a significant precipitation risk. Another low pressure over central Canada should also be too far away from our region to have a large influence. Still thinking there is the potential for a wintry mix of light rain and snow Tuesday night. On Wednesday, precipitation should diminish, especially during the afternoon. Wednesday night should be mainly dry, with our area in between systems. Thursday and Friday...Moderate confidence. Guidance starting to come into better agreement. But, with near zonal flow in our neck of the woods, some timing differences remain. Still looking like a cold front will pass by sometime before daybreak Friday. The greatest risk for precipitation should be toward the coasts, where better access to moisture exists. Chance for rain and/or snow showers across most of the region. Colder air will start to arrive Friday, as gusty west to northwest winds develop. This could lead to some lake-effect snow showers across portions of western MA. However, not confident these snow showers would move too far east of the Berkshires. This will need to be monitored. The gusty winds will lead to minimum wind chills in the single digits and teens Friday night. Saturday and Sunday...Moderate confidence. Dry weather expected Saturday, as a high pressure takes control of our weather. As this high pressure moves east Sunday, a weak low pressure may move close enough to provide a chance of some rain and snow showers. Moisture is still forecast to be limited, so this potential will need to be watched. The most noticeable change this weekend will be the temperature drop. Expecting high temperatures Saturday to be lower than our normal low temperature. Gusty west to northwest winds continue, especially near the coasts. It should feel like the teens during the day Saturday. Slightly warmer Sunday, as winds become more from the southwest. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12Z update...high forecast confidence on trends, but some uncertainty on exact timing of details. Leading edge of precip on radar not reaching the ground due to dry air eroding precip before reaching the surface. Steady/moderate snow in MVFR-IFR for only an hour or two before becoming lighter and intermittent between 12z-15z. Then snow changes to rain 15z-18z across RI and eastern MA, and possibly much of CT, too. Before then, snow accumulations of a coating to an inch for RI and eastern MA and 1-2" elsewhere, with a low risk of a few amounts up to 3" across western CT/MA. Also low risk for a period of freezing drizzle 12z-15z across the interior. Precip ends 18z-21z from west to east with conditions improving to MVFR/VFR 21z-24z from west to east. Tonight...high forecast confidence. VFR likely along with increasing NW winds up to 25 kt across Cape Cod and Islands. Lighter winds elsewhere. Dry weather prevails. Tuesday...high forecast confidence. VFR and diminishing winds. Dry weather prevails with any risk for precip holding off until after sunset. KBOS TAF...High confidence in trends. Moderate confidence in timing. MVFR with -SN around 12z-15z before mixing with and changing over to -RA 15z-18z. Snow accumulations of a coating to an inch possible. MVFR may improve to VFR toward 00z. KBDL TAF...High confidence in trends. Moderate confidence in timing. MVFR in SN 09z-12z then possibly mixing or changing to rain 15z- 18z. Snow accumulations of 1 to 2 inches with a low risk of snowfall amounts up to 3 inches. Also low risk for a period of freezing drizzle 12z-15z. Outlook /Tuesday Night through Friday/... Tuesday night and Wednesday...Moderate confidence. MVFR to local IFR ceilings Tue night into Wed morning in light rain RI/SE Mass, and a mix of rain/snow elsewhere. Improving to VFR Wed afternoon and night. Thursday...Moderate confidence. Mainly MVFR ceilings in areas of light rain showers ahead of approaching polar front. Depending on timing, rain showers could change to snow showers with local IFR conditions Thu afternoon/night. Friday...Moderate confidence. VFR except localized MVFR in isolated snow showers far interior MA. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... 345 am update... Today...high forecast confidence. Main issue will be vsby restriction in snow early this morning then snow changing to rain later this morning. Vsbys improve late today from west to east. Otherwise light winds today. Tonight...high forecast confidence. Low pressure develops east of New England resulting in NW winds of 20-25 kt. Thus SCA posted. Dry weather and good vsby. Tuesday...high forecast confidence. Diminishing winds as high pressure builds over New England. Dry weather and good vsby. Outlook /Tuesday Night through Friday/...Moderate confidence. Expecting winds and seas to remain below Small Craft Advisory levels Tuesday Night through much of Thursday. Winds and seas increase late Thursday into Thursday Night, particularly across the outer coastal waters. Small Craft Advisories may be needed sometime Thursday Night into Friday. Low risk of gale force winds Friday. That potential will need to be watched as this week progresses. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 7 PM this evening to 7 AM EST Tuesday for ANZ231>235-237-250-251-254>256. && $$ SYNOPSIS...Belk/Nocera NEAR TERM...Nocera/Doody SHORT TERM...Nocera LONG TERM...Belk AVIATION...Belk/Nocera MARINE...Belk/Nocera

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