Area Forecast Discussion
Issued by NWS Boston, MA

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325 FXUS61 KBOX 042108 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 408 PM EST Sun Dec 4 2016 .SYNOPSIS...
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Accumulating snow for Monday morning which will make for a messy AM commute as roads will become slick and travel hazardous. Will mix and/or change over to snow before tapering off late. Dry Monday night into Tuesday with 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 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.
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&& .NEAR TERM /UNTIL 3 AM MONDAY MORNING/... High pressure along with clear conditions and light winds pushes E across the region. Prior to increasing mid to high level clouds towards midnight there will be a period of radiational cooling which should allow temperatures to drop rather abruptly down into the 20s before slowing with the increasing blanket overhead. Over- all dry with light N winds. && .SHORT TERM /3 AM MONDAY MORNING THROUGH MONDAY NIGHT/... */ Highlights... * Light to moderate snowfall forecast * Timing roughly around 4 am to 10 am * Impacts to the AM commute with slick roads, reduced visibilities * Highest snowfall amounts around 1-2" over Berks, Worcester Hills */ Discussion... Evaluating synoptics: Decent isentropic upslope and NW-SE fronto- genetical banding along the 280-310K theta surfaces (H9-5) through favorable snow growth regions of a continental tropical airmass. Appears secondary low development off the Mid-Atlantic will rob better moisture from proceeding N. Add in orographic support given the general SW flow through the low levels. This as the mid-level vortex across the NE CONUS flattens and deamplifies, sheared and stretched by progressive W flow with an accompanying punch of drier air. N winds keep cold, dry air parked ahead of synoptics turning S with passage allowing warm, moist air (around H925) to surge N. Altogether: along a diffuse warm-frontal boundary / inverted trough will see an over-running setup that weakens W to E due to diminishing synoptic lift and forcing and increasing top-down drying through the column. Perhaps a onset delay with lingering drier air, expect an initial light to moderate snow that weakens in intensity W to E as it mixes with and/or changes over to rain, especially closer to the coast. The N/W interior of S New England is likely to remain as all snow. Snow ratios averaging around 9:1. Timing: Focused around 4 am to 10 am Monday. Impacts: A slick AM commute with reduced visibilities down to a mile or two at times, especially for CT as well as western and central portions of MA. Anticipate snow accumulation along area roadways, especially for the Greater Hartford - Springfield - Worcester metro areas. Snowfall amounts: Around 1 to 2 inches for the high terrain of the Berkshires and Worcester Hills, around 0.5 to 1.5 for low elevation locations of western and central MA and N/NE CT, and a coating to an inch for the coastal plain, inclusive of E MA and all of RI, mainly around and S/E of the I-95 corridor. This in good agreement with ensemble probabilistics and CIPS analogs which showed similar signals. Monday afternoon into Monday night... Conditions begin to improve W to E beginning around midday. Will see winds increase out of the W before turning NW to N. Temperatures a bit tricky given winds and forecast snow cover which will likely hamper temperatures from warming much during midday-afternoon partial clearing. Thinking mid to upper 30s across the central and N-tier of S New England with low 40s along the S coast. Overnight, depending on the winds, if it remains breezy, if interior snow covered locales see light winds, then its possible temperatures will drop well down into the lower 20s. Clouds could be an issue from the N/W hampering radiational cooling. && .LONG TERM /TUESDAY THROUGH SUNDAY/...
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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...Moderate confidence. Area of high pressure extending from Quebec southward thru New England is expected to keep the majority of this period dry. However, given the expectation of low pressure advancing towards our area from the south/west, overrunning moisture will advance northeastward into our area, with an increase in high-mid deck clouds likely during the afternoon. Will continue to go with a slight/low chance pop for light rain/light snow late in the day in SW zones. Expect highs upper 30s-mid 40s. Tuesday night through Wednesday night...Moderate confidence. Models continue to show a fast moving, weakening mid level short wave in SW flow moving thru our area, while a weak low pressure system emerges off the mid Atlantic coast Tuesday night. A potential secondary low pressure in southern Great Lakes region is expected to weaken as it moves E. Mid-Atlantic low center is forecast to track well south of 40N/70W, with our area having relatively light QPF totals. Best chance for measurable precip should be Tuesday night into Wed morning, then as low moves further offshore, precip becomes more spotty/diminishing for Wed afternoon. At this time thinking there is the potential for a wintry mix of light rain and snow to all light snow across much of the area Tue night, then on Wed precip winding down but a changeover to light rain at least along the coastal plain. Wednesday night should be mainly dry with our area in between systems. Thursday and Friday...Moderate confidence. There continue to be some timing and track differences regarding polar front wrapping around cutoff upper low across Hudson Bay. Digging mid level long wave trof will bring a surge of very cold air into our area. GFS timing of front is Thu day, while ECMWF timing is more Thu night. Will go with a model compromise for this timeframe. Mainly chance pops since exact timing of frontal passage is still uncertain. Chance for rain and/or snow showers across most of the region. Colder air will continue to sweep into the region during Friday as W- NW winds quickly increase. While at this time most of Friday/Friday night should be dry, cold air advection combined with WNW flow going thru eastern Great Lakes Region could translate to a few snow showers making their way into the east slopes of the Berkshires. Also the potential exists for a tightening pressure gradient as a low over the Canadian Maritimes intensifies, which could allow for gusts of 25-35 kts. Wind chills Fri night dip into the teens/single digits. Saturday and Sunday...Moderate confidence. While mainly dry conditions are anticipated for Saturday with high pressure working eastward, may see lingering snow showers across the east slopes of the Berkshires, as well as ocean effect snow showers off the coast. Our area could still be caught in a tight pressure gradient with a deepening low pressure over the Canadian Maritimes. Thus could have winds gusting to 25-35 kt, strongest winds along east coastal MA and the nearby coastal waters. During Sunday models show potential for a weak low pressure system moving eastward towards/into our area. While moisture does not look abundant with this feature, it could bring some light snowfall to the area.
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&& .AVIATION /21Z SUNDAY THROUGH FRIDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 18z update... Short Term /through Monday Night/...High confidence. Rest of Today... VFR. NW winds diminishing. Tonight into Monday morning... Cigs lowering to MVFR with onset of -SN around 9-12z over W and Central MA and all of CT, around 12-15z for E MA and all of RI. IFR vsbys at 2SM with -SN. -RA mixing with -SN for coastal terminals with lesser impacts to visibility. Snow accumulations anticipated on non-coastal runways, around 1-2 inches forecast for the high terrain, around 0.5-1.5 inch for W and Central, coating to an inch for coastal plain terminals S/E. Monday afternoon into Monday night... Improving to VFR as conditions clear. MVFR cigs lingering across the Berkshires and developing across the Outer Cape as winds turn N. KBOS TAF... MVFR with -SN around 12z-15z Monday mixing with and changing over to -RA. Snow accumulations possible, expecting mainly a coating but possibly up to half an inch. KBDL TAF... MVFR with -SN 9z-12z Monday. IFR visibilities with snow, 2SM. Snow accumulations possible around half an inch to 1 inch. Lower risk of snowfall amounts in excess of 2 inches. Outlook /Tuesday through Friday/... Tuesday...High confidence. VFR. Tuesday night and Wednesday...Low to moderate confidence. MVFR to local IFR ceilings Tue night into Wed morning in light rain RI/SE Mass, a mix of rain/snow elsewhere. Improving to VFR Wed afternoon/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.
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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 Monday Night/... Light winds throughout as an area of disturbed weather sweeps across the waters Monday with showers, possibly mixing with snow along the coast. It`s behind this disturbance that winds will increase out of the NW and we`re likely to see gusts up to 20 kts along with some ocean-effect clouds. Seas remain below 5 feet throughout. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday through Wednesday night...Winds and seas remain below small craft criteria for most of the waters. NW winds Mon night will shift to E, gusting up to 20 knots at times. Thursday...Small craft advisories may be needed. Winds will shift to the SW-W and may gust up to 25 to 30 kt by late Thursday afternoon/night. Seas will be building to 4-6 ft over the outer waters. Friday...Strong W-NW gales expected to develop. There is the potential for some storm force gusts. Stay tuned for the latest forecast updates.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Sipprell/NMB NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...NMB AVIATION...Sipprell/NMB MARINE...Sipprell/NMB

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