Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 042024 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 324 PM EST Sun Dec 4 2016 .SYNOPSIS... * Near- and Short-term updates only... 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 night. A polar front crosses the region Thursday with rain showers possibly changing to snow showers. Very windy and cold conditions forecast for Friday and next Saturday with wind chill indices in the single digits and teens Friday night. && .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/... Highlights... * Mixed precipitation possible late Tuesday night and Wednesday * Very strong front blasts across the region Thursday * Windy and very cold into next weekend with strong gales over the waters Monday night...High confidence. Expecting quiet conditions as high pressure builds across the region. Skies becoming mostly clear, except for a few lingering clouds across the E slopes of the Berkshires. N winds will be a bit gusty early along E coastal areas, then will diminish as the high takes over. With the lighter winds, should see some radiational cooling especially in the normally colder locations. Temps will fall back to the mid and upper 20s across most locations, ranging to the 30s along the immediate coast. Tuesday...Moderate confidence. High pressure ridge extending from a central across Quebec down the eastern seaboard will lift NE during the day. Winds become light NE-E. Weak low pressure moving NE out of the SE U.S. will bring some clouds across the region during the afternoon. Overrunning moisture will push NE toward the region after 18Z, so carried just chance POPs across N CT into SW RI by sunset. Some question on the timing and extent of the moisture into the region, however. Expect highs in the mid and upper 30s across the higher inland terrain, ranging to the lower-mid 40s along the coast. Tuesday night through Wednesday night...Moderate confidence. Fast moving weakening H5 short wave in the SW flow behind the exiting ridge will cross the region Tuesday night, with weak low pressure off the mid Atlantic coast. With onshore easterly flow, will see light precip starting off as light snow, then mixing with or changing to rain across the coastal plain with the light but steady onshore winds. As the short wave lifts NE into Wednesday morning, the surface low off the mid Atlantic will shift E well south of the region taking its moisture field offshore. So, will see precip become rather spotty by Wednesday evening. Temperatures on Wednesday will rise to the upper 30s to mid 40s across most areas, but remaining a bit cooler across the higher inland terrain. Looks like precip should change to mainly light rain by midday Wednesday. By Wednesday night...most models and ensembles start to bring more moisture to the region after midnight. So, brought chance POPs back in for now. QPF amounts remain low with lack to deep moisture field. As temps fall back, any mixed rain and/or snow across the interior will change back to light snow, with the best chance across the E slopes of the Berkshires. Thursday and Thursday night...Moderate confidence. Timing and track differences continue as a polar front wraps around the cutoff low pressure across Hudson Bay. A digging H5 long wave trough, bringing surge of very cold air from western Canada and Alaska, will work eastward. Will see southerly winds ahead of the front Thursday, but exact timing of the frontal passage in question. GFS tending to be faster with this feature than the ECMWF, which was similar to the previous forecast. Continued with a model blend this forecast. The front should pass through Thursday night, accompanied by a chance of snow showers across most of the region, except rain in southern RI, Cape Cod, and the Islands. Lows 25 to 30 except mid to upper 30s Cape Cod and Islands. Friday and Saturday...Moderate confidence. Colder air will sweep in across the region Friday as W-NW winds quickly increase. Models differ with the development of strong low pressure over the Maritimes during this timeframe. However, with the strong pressure gradient, expect winds to gust up to 25-35 kt, highest along the east coast, which looks to continue through Saturday. May see gusts around 40 kt along the coastal waters Friday night and Saturday. May also see lingering snow showers across the E slopes of the Berkshires, as well as ocean effect snow showers off the coast. With the strong cold air advection moving in, expect highs on Saturday to run up to 10 degrees below seasonal normals. Will also see wind chill values Friday night into early Saturday morning down to the single digits and teens. && .AVIATION /20Z SUNDAY THROUGH FRIDAY/... 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 through Wednesday night...Low to moderate confidence. MVFR to local IFR ceilings in light rain RI/SE Mass, a mix of rain/snow northern CT, central and northeast Mass, and light snow northwest MA. May briefly change over to all rain across most areas late Wednesday morning into the afternoon. Thursday...Low to 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 in western MA late in the afternoon. && .MARINE... 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. Seas may build to near 5 ft over the southern outer waters by Wednesday night. Thursday...Small craft advisories will likely be needed. Winds will shift to the S-SE and may gust up to 25 to 30 kt by late Thursday afternoon. Winds may shift to SW toward Thu evening. Seas will be building to 5-7 ft over the outer waters. Attention mariners...Expect strong W-NW gales with some potential for storm force gusts are forecast to develop Friday into Saturday. Stay tuned for the latest forecast updates. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Sipprell/EVT NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...EVT AVIATION...Sipprell/EVT MARINE...Sipprell/EVT

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