Area Forecast Discussion
Issued by NWS Boston, MA

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689 FXUS61 KBOX 041158 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 658 AM EST Sun Dec 4 2016 .SYNOPSIS... High pressure will build across the area today, resulting in diminishing winds along with dry but cold conditions. A period of light snow Monday morning will bring a minor accumulation before tapering off to a mix of light rain and snow in the afternoon. Mainly dry conditions expected Monday night into Tuesday as high pressure moves across. 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 6 PM THIS EVENING/... 645 AM Update... Bands of stratocumulus clouds moving across most of the region early this morning as seen on latest IR satellite imagery and surface OBS. General trend is for the clouds to dissipate this morning, though will linger across outer Cape Cod and occasionally into Nantucket off and on through midday or so. NW winds sustained at 10-20 mph, with gusts up to 25-30 mph across the outer Cape and Nantucket. High pressure centered over western Quebec, extending across W NY/W PA to central VA at 11Z. The ridge will slowly build E this morning, so expect winds to diminish. Pretty much same trend from the previous forecast. With the clouds around, temperatures a bit milder than previous forecast, ranging from 30-35 for most locations at 11Z, but to the upper 30s at KPVC and KCQX. Near term forecast pretty much on track except for the cloudiness, so have updated to bring conditions current and incorporate into today`s trends. Previous Discussion... As clouds dissipate with rising mid level heights, will see more sunshine this afternoon than during the morning. Sunshine will be ineffective today given the low Dec sun angle and cold airmass in place with H85 temps down to about -9C this morning and -5C at 925 mb. Model soundings indicate blyr mixing will be limited to about 875 mb due to subsidence inversion. This will yield highs only in the upper 30s to lower 40s...about 5 degs colder than normal. However less wind than yesterday will make it more tolerable. Ocean effect snow showers early this morning affecting the outer Cape and possibly into Nantucket will come to an end later this morning as subsidence inversion lowers and squashes updraft potential as high pres crest over the region. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Snow Potential... Good model agreement on snow overspreading CT and western-central MA between 09z-12z...then into RI and eastern MA 12z-15z. Here are a few limiting factors that will keep snowfall potential low... 1) short wave trough is very progressive, thus short duration of snowfall 2) deep layer moisture is brief as well 3) POPS from guidance are only high chance/low likely. This could be a potential red flag. Prefer to see guid offering category pops if expecting accumulating snow. 4) Forcing for ascent is fairly weak and brief in the snow growth region /-10c to -18c/ with dry air entering this layer rapidly from 15z- 18z. Given this not expecting large snow ratios. 5) short wave trough is deamplifying along with weakening wind fields with time as it approaches and moves across southern New England. This will limit forcing for ascent. 6) warm air at 925 mb surging northward will likely change snow to rain toward midday across CT/RI and south of the MA turnpike. This will also limit snowfall potential. While blyr wet bulb temps are below freezing...precip may be too light to take advantage of evaporative cooling or from melting precip. Given all of the above factors will play this portion of the forecast conservative for now. Potential snowfall looks to be from a coating to an inch for most locations. A few locations in the higher terrain of CT/MA will have a low risk for up to 2 inches. Overall not a high impact event. Best chance of roads briefly becoming snow covered will be across CT/western-central MA including the Greater Hartford-Springfield areas before snow shield weakens as it moves eastward. Given these factors above will hold off on any headlines or statements for the moment. Temperatures... Temps will fall rapidly this evening with sunset given cold/dry airmass in place combined with clear skies and light winds. Thus blended some of the colder MOS guidance to account for this. Then clouds arrive around or shortly after midnight resulting in temps leveling off or perhaps rising a few degs toward daybreak. Temps begin to rise late Mon morning and afternoon as blyr winds become light SE. Winds... Winds will be light given weak pres gradient as feeble low pres tracks south of New England. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... 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 /12Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 1 am update... Short Term /through Monday/...High confidence. Today...VFR with CIGS from 3500-4500 ft. The exception will be over the outer Cape and possibly into Nantucket where snow showers at times will yield MVFR conditions through mid morning. Any lower conditions should improve to VFR by midday. Light NW winds, except gusty N-NW winds over Cape Cod and the islands with gusts up to 30 kt. Winds will diminish during the day Tonight...VFR and light winds. MVFR in snow overspreading CT and western-central MA between 09z-12z...holding off til 12z-15z across RI and eastern MA. Monday...a period of light snow in the morning then then tapering off during the afternoon. Light snow will mix with or change to rain along the south coast and possibly north to PVD-BOS. Snow accumulations will be light...mainly a coating to an inch. Low risk for a few spots across CT receiving up to 2 inches. MVFR conditions will trend to VFR toward sunset across CT and western- central MA...MVFR likely persisting across RI and eastern MA. KBOS TAF...high confidence for VFR/dry runways and NW winds not nearly as gusty as Sat. MVFR in light snow overspreads Logan 12z-15z Monday. Snow accumulations from a coating to an inch possible. KBDL TAF...high confidence for VFR/dry runways and NW winds not nearly as gusty as Sat. MVFR in light snow overspreads BDL 09z- 12z Monday. Snow accumulations of a coating to 2 inches possible. Outlook /Monday Night through Thursday/... Monday night...VFR. Mainly clear skies, except lingering clouds early across the E slopes of the Berkshires and across E coastal Mass through around midnight. Light NW winds. 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/... Today... Gusty NW winds diminish as the day progresses. Ocean effect snow showers east of Cape Cod will dissipate by midday. Small crafts should end by mid to late morning most of the waters, but will linger through the day over the eastern outer waters. Tonight... Light winds as high pres crest over the waters. Dry weather and good vsby. Monday... Light SE winds as weak low pres tracks near the southern New England coast. Outlook /Monday Night through Thursday/...Moderate confidence. Monday night 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. 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...Small Craft Advisory until 10 AM EST this morning for ANZ230>237-251-256. Small Craft Advisory until 4 PM EST this afternoon for ANZ250- 254-255. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera/EVT SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.