Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 121154 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 654 AM EST Tue Dec 12 2017 .SYNOPSIS... Strengthening low pressure in the Ohio Valley early this morning redevelops over northeast Massachusetts this evening and then intensifies into at least a Gale Center over New Brunswick Wed. This storm system will bring a wintry mix of snow and ice to the interior with mainly rain in the coastal plain today. A shot of arctic air wraps around the exiting low Wednesday and Thursday, with a period of strong winds and bitterly cold wind chills. Another low may bring some light snow across south coastal areas Friday night into Saturday if it tracks close enough to the coast. Somewhat milder weather will arrive by the end of the upcoming weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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*** A Wintry Mix Across The Interior This Morning *** 645 AM update... Low pressure across W NY/PA at 11Z will continue to intensify as it moves steadily E during the day. Light snow continues off and on across N central/W Mass and as far E as KBVY at 11Z, as noted on obs over the last few hours. Area of spotty light precip noted on latest NE regional 88D radar imagery since 09Z, with steadier precip across E NY into W CT which will push east during the remainder of the morning. Noting light winds shifting to E-SE across E coastal Mass into RI, which will continue to pick up as the low deepens and approaches. Precip is a bit slower than previous forecast, but otherwise remainder is pretty much on track. Have updated to bring conditions current and incorporate trends through midday. Previous Discussion... Significant trough amplification occurs today over the eastern Great Lakes, with mid level trough taking on a negative tilt tonight before closing off to a 506 dam low over western MA by 12z Wed. For this morning, a modest cold airmass in place over southern New England with temps below freezing inland and dew pts down to 16F at Worcester. 1020 mb high over Maine early this morning builds to about 1026 mb later today in response to upstream trough amplification. This will provide cold air damming into interior MA and possibly farther south into northern CT and northwest RI. Column is cold enough to support snow. However developing low level southeast jet provides strong WAA and will result in snow changing over to rain quickly in the coastal plain. However northwest MA will have the longest duration of snow and could be moderate to perhaps heavy for a time this morning toward 15z. In addition, mid level lapse rates approach 6.5-7C/KM combined with bulk of forcing for ascent occurring in the snow growth region yields the risk for a period of moderate to heavy snow 12z-15z. Thus this event could overperform a bit across northwest MA where a brief period of moderate to heavy snow could produce 1-3" snowfall amounts FROM 12Z- 15Z, possibly a few 4 or 5 inches in northwest Franklin county. Otherwise most locations will see snow quickly change to rain this morning as warm nose at 925 mb advects across the area. The issue will be across the interior (along and northwest of I-90 and I-495) where shallow cold air will be stubborn to dislodge. This will result in a period of freezing rain/ice. A shorter duration of freezing rain/ice is possible farther south across northern CT, northwest RI into the I-495 corridor. Given current temps remain below freezing across interior Essex county along with dew pts in the low 20s and winds calm, we`ll expand winter weather advisory to this region for a brief period of mixed wintry precip 7 am - 10 am. Keep in mind it doesn`t take much freezing rain/ice to result in slippery travel. Farther south into the coastal plain including Boston and Providence, any snow will quickly changeover to rain given increasing southeast winds. Rain will likely be heavy at times this afternoon given trough amplification/strong jet dynamics with LFQ of upper level jet streak overspreading the area, enhancing QG forcing. In addition some instability aloft with mid level lapse rates increasing to 6.5-7.0C/KM (TTs 50-55). This combined with fairly strong forcing may result in embedded convection, enhancing rainfall potential. It will become breezy along the eastern MA coast especially Cape Ann as low level jet increases as it departs offshore. This low level jet will warm the coastal plain to 45-50 degs this afternoon, this includes the Boston to Providence corridor. Meanwhile farther inland late day highs of 35-40.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
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Early morning update... Vigorous negatively tilted short wave trough approaches from the southwest. Moisture is limited with this feature given attending dry slot. However could be sufficient low level moisture for a few low top rain/snow showers ahead of the dry slot. Behind this departing short wave arctic air blast across the region with very strong CAA as H925 temps crash from +7C over Cape Cod at 7 pm to -7C by 12z Wed. Any standing water and/or slush will freeze. Thus this afternoon is your chance to remove any leftover snow/ice.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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Overview... Deep H5 long wave trough sets up by mid week from the Arctic Circle southward to the mid Atlantic coast, while high amplitude ridge continues from the Northwest Territories through the Pacific Coast. This will bring the coldest air so far this season to the region along with strong W-NW winds through Thursday. As the cold pool retreats into eastern Canada late this week, appears that the western ridge may break down as a short wave moves into the Pacific NW late Fri or Sat. Also noting a weak mid level short wave that may wrap around base of the eastern Canadian cutoff low and cross the region. May also see a strengthening southern stream system try to ride up across the western Atlantic. Rather wide model solution spread at this point, so track is in question along with the possibility of the northern fringe of the precip shield approaching portions of the south coast late in the week or into the weekend. 00Z GFS is farthest W with this system, while the Canadian GGEM is further offshore. Something to keep an eye on. With rather wide solution spreads amongst the model suite from about Friday into early next week, lower forecast confidence for this portion of the forecast. Will use a blend of available model guidance into Friday, then transition over the model ensembles for the latter portion of this period. Details... Wednesday and Wednesday night... Strong low pressure will spin across the Maritimes into Labrador through Wednesday night, bringing a strong surge of arctic air to the region. Noting H85 temps dropping through the day, due to -12C to -16C by 00Z Thu. While the core of the arctic air remains N of the region, will see increasing W-NW winds with the sharp pressure gradient across the region from the eastern Canadian vortex. Noting H85 jet up to 40-50 kt moving across the region by Wed afternoon, with excellent low level lapse rates, so will see a good chunk of these winds mixing down. Could see gusts up to 40-50 mph during Wed, so wind advisory headlines will probably be needed. Some leftover snow showers may linger across the E slopes of the Berkshires through midday Wed, with a few lingering along the S coast early. Winds may drop off a bit Wed night as better mixing moves offshore, but temps will tumble with mostly clear conditions. Wind chills away from the immediate coast will drop to the single digits, with readings as low as -5 toward daybreak across the E slopes of the Berkshires. Thursday... Expect the gusty W-NW winds to continue through midday Thursday, but will start to weaken as the eastern Canadian low slowly exits. Models continue to signal an H5 short wave passes S of New England. The northern fringe of the precip shield may clip the S coast Thursday morning through midday as the weak low passes close to or just S of the 40N/70W benchmark. Have carried CHC POPs across Nantucket and Martha`s Vineyard. Will see snow showers or a period of light snow for a few hours as the low passes. Little if any snow accumulations are expected. The arctic air will continue across the region, with high temps only in the upper teens and 20s across the higher inland terrain, ranging to the lower-mid 30s at the shore. As winds drop off during the day, wind chills will not be quite as harsh as on Wed. However, with mainly clear skies Thu night, temps will fall back to the single digits well inland, ranging to 20-25 across the outer Cape and islands. Friday through Saturday... Another weak short wave develops across the Great Lakes Fri, while stronger low pressure forms off the SE U.S. coast. The weak low will push E while the coastal low moves NE off the eastern seaboard. Models showing some spread with their development and track of the coastal low, with the GFS closest to the coast but still passing S and E of the 40N/70W benchmark. Could still see some light snow or snow showers push into SE Mass/E RI, but this is still up in the air for exact track and impacts later Friday into early Saturday. Should see improving conditions with slowly moderating temps Sat. Sunday and Monday... High pressure ridge builds across the region Sunday. Winds back from W-NW to SW, which will bring milder temps, actually near or just a few degrees below seasonal normals. May see another short wave bring some light precip by Sun night and continuing into Monday, but timing and track of this system is in question.
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&& .AVIATION /12Z 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. Short Term /through Tonight/... 1045Z update... Through Today...High confidence on trends but lower on exact details. MVFR CIGS across central and western areas will push E through 14Z, with local IFR-LIFR at times across N central and western areas. Light snow/sleet transitions to rain along the coast, with some freezing rain at times inland. S-SE winds increase along the coast. Tonight...High confidence on trends, lower on details. MVFR CIGS/MVFR-IFR VSBYS improve to VFR by around 05Z-07Z, except lingering lower CIGS across higher terrain. Gusty W-NW winds develop. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Wednesday through Saturday/... Wednesday: Mainly VFR, with local MVFR possible. Strong winds with gusts to 40 kt. Wednesday Night: VFR. Strong winds with gusts to 40 kt. Thursday: VFR. Windy with local gusts to 30 kt. Thursday Night: VFR. Breezy. Friday: VFR. Friday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SN. Saturday: VFR. Breezy.
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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 Tonight/... 645 AM update... Today... SE winds increase to 20-30 kt with strongest winds across eastern MA waters. Snow early changes to rain with reduced vsby. Low pres intensifies as it moves from W NY to northern MA. Tonight... Arctic cold front sweeps across the area with SE winds shifting to W-SW up to 20-30 kt. Vsby improves after evening rain/snow showers. Outlook /Wednesday through Saturday/... Wednesday and Wednesday night: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Slight chance of snow showers Wednesday. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of snow showers. Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of snow. Areas of visibility 1 to 3 nm. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of snow. Visibility 1 to 3 nm. Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Slight chance of snow. Areas of visibility 1 to 3 nm.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory until 10 AM EST this morning for CTZ002>004. MA...Winter Weather Advisory until 1 PM EST this afternoon for MAZ005-006-010>012. Winter Weather Advisory until 4 PM EST this afternoon for MAZ002>004-008-009-026. RI...Winter Weather Advisory until 10 AM EST this morning for RIZ001. MARINE...Gale Warning from 9 AM Wednesday to 6 AM EST Thursday for ANZ232. Gale Warning from 9 AM Wednesday to 4 AM EST Thursday for ANZ233-234. Gale Warning from 1 PM Wednesday to 1 AM EST Thursday for ANZ230-236. Gale Warning from 10 AM Wednesday to 6 AM EST Thursday for ANZ231. Gale Warning from 8 AM Wednesday to 6 AM EST Thursday for ANZ235-237-251. Gale Warning from 6 AM Wednesday to 6 AM EST Thursday for ANZ250-254. Gale Warning from 5 AM Wednesday to 6 AM EST Thursday for ANZ255-256. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera/EVT SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT

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