Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 151458 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 958 AM EST Fri Dec 15 2017 .SYNOPSIS... High pressure will move offshore today but bring cold and dry weather for most of the day today. Low pressure off the mid Atlc coast will track well southeast of Nantucket but close enough to bring a period of accumulating snow to the coastal plain this evening, ending overnight. Expect dry but cold conditions Saturday night into Sunday. Another round of light snow or a wintry mix before changing to rain is possible early next week. Cold and blustery conditions look to return around mid week. && .NEAR TERM /UNTIL 2 PM THIS AFTERNOON/...
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10 am update... For the most part AM forecast is on track. Will adjust temps/dwpts slightly as obs are running a bit warmer than forecast, likely given that CI is rather thin. This will also yield a slight upward adjustment in afternoon highs. However, still feel most locations will remain in the 20s at their warmest point. Only other adjustment was an ever so slight delay in POP timing late this afternoon and evening, dwpt depressions are expected to be a bit higher than previously forecast, which will likely delay the onset a bit. Previous Discussion... High pres moves offshore this afternoon. Morning sunshine will fade behind increasing clouds this afternoon. Developing low pres emerging off the mid Atlc coast this afternoon but it should remain dry through sunset as deeper moisture still to the south. Just a low risk for a few flurries or light snow developing near the south coast around sunset. 925 mb temps -8 to -9C so another cold day with highs mostly mid-upper 20s with some lower 30s outer Cape/Islands.
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&& .SHORT TERM /2 PM THIS AFTERNOON THROUGH 6 PM SATURDAY/... *** A period of accumulating snow likely across RI and southeast MA this evening *** Late today and tonight... Developing low pres off the mid atlc coast will be tracking NE late today and this evening. Meanwhile, another digging northern stream shortwave will be moving through the Gt Lakes and into the northeast. How these two shortwaves interact will determine how far north and west accumulating snow gets. Models have trended stronger and more amplified with this northern stream shortwave as it is becoming better sampled within the upper air network. This allows the mid level flow to back enough for deeper moisture to move further north and west into SNE. While the sfc low is tracking southeast of the benchmark, inverted trough extends back to the NW into SNE assocd with approaching northern stream trough. Models showing large scale synoptic forcing ahead of the shortwave along with favorable upper jet dynamics within the left exit region of 170 kt upper jet. Cross sections indicate a period of modest omega within the DGZ this evening, especially across SE New Eng. Given the deep moisture in place, expect a period of light to moderate snow across much of RI and SE MA this evening where snowfall of 1-3 inches is expected with a low risk for a few 4 inch amounts near the south coast and Cape Cod if trough is more amplified. Onset of the snow will be 5-8 pm so part of the evening commute may be impacted. Will let next forecast team examine the 12z model suite to decide whether an advisory is necessary. Expect light snow to extend north across much of SNE but amounts will be limited to less than an inch in the distant interior. This will be a progressive system and snow will be exiting SE New Eng coast after midnight. Saturday... Another northern stream shortwave will bring a reinforcing shot of cold air and blustery WNW winds to SNE. 850 mb temps -12C support highs from upper 20s to mid 30s with a mix of sun/clouds and wind gusts to 30 mph expected. Mainly dry weather is expected other than a few flurries in western MA as lake effect snow showers try to spill over the Berkshires. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Overview... Overall steering pattern across the lower 48 transitions to relatively flat mid level flow, which remains progressive even as a long wave trough digs across the western states late this weekend, then trends toward the southern stream as it slowly moves along in the flow. The northern stream remains progressive, with cold temps continuing into early next week. Another H5 trough moves ashore across the Pacific NW Tue night/Wed, with rather good agreement amongst the 00Z model suite. While this moves east through late next week, appears that rising H5 heights as the return SW flow may bring somewhat milder temps to eastern areas toward the end of the period. Models and ensembles continue to show weak short waves moving across the region in the fast flow aloft, with timing issues coming into play starting early next week as model solution spread increases. Also noting some PTYPE issues as the mid level pattern tries to transition back to near or slightly above normal temps toward the middle of next week. Details... Saturday night and Sunday... As H5 short wave shifts offshore Sat night, will see any leftover precip exit as well. Leftover NW low level jet up to H85 will push offshore. However, will see gusty winds linger Sat evening, up to 25-30 kt. Good low level lapse rates, on order 8-9C/km mainly across central and eastern areas which will allow for good mixing. The lapse rates will drop off between 03Z-06Z as the low level jet also moves offshore, so winds will drop off rather quickly by around midnight. Skies will be mostly clear as temps drop to the single digits and teens, except the upper teens to lower 20s along the immediate coast. Large high pressure, extending from Quebec down the eastern seaboard, will move across the region late Sat night and Sunday. Light W-NW winds will become variable as ridge axis crosses the during Sunday. Mid and high clouds start to stream across the region Sunday afternoon. It will be another chilly day, with temps remaining below freezing across the interior, reaching to lower-mid 30s along the immediate coast. Sunday night into Monday... Some question about next approaching system from the Ohio Valley during this timeframe. Both the GGEM and ECMWF were pretty robust with the development of overrunning precip, while the GFS and NAM keeps the mid level energy rather diffuse in the nearly zonal flow. Went along with an overall model blend, which has precip developing overnight Sun night and continuing into Monday. Should see some light snow break out, but overall this is another moisture starved system with QPF amounts generally less than 0.1 inches, which yields an inch or less of snow as it mixes with or changes to rain before ending across eastern areas. Have most of the precip pushing offshore Mon afternoon, though timing is in question due to model solution spread with this system. Monday night through Tuesday night... Another short wave moves along in the zonal flow, but again more questions about whether there is enough energy and moisture to bring another round of precipitation during this time. Could also see PTYPE issues with this system as low and mid level winds appear to back to S-SW as long wave trough starts to dig across the northern Plains states, with possibly a wintry mix developing late Mon night into early Tue before changing to rain during Tue. Current forecast suggests temps will actually rise during the day, with highs running around 5 degrees above seasonal normals. Will see the strong low pushing NE across the Great Lakes into Quebec. Attendant cold front should push through, bringing a return to colder temps Tue night. Wednesday and Thursday... Blustery and cold conditions return Wed as the strong low will exit into the Maritimes. May see some snow showers develop in the W-NW flow, possibly lake effect induced, moving across the E slopes of the Berkshires. May also see ocean effect snow showers develop offshore. High pres will push eastward Wed night and Thu, with winds diminishing and temps close to or slightly below seasonal normals. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... Today...High confidence. VFR through most of the day. Some late day MVFR cigs developing near the south coast after 19Z with chc -SN. Tonight... High confidence in a period of snow with IFR impacts developing across RI/SE MA this evening. Localized LIFR in moderate snow possible near the south coast. Accums 1-3 inches. Northern and western extent of the snow still uncertain, but it will likely reach BDL-ORH-BOS for a short time with brief MVFR possible and a coating to less than an inch. Snow exiting SE New Eng coast around 06z with improving conditions to VFR thereafter. Increasing NW winds along the coast 08Z-12Z with gusts to 25 kt. Saturday...High confidence. VFR with SCT-BKN 050-080 developing. WNW gusts to 25 kt. KBOS Terminal...Moderate confidence. Period of light snow with MVFR conditions this evening. Brief IFR vsbys possible. KBDL Terminal...Moderate confidence. Period of light snow with MVFR conditions this evening. Low risk for brief IFR vsbys Outlook /Saturday Night through Tuesday/... Saturday Night: VFR. Breezy. Sunday: VFR. Sunday Night: Mainly MVFR, with areas VFR possible. Chance SN. Monday: Mainly MVFR, with areas VFR possible. Chance SN, slight chance RA. Monday Night: Mainly VFR, with local MVFR possible. Slight chance FZRA, slight chance SN. Tuesday: Mainly MVFR, with local IFR possible. Breezy. Chance RA, chance FZRA, chance SN. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Today... W-NW winds 10-15 kt shifting to SW this afternoon. Seas below SCA. Vsbys may lower in light snow south coastal waters late in the day. Tonight... Increasing NW winds developing after midnight as coastal low moves offshore. Gusts to 25-30 kt expected late tonight. Vsbys reduced in developing snow this evening, especially south coastal waters, improving after midnight. Saturday... SCA conditions with WNW gusts to 30 kt with low risk for marginal gale force gusts over southern waters. Outlook /Saturday Night through Tuesday/... Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Sunday: Winds less than 25 kt. Sunday Night through Monday: Winds less than 25 kt. Chance of rain, chance of snow. Monday Night: Winds less than 25 kt. Slight chance of rain. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC/Doody SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.