Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 161228 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 728 AM EST Thu Feb 16 2017 .SYNOPSIS... Rapidly deepening area of low pressure lifts northeast across the southeast Canadian maritimes behind which snow lingers as winds out of the northwest remain blustery. Dry weather arrives Friday into Monday. A significant moderation in temperatures expected Sunday, when much of the region may see highs around 50 degrees. && .NEAR TERM /UNTIL NOON TODAY/...
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Through Midday Thursday... * Accumulating snow for East Coastal Massachusetts through the morning commute...Winter Weather Advisories continue through 10 am 727 am update... Thinking the latest runs of the HRRR have the right idea timing-wise for the snow this morning. Didn`t think the area was quite as good based on radar. Heaviest snowfall was along a line from just southeast of Boston to about Chatham and Orleans. Expecting snowfall to continue to diminish through the morning, although not likely in time for the rest of the morning commute across eastern MA. Will allow Winter Weather Advisory to continue through its current expiration time. Tweaked most aspects of the forecast this morning to reflect observed trends. 400 am update... Quite the tough forecast. Focus has been upon identifying where sufficient deep layer moisture (surface - H7) is collocated with convergent flow associated with the inverted / norlun trough in regions of steep lapse rates and weak instability. Early on none of the near-term high-res guidance was performing well, missing out completely on the 980 mb meso-low over Nantucket Sound. As of this hour the HRRR is still underperforming, skewed slightly E of actual observations which show at the surface the trough axis extending from roughly Keene NH, southeastward between Fitchburg and Worcester, off the Plymouth County coast and out across the outer arm of the Cape south of Provincetown. Feel a blend of the 16.06z NAM and the latest RAP has a better handle on the low-level moisture situation, with the RAP leading the way in the temporal and spatial resolution of the favorable omega regions within the dendritic growth zone along with weak instability and steep lapse rates. However not absolutely sure what to expect. Per WSR-88D the snow banding ongoing is pushing S/W whereas the RAP suggests the focus will shift E and offshore. Feel there is some correlation to a band of potent impulse energy at H5 sweeping the region presently providing lift through the dendritic growth zone which surprisingly none of the high-res forecast guidance seemingly is capturing well. Meanwhile accumulation has been a headache. Cold air has not pressed in from the W as fast as originally thought. Per ground-truth, appears accumulation is not effective until the surface temperature is roughly 32 to 33 degrees. Have accounted for this in the forecast to produce the desired storm total snowfall. So the take-away`s: Snow will persist across the Winter Weather Advised area through the morning commute. At a minimum it will result in visibility restrictions down to a mile. Accumulation on area roadways is dependent on both the surface temperature and the intensity of the snowfall. While forecast snowfall amounts are below advisory levels, the Winter Weather Advisory is continued based on the threat. Forecast storm total snowfall amounts range around 2 to 4 inches over Essex County MA, with an inch or two within the I-95 beltway and the Route 3 corridor from Boston to Plymouth. Recognize that within the city of Boston and likely some of its neighboring metro areas that accumulation is hard to come by and again this is the result of surface temperatures limiting accumulation. Colder air is pushing in from the W gradually with deformation taking place in response to the convergence along the trough. W winds also picking up per isallobaric / gradient response with the associated low pressure rapidly deepening towards the SE Canadian Maritimes. Subsequently, there is the possibility that snowfall accumulation could become more effective. Already as of 4 am the temperature at our office in Taunton has dropped 2 degrees.
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&& .SHORT TERM /NOON TODAY THROUGH 6 AM FRIDAY/... Midday Thursday into Thursday night... * Colder conditions pushing in, and along with blustery NW winds, will make for very cold wind chills Confident snow shower activity lingers over the E coast and the Berkshires. But there remains a risk of continued snow activity over the interior as seemingly what appears to be an inverted / norlun trough remnants sweep offshore per latest HRRR. Localization of decent moisture up to H7 within perhaps some weak confluent flow as lapse rates remain steep especially with any diurnal heating. Better risk during the early half of the day parent with trailing potent impulse energy associated with the N-stream, sweeping offshore during the later half of the day. Given negative differential vorticity advection (NVA) am leaning with a drier forecast for the later half of the day with lingering cloud decks. Continued to favor the RAP with respect to near-term high-res forecast solutions. Cold air advection continuing behind the deepening low, with a boundary layer conditionally unstable beneath the tropopause fold up around H4, well mixed up to H85 where 25 to 35 mph NW winds reside, expecting a blustery day into the overnight period. Highs around the low to mid 30s, however given the cold northwest flow, it will feel 10 to 15 degrees lower, down in the 20s. As to the overnight period, though winds somewhat diminished with the possibility of boundary layer decoupling, with lows in the teens to the 20s by Friday morning, it`ll feel for some locales in the single digits. Strongest winds over the high terrain and along the immediate shoreline, especially over the Cape and Islands. Strongest wind gusts up close to 40 mph, however thinking a low risk of reaching wind advisory criteria. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Generally dry conditions through most of this period * Blustery and cold Friday * Moderating temperatures Saturday, and unseasonably mild Sunday * Above normal temperatures continue into early next week Details... Friday and Saturday... High pressure centered to our south becomes more slightly dominant in our neck of the woods. With this high pressure not directly overhead, expecting west winds to continue Friday, then shift southwest during Saturday. Dry weather should prevail, but we`ll have some clouds at times, moreso on Saturday as the warm air advection get underway and a weak shortwave arrives. Below normal temperatures Friday gradually rise to near normal levels for Friday night into Saturday. Saturday night and Sunday... Core of the deep closed low moves from the Maritimes toward southern Greenland. Meanwhile, a potent shortwave from the mid Mississippi Valley moves offshore through the Mid Atlantic states. The combined movement of these two features alters the mid level flow in our region to be more zonal for a time. This in turn will direct a low pressure to move from James Bay through the Saint Lawrence River valley. This will mean higher temperatures for southern New England, as we would be on the warm side of this low pressure. Some locations may approach 50 degrees Sunday, most likely across the lower terrain of eastern MA, RI and the lower CT River valley. Dry weather continues. Monday... High pressure expected to be in place across New England. This should mean near normal temperatures and mainly dry weather. Tuesday through Wednesday... This is the period of greatest uncertainty for this forecast, which is not atypical for this time range. The forecast for our region will depend on the extent of amplification of a downstream mid level trough over the Maritimes. 16/00Z ECMWF was quite aggressive in bringing its closed low well south of the other guidance, almost 10 degrees of latitude. After looking at the ensembles, am going to discount the ECMWF deterministic solution. Some hints there might be a warm frontal passage Tuesday, which could give us a slight chance for showers. Not a lot of humidity feature, so just have increasing clouds Tuesday, for now. Another high pressure to our south should then nudge a ridge into southern New England for Wednesday. Near normal temperatures expected. && .AVIATION /12Z THURSDAY THROUGH MONDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Range /through Thursday/... Today...Moderate confidence. Low-end VFR / MVFR west, IFR with SN east this morning. Mainly holding low-end VFR / MVFR for all terminals the remainder of the day. Biggest impact to E MA terminals for the morning push. Precipitation dissipating into the afternoon hours. NW winds, sustained around 10 to 15 kts, gusts 20 to 30 kts, strongest winds over the high terrain and along the coast. Tonight... MVFR to low-end VFR BKN cigs linger, gradually dissipating through the evening period. However blustery NW winds continue with sustained flow around 10 kts and gusts around 25 kts. KBOS TAF...Messy morning push. Improving conditions during the late morning hours into the afternoon with blustery NW winds throughout, especially after SN comes to an end, gusts up to 30 kts possible. KBDL TAF...Not much impact with respect to precipitation. Terminal should remain dry. Winds remaining blustery out of the NW. Outlook /Friday through Monday/... Friday and Saturday...High confidence. Mainly VFR conditions. Saturday night through Sunday night...Moderate confidence. May see patchy MVFR-IFR CIGS/VSBYS develop Saturday night across the CT River valley and east slopes of the Berkshires ahead of warm front. MVFR-IFR CIGS may linger Sunday and early Sunday night along the east slopes of the Berkshires. Elsewhere, mainly VFR. Monday...High confidence. Mainly VFR conditions.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Range /through tonight/...Moderate Confidence. Blustery NW winds to gale force. Gale warnings posted for all waters except small craft headlines for Boston Harbor/Narr Bay. Outlook /Friday through Monday/... Friday and Friday night...High confidence. W-NW winds will continue to gust up to 25-30 kt, so Small Craft Advisories will likely continue for many of the waters. Light freezing spray will may persist into Friday morning, but should become less of a risk as winds diminish. Rough seas early, then will slowly subside but remain at or above 5 ft through Fri night across the outer coastal waters. Saturday through Sunday...High confidence. W-SW winds gusting up to 20 kt. Seas below 5 ft. Monday...Moderate confidence. NW winds may approach Small Craft Advisory levels across the eastern outer coastal waters. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Winter Weather Advisory until 10 AM EST this morning for MAZ005>007-013>022. RI...None. MARINE...Gale Warning until 10 AM EST this morning for ANZ231>235-237- 251. Small Craft Advisory until 7 PM EST this evening for ANZ230- 236. Gale Warning until 7 PM EST this evening for ANZ250-254>256.
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&& $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk/Sipprell SHORT TERM...Sipprell LONG TERM...Belk AVIATION...Belk/Sipprell MARINE...Belk/Sipprell

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