Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 182030 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 330 PM EST Sun Nov 18 2018 .SYNOPSIS... A weak storm system will sweep across New England tonight into Monday with a rain / snow mix. Light snow accumulations forecast for the high terrain along with impacts for the Monday morning commute. A more potent storm system is on tap for Monday night through Tuesday night. More appreciable snow accumulations are possible. Thereafter, very cold air moves for mid to late week, potentially record-breaking, while remaining dry. && .NEAR TERM /THROUGH MONDAY/... 330 pm update... Thickening clouds across the region as temperatures have topped out around the upper 30s to low 40s. Expect clouds to lower towards evening as winds shift out of the S if they have not done so already as high pressure pushes offshore. A dry forecast going through the remainder of the afternoon into evening. As alluded to earlier, it`ll take awhile for the column to moisten top down. Decent low level dry air evident in the 12z Albany sounding. We are seeing increasing returns and surface outcomes out across Upstate NY into Southern NH presently. Isentropic ascent along 285-295K surfaces of a moist H6-9 layer, collision and coalescence resulting in precipitation that is moistening the lower levels near the surface. Increasing PoP chances towards likely into evening. Tonight through Monday... Weak wave impulse invoking light S to N transitioning snow to rain outcomes through the forecast period. Coating to 2 inches with the majority of accumulation over high terrain. Especially in NW MA and CT, Monday AM commute impacts expected with reduced visibility and snow covered roads. Sub-advisory event. Weak low to mid level trof with localized enhancement of advance low- level H95-85 convergent anticyclonic flow along 285-295K isallobaric surfaces. Front end thump of H6-9 moisture lofted with some support via mid-level ascent of stretched mid-level vortmax through broader H5 cyclonic flow. Weak deformation, expected light outcomes, very diffuse system and weak omega through the low-mid level column, even the dendritic growth zone. Maybe some support via the RRQ of the H3 jet streak. More likely outcomes are via a thermal wind response and low-level convergent forcing with aforementioned weak trof invoking a slight measure of tighter thermal packing. Definitely along the W slopes of high terrain via orographic lift. But some challenges with thicknesses, perhaps a warm nose around H95 above shallow surface cold air. A nod to wet-bulbing. Even as HREF points out, can`t rule out mixed wintry precipitation types ever briefly from snow to freezing rain to rain with S-N transition as there is prevailing S wind, a lack of northerly isallobaric flow. Will go with chance wording on possible freezing rain outcomes. Ever brief, will hold off on headlines and let later shifts evaluate whether the freezing rain event will be prolonged. Better model agreement with 18.12z forecast suite, tighter clustering of outcomes. Probabilities included, a coating to 2 inches forecast with highest amounts over the Worcester Hills and Berkshires where winter weather travel impacts are more likely. Hint some freezing rain mixed in but brief, an hour or two, with little impact. A light event, favor snow accumulation on surfaces at 32 to 33 degrees, 32 especially daytime. Consensus weighted forecast with preference to WPC / NERFC liquid-equivalent precip forecasts. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... Monday night through Tuesday night... More potent storm system impacting S New England. The potential for advisory level snows, with less confidence concerning warning level snows, centered around early Tuesday. Travel impacts for the Tuesday AM commute seemingly likely perhaps lingering through the day for Northeast MA with wrap-around outcomes as the storm exits. Sharper trof axis with strong low-level convergent focus drawing in deep moisture along the anti-cyclonic warm-moist conveyor belt. Lift along well-defined surface frontal boundaries with a deep low center undergoing ascent in advance of a potent mid-level vortmax, thermal wind response and forcing yielding a front-end frontogenetical thump in vicinity of S New England, perhaps collocated with decent -EPV. Expecting ice within the column, forcing thru the dendritic growth zone exhibiting steep lapse rates. Indications that moderate, maybe heavy snow at times is possible. More efficient, more fluffy snow accumulating N/W while a more heavier, wetter S/E. But forecast models ramping up with 18.12z suite, perhaps struggling with a secondary baroclinic low along an offshore sub-tropical axis. Variance with low positioning, thicknesses coinciding with warmer air intruding at H85, and location of deepest forcing / omega, all tied into differences in synoptics discussed above. Deformation / frontogenesis stronger in some models than others. Noteworthy spread in plume diagrams that stretch across headline criteria. Not to mention challenges with snow intensity given a mostly daytime event. Lean with daytime snow accumulations at 32 or lower if light outcomes, however more moderate to heavy it probably won`t matter, that likely such intensity will drag colder air downward. Then there is the possibility of mixed precip types with invading warmer air aloft. Forecast headaches for sure. Preferred to hint as some sleet and/or freezing rain mixing in for isolated locations, but think this will be mostly rain / snow. Again, let later shifts take a look given there is not a high level of confidence with this forecast, a level of uncertainty. Recently, GFS has exhibited better mid-range performance with the EC coming in a close second. Even with the oncoming system, the GFS has shown better consistency and likelihood of outcomes over all other high-res guidance. In collaboration with other WFOs / NERFC / WPC, prefer a global forecast guidance consensus blend swaying away from high-res guidance including the warmer NAM. Overall, leaning colder aloft. A sway northward with the main precip axis forecast, in the 18.12z EC as well, with GFS/EC thermal fields have a general coating to 3 inch event N/W of the I-95 corridor out of roughly 0.25 - 0.50 liquid. Locally higher amounts over the high terrain and within NE MA interior as the storm exits, dragging cold air S as precipitation continues to wrap around, the possibility of fluffy, quick-accumulating snow along the I-95 corridor late Tuesday N of Boston. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Highlights... * High pressure brings dry and cold conditions for Wednesday through Thanksgiving Day * Signs a potential warm up late next week into next weekend, but still below normal Overview... 18/12Z guidance suite remains in rather good agreement through the much of this upcoming week. Timing issues arise with the handling of shortwave energy approaching the West Coast of the USA towards Wednesday. This will have implications for our forecast late next weekend into early next week. Given the time range, have little confidence in the details that far out. Will take a consensus approach. It will likely be several days before this shortwave energy can get better sampled, and the details better resolved. Temperatures... Have high confidence in below normal temperatures through this portion of the forecast. Expecting the coldest air so far this season Wednesday night into Friday morning. There is the possibility for near to record cold during that time. Precipitation... Mainly dry weather much of the time Wednesday through Friday night. Will have to monitor the potential for lingering snow showers across northern MA Wednesday into Wednesday night, as the lake effect snow machine should be active. With such cold air overspreading our region, will also have to watch out for ocean effect snow/rain showers across the outer Cape and Nantucket. Neither of these should result in significant snowfall. 18/12Z guidance suite started to hint at the possibility for another coastal storm to impact southern New England late next weekend. Large uncertainty in the track and timing due to the handling of a feature on the West Coast, as previously discussed. Will keep a broad window for precipitation chances next weekend to cover the possible timing. However, not expecting the entire weekend to be a washout. Current probabilities favor Saturday to be the drier of the two days. && .AVIATION /20Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High Confidence. Rest of today... VFR. Light W winds turning S. Lowering and thickening mid-high level CIGs towards evening. SCT-BKN low-end VFR CIGs as well beneath the mid-high layer around 4 kft. Tonight into Monday... Conditions lowering with widespread MVFR, IFR-LIFR mainly confined to the high terrain and wherever -SN/SN develops. Initial SN from GHG-PVD-OXC northward, the RA/SN line retreating N into the Monday morning hours towards BVY-ORH-PSF, outcomes concluding into the afternoon. Overall light precipitation, snow outcomes of a coating up to around 2" with highest amounts confined to high terrain terminals. Low risk -FZRA. Thinking most runways remain wet. S/SW winds shifting W/NW through the day, gusts as high around 15 kts mainly over coastal terminals. Monday night through Tuesday night... Brief clearing into Monday night with lower CIGs lingering over SE MA. However, widespread lower MVFR towards IFR return with onset of -SN/SN once again. Can`t rule out +SN. IFR-LIFR CIG and VSBY impacts with any SN. SN for all terminals but Cape and Islands initially, will see the RA/SN line retreat towards BVY-SFZ-HFD towards the end of the event late Tuesday afternoon, conditions improving thereafter into Tuesday night. N winds throughout, heightening towards late Tuesday with gusts up as high as around 25 kts especially over coastal terminals, becoming NW and remaining gusty through Tuesday night. KBOS Terminal... Downward trend beginning 0z Monday. Potential -SN during the Monday AM push, however expect the changeover to -RA to be ongoing during the timeframe. VSBY impacts if anything but expect runways to remain wet. Monday night into Tuesday of greater concern. Accumulating snows and more significant impacts possible during the Tuesday AM push and through Tuesday afternoon. KBDL Terminal... Downward trend beginning 0z Monday. Likely -SN during the Monday AM push, however it will not be long before seeing a changeover to -RA. VSBY impacts anticipated but runways remaining wet. Monday night into Tuesday of greater concern. Accumulating snows and more significant impacts possible during the Tuesday AM push and through Tuesday afternoon. Outlook /Wednesday through Friday/... Wednesday: VFR. Breezy. Slight chance SHSN. Wednesday Night through Thanksgiving Day: VFR. Windy with gusts to 30 kt. Thursday Night: VFR. Windy with local gusts to 30 kt. Friday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High confidence. Rest of today... Good boating weather. High pressure pushing out into the Gulf of ME. Light winds turning S. Tonight through Monday... Weak area of low pressure sweeping across S New England. S winds in advance turning W/NW with passage. Gusts up as high as 15 kts, seas remaining below 5 feet. With accompanying rain, potential visibility impacts on the waters. Monday night through Tuesday night... More potent low pressure center sweeping NE in vicinity across Cape Cod and the Islands late Tuesday. N winds out ahead followed by W winds behind, sustained up around 10 to 15 kts with gusts up around 25 kts. Headlines will likely be necessary as seas build 5 to 7 feet on the outer waters. More moderate rain / snow outcomes, with snow more likely over the cooler E waters, likely visibility impacts for boaters. Outlook /Wednesday through Friday/... Wednesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Wednesday Night: Low risk for gale force winds with gusts up to 35 kt. Seas up to 5 ft. Slight chance of rain showers. Thanksgiving Day: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of snow showers. Thursday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. && .CLIMATE... With the potential of the coldest air of the season moving into the region around Thanksgiving, here are the record temperatures for November 22nd and the holiday of Thanksgiving. November 22nd Location / Record Low / Record Minimum High Boston..........9 (1879) / 24 (1880) Hartford.......14 (1969) / 27 (1978) Providence.....16 (1987) / 30 (2008) Worcester......11 (1987) / 24 (2008) Thanksgiving Location / Record Low / Record Minimum High Boston.........11 (Nov 27, 1873) / 24 (Nov 28, 1901) Hartford.......12 (Nov 28, 2002) / 27 (Nov 23, 1989) Providence.....14 (Nov 23, 1972) / 30 (Nov 28, 1996) Worcester.......9 (Nov 23, 1989) / 22 (Nov 23, 1989) && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Belk NEAR TERM...Sipprell SHORT TERM...Belk LONG TERM...Belk AVIATION...Belk/Sipprell MARINE...Belk/Sipprell CLIMATE...Sipprell

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