Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210538 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1238 AM EST Wed Nov 21 2018 .SYNOPSIS... Low pressure exits tonight followed by weak high pressure. An arctic cold front may bring a few snow showers/squalls late Wednesday into Wednesday evening, followed by record cold and bitter cold wind chills Wednesday night into Thursday. Dry weather expected Friday into Saturday afternoon with moderating temperatures. Periods of unsettled weather appear to be in the cards at times Saturday night through next Monday although specific timing is uncertain. Odds favor precipitation mainly being in the form of rain...but some ice/snow is possible at the onset across the interior. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 1015 PM Update... Satellite imagery shows the back edge of clouds moving offshore of the Cape at this hour, leaving mostly clear skies across SNE. Some mid level clouds linger over the high terrain of western MA but should see mostly clear skies for the first part of the night before mid and high level moisture increases from the northwest brigning increasing clouds Wednesday morning.Main concern will be continued risk of black ice. Surface observations indicate most locations hovering at or below freezing, and road surface temps are forecast to dip below freezing in the next few hours. Given many roads haven`t had the chance to dry out, could be an issue with slick roads through the morning. Previous Discussion... As weak low pres continues to deepen it has allowed wrap- around precip to linger and change to SN as colder/drier air moves into the lower column. This should be ending quickly as the drier air wins out. Already noting clearing across W/Central MA/CT per latest satellite imagery. The main concern overnight is the drop in temps. With damp roadways lingering and only light flow, do not anticipate all areas will be able to dry before freezing temps arrive. Have already raised SPS for patchy black ice through the overnight hours. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/... Wednesday... Next robust northern stream shortwave will be diving into the eastern Lakes and northern New Eng. The attending sfc low pres tracks along the international border dragging an arctic front across SNE late in the day and early evening. Best low level forcing and steep 0-3km lapse rates 7-8 C/km are confined to northern New Eng where best chance for a few heavier snow showers/squalls. This activity should weaken as it approaches SNE where environment is a bit less favorable, but a few of these snow showers or perhaps a brief heavier squall may survive the trip into SNE late Wed/Wed evening, especially northern and western MA. The other concern for late Wed is a period of brief strong wind gusts behind the arctic front. There is a risk for brief 35-45 mph wind gusts right behind the front late Wed, especially western New Eng and higher terrain. Highs will range from the mid 30s higher terrain to lower 40s coastal plain. Wednesday night... * Near record cold & bitter cold wind chills late Wed night into Thanksgiving morning * An isolated snow shower possible early in the evening, otherwise strong cold advection will deliver an arctic blast with near record cold. Highly anomalous airmass with 850 mb temps down to -20 to -22C at 12z Thu which is about as cold as it gets for this time of year. Lows will drop into the single numbers central/W MA with teens elsewhere. Gusty NW winds will result in wind chills zero to 10 below across much of SNE by Thu morning. The arctic air over the still mild waters will result in extreme instability developing late Wed night with delta T from SST to top of the mixed layer around 30C with ocean induced Capes approaching 1000 J/kg! Given trajectory around 320 degrees, the bulk of the ocean effect snow should remain offshore, but snow showers may clip the outer Cape late Wed night. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Highlights... * Cold Thursday. Some snow possible on the outer arm of Cape Cod. * Dry with moderating temps Fri and especially Saturday afternoon * Unsettled at times Sat night-Mon with periods of rain favored but some ice/snow possible at the onset mainly across the interior Overview and model preferences... Arctic influenced vortex spins S from roughly the Hudson`s Bay region late week. Given its origins, continue to anticipate anomalous could along with given this vortex has H5 heights nearly 3 std deviations below normal and H85 temps as much as 20C below normal. The influence is felt not only in the cold ambient temperatures but with deepening low shifting into the Maritimes, blustery winds for Thanksgiving Day. Do continue to note some breakdown in the synoptic pattern however toward the weekend as the strongly positive PNA tends toward neutral allowing weakening ridge to eject eastward. This will help the airmass moderate in preparation for another Aleutian wave moving E early next week. Whether or not this phases with the N stream and to what extent will have to be watched as it is expected to extract more Gulf of Mexico moisture than previous waves. In spite of the anomalies, guidance shows fair agreement through late week, so feel that continuing with a consensus blend of guidance will still work. There is still a fair amount of spread early next week such that will add more weight to the ensembles to account for most solutions across the envelope. Details... Thu and Thu night... The coldest stretch. Any lingering SHSN/squalls should dissipate with the intrusion of dry air into the lower lvls overnight. However, strong pres rise/fall couplet is noted yielding blustery cold advection which will continue into Thu. H85 temps drop to nearly -25C through Thu night. Even with fully effective mixing this will limit highs Thu to the upper teens and low 20s. Combine this with the 45kt LLJ, expect gusty winds which will force wind chills into the single digits through much of the day. Ambient mins in the single digits to low teens can be expected both Wed night and Thu night, Wed night due to strong cold advection, Thu night with the assistance of some radiational cooling. As previous forecaster noted, will need to watch particularly from HYA eastward on the Cape, nearly 30C delta-T values suggest a good risk for ocean effect snows particularly late day and overnight as the wind takes on a more northerly component. Accumulations are likely given the setup. Fri into Sat... Gradual moderation of the airmass as high pres above 1030mb crests across the region. This will be a gradual process, and highs/lows on Fri are still expected to remain below normal (peaking in the 30s by day, low 20s by night). Slightly warmer Sat as return flow develops. Looking at highs back into the 40s in advance of cloud shield from the SW. Sun through Tue... Complex scenario here given two shortwaves indicated by models. The first slides E out of the Aleutians and shifts toward the S CONUS, meanwhile a second weaker northern stream wave may slide ESE from BC and the Northwest Territories. Whether or not, and how, these two phase is at question, and given neither are well sampled models continue their usual longwave struggle. In any case a risk for more unsettled and wet wx is expected with potentially two rounds of precip. Moisture content will be high enough given the tapping of Gulf of Mexico moisture from the first wave. Agree that the Sun/Sun night wave may be one to watch given that near sfc temps may still be cold enough for some mix of ice before a change to all rain. The second will be dependent on low pres track, as an inland track would favor a warmer solution, while a coastal track may introduce enough cold air for winter to be in play. More on these as we approach. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High Confidence. Early morning... VFR with clear skies at all TAF sites. Winds WSW less than 10 kts. Given remaining moisture from rain on Tuesday and below freezing temperatures, there is a risk for patchy ice on untreated runways. Today... VFR with increasing BKN cigs developing in the afternoon. There is potential for an isolated brief snow shower to bring conditions down to MVFR, mainly in NW MA after 20Z. W/SW wind gusts to 25 kt developing in the afternoon. Brief 35-40 kt gusts possible late day in the interior behind arctic front. Wednesday night... Mainly VFR, but areas of MVFR cigs developing over the outer Cape late along with risk of snow showers. NW gusts to 25-35 kt at times. KBOS Terminal... High confidence in trends. KBDL Terminal... High confidence in trends. Outlook /Thursday Night through Sunday/... Thanksgiving Day and Night: VFR. Windy with local gusts to 30 kt. Friday and Friday Night: VFR. Saturday: Mainly VFR. Saturday Night and Sunday: Mainly MVFR, with local IFR possible. Breezy. Chance RA, chance FZRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. NW gusts to 25 kt expected at times tonight. Winds will temporarily diminish Wed morning while shifting to W/SW, but will increase Wed afternoon and especially Wed night while shifting back to W then NW behind arctic front. Gusts to 35-40 kt likely, peaking Wed night. Gale warnings will be issued. Vsbys may be reduced in snow showers late Wed night across waters east and NE of Cape Cod. Outlook /Thursday Night through Sunday/... Thanksgiving Day: High risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Slight chance of snow. Areas of visibility 1 to 3 nm. Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of snow. Areas of visibility 1 to 3 nm. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night: Winds less than 25 kt. Saturday: Winds less than 25 kt. Chance of rain showers. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain. Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain. && .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...Gale Warning from 4 PM this afternoon to 7 PM EST Thursday for ANZ231>235-237-250-251-254>256. Gale Watch from this evening through Thursday evening for ANZ230-236. Small Craft Advisory from noon today to 7 PM EST this evening for ANZ230-236. && $$ SYNOPSIS...KJC/Doody NEAR TERM...Doody/BW SHORT TERM...KJC LONG TERM...Doody AVIATION...Doody/BW MARINE...KJC/Doody CLIMATE...

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