Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191923 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 223 PM EST Mon Nov 19 2018 .SYNOPSIS... */ Near- and short-term forecast updates only... Developing storm center tonight through Tuesday night will bring accumulating snows mainly along and N of the MA-Turnpike, impacting the Tuesday AM commute for some. Otherwise an all rain or rain/snow mix. Turning cooler and drier behind the storm, breezy, as an Arctic front sweeps across the region late Wednesday followed by near record cold and bitterly cold wind chills Wed night through Thanksgiving Day. Dry weather with less wind and moderating temperatures Friday and Saturday. However wintry weather may return to Southern New England Saturday night into Sunday. && .NEAR TERM /UNTIL MIDNIGHT TONIGHT/... 230 pm update... Break in the action. Between the waves, quiet conditions prevailing. But precipitation moving back in during the evening hours, becoming widespread towards midnight. Overall light to start, and we`ve got some warmer air to overcome out ahead. Anticipating a start to snow for the high terrain whereas as lower elevations around the Berkshires and Worcester Hills it`ll start off as a rain / snow mix prior to the lower column cooling and snow prevailing. N winds will be driving through this forecast period, no longer light and variable, and likely wet-bulbing will prevail. Most of the impacts along and N of the MA-pike, can`t rule out some travel hazard conditions developing especially in the way of reduced visibility. && .SHORT TERM /MIDNIGHT TONIGHT THROUGH TUESDAY NIGHT/... Tonight through Tuesday... Strengthening storm system across New England bringing accumulating around 3-inch snows mostly N of the MA Turnpike, especially the high terrain. Tuesday AM travel impacts with reduced visibility and snow on area roadways for those areas noted. Synoptically. Initial low sweeps New England with maturation further downstream. Mainly open wave, not so much closed H925-85, tracking over interior New England around the MA / VT / NH border, low level convergent focus of the anti-cyclonic warm-moist conveyor belt out ahead of a digging, sharpening mid-level trof axis and accompanying vortmax energy. Initiated low to mid level thermal packing as well as deformation / frontogenesis beneath mid-level ascent and initial diffluence aloft, majority low-level forcing up thru the dendritic growth zone, H6-7, so not impressively deep. Omega values through the column weak, especially through the dendritic growth zone which happens to exhibit some fairly steep lapse rates. Overall, a decent front-end thump leading out ahead of the surface to H85 low. The storm becomes better organized through Tuesday as it exits into the Gulf of Maine having swept across the Cape Cod Canal around 1 pm. Challenges. Thermal fields / thickness tell the story. Right on the freezing cusp with deeper Arctic air absence N, hardly a N/NE high over Canada. While N isallobaric flow, it`ll only drain S surface temperatures ranging around 30 with dewpoints a tad lower such that wet-bulbing can only do so much. Low-level column nearly isothermal up through H8, warmer air intruding as the H925-85 flow flutters along the MA / NH / VT border, possibly some drier air working in through the dendritic growth zone, while fluttering around freezing as the event evolves into the daylight hours, dealing with forecast headaches concerning precipitation types and snowfall accumulations as well as snowfall intensity. Speaking of, as model forecast mass fields look pretty similar, small-scale focusing forcing mechanisms vary as discerned via model forecast plume diagrams. Considerable spread in respect to liquid-equivalent precipitation and snowfall, the latter ranging from sub-advisory to warning levels in particular locations. Relates back to snowfall intensity, some low probability of 1"/hr noted in high-res guidance, and thermal field alignment as to where the rain / snow cut-off line will be at any one particular time. Forcing majority is, however, below the dendritic growth zone with this synoptic setup. Of last note, model forecasts assume that everything which falls accumulates on the ground which we know is not always the case. So it`s definitely not easy but we`re going to do our best as we always do. Decisions. Despite uncertainties and challenges noted, a consensus weighted forecast does not seem like a bad idea. Gave greater weight towards the HREF blended within. With probabilities noted, and the 19.12z forecast suite slightly jogging S, focus is around the MA / NH / VT border. Coating N of the MA-pike, with 1-3" lower elevations while 3-6" higher, 500 feet and above especially. Potentially near warning-level snows up around the N MA towns of Ashburnham, Ashby, and Townsend. Keep with present N/W MA WINTER WEATHER ADVISORIES. The storm center sweeping NE across the Cape Cod Canal important with respect to blustery N/NE winds out ahead. Impacts. Expect hazardous travel over N/W MA especially the higher terrain of Worcester Hills and Berkshires. Route 2, western-half of MA I-90, N MA I-95, I-190 out of Worcester, and the NW beltway of 495 all have the risk of accumulating snows on roadways. Reduced visibility as well especially within any moderate to heavy snow as near 1"/hr snowfall rates are possible. Thinking a very low risk of any mixed wintry precip-types mainly in the form of freezing rain. Tuesday night... Turning colder and drier. The storm continuing to deepen into the Gulf of ME, blustery NW winds follow. Some lingering snow showers possible, especially for the high terrain. Otherwise the main storm is the driving cold along with breezy conditions that`ll make it feel 5 to 10 degrees colder than the forecast nighttime lows around the low to mid 20s, possibly upper teens for the high terrain. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... 4 AM update... Highlights... * Arctic front late Wed with snow squalls possible * Near record cold & bitterly cold wind chills Wed night/Thanksgiving * Moderating temps and dry Friday into Saturday * Wintry weather possible Sat night into Sunday Wednesday... Arctic front moves thru the region 18z-00z. Models differ on track of arctic frontal wave. The NAM is farthest to the south across the NH/MA border, while the remainder of the guidance is across northern New England. The NAM solution would yield more frontal convergence into northern MA resulting in a greater risk of snow squalls. While reminder of guidance keeps greater risk across northern New England. Still almost 72 hrs out so will have to see how later model runs trend. NAM solution supports steep low level 0-2 km theta-E lapse rates and plenty of low level moisture for instability to be realized. This combined with shallow robust convergence would support potential for a few strong snow squalls. Given very busy holiday travel Wed will have to watch this closely. Regarding high temps, 925 mb temps peak around -4C ahead of the arctic front, so could see highs 35-40 before the frontal passage. Wednesday night and Thanksgiving... Near record cold and accompanied by bitterly cold wind chills as arctic airmass overspreads the region. By 12z Thu model guid has 850 temps down to -20C to -22C across the region! These values are -3 standard deviations colder than climo. 1000-500 thicknesses are impressive as well, with 498 dam contour traversing northern MA! Given the magnitude of arctic airmass we followed by the coldest guid which yields mins just before sunrise Thu of 5 to 15 followed by temps only rebounding into the upper teens and low 20s Thanksgiving Day. In addition strong northwest winds will provide bitterly cold wind chills. For outer Cape Cod, arctic air streaming in on NNW winds will resulting in possible ocean effect snow showers with the greatest probability from PVC to CQX given wind trajectory. Friday... Cold anomalous trough moves offshore with arctic 1037 mb high settling over the region. Very cold morning with lows in the single digits and teens! Temp moderate into the upper 20s and lower 30s. Thus not as cold as Thu and especially less wind. However still much colder than normal as normal highs should be around 50! Nevertheless much more tolerable than Thu. Next Weekend... Dry weather possibly lingers into much of Sat. However active weather pattern with next trough approaching the region Sat night into Sunday. Its a southern stream system so abundant moisture is available. Given cold air from Thu/Fri retreats looks warm enough for rain however still 6-7 days away. Nonetheless heavy rain potential that would impact heavy travel day Sunday. && .AVIATION /19Z MONDAY THROUGH SATURDAY/... 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... Mix MVFR to low-end VFR. Will hold IFR for the high terrain and closer to the MA / NH / VT border with respect to CIGs and VSBYs. Light winds overall. Pockets -FZRA for high terrain terminals through 16z, otherwise -RA lingering through early afternoon then a break before the next storm into this evening. Tonight and Tuesday... A mix of MVFR-IFR to start except SE New England terminals, conditions lowering with onset RA/SN. SN for N/W MA terminals especially over the high terrain. RA/SN mix around the MA-pike while rain elsewhere or transitioning to rain. VSBYs more than likely impacted by SN. Can`t rule out IFR-LIFR conditions. N winds prevailing, stronger NE along the E MA coastline as a storm center passes over the Cape Cod Canal around 18z. Gusts up around 25 kts possible for E coastline terminals. Tuesday night... Winds shifting NW as conditions improve towards VFR, the 18z storm center lifting out into the Gulf of ME. Gusts up to around 25 kts possible especially over the high terrain and along coastline terminals. KBOS Terminal... A break but CIGs only lifting 2-3 kft with a lull in outcomes from earlier -RA. Then looking at lowering conditions evening and over- night with SNRA mix through early morning before changing over to all RA. MVFR-IFR CIGs and VSBYs with increasing N/NE winds through the day shifting NW into evening and overnight. Gusts potentially as high as 25 kts with NE winds around 18z before turning NW and remaining blustery. KBDL Terminal... Keep MVFR prevailing. Then lower conditions moving back in towards evening continuing through overnight. RASN mix right through the Tuesday morning push, becoming all rain towards midday. Mostly IFR, improving MVFR to VFR late Tuesday into Tuesday evening. N winds throughout shifting NW late. Somewhat breezy, especially late with potential gusts up to 20 kts. Outlook /Wednesday through Friday/... Wednesday: Mainly VFR, with local IFR possible. Windy with gusts to 30 kt. Slight chance SHSN. Wednesday Night through Thanksgiving Day: VFR. Windy with gusts to 35 kt. Thursday Night: VFR. Windy with 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. Storm center passing over the Cape Cod Canal around 18z Tuesday. Will see increasing E/NE winds out ahead especially over the E waters, S/SW for the S waters. Behind the storm center, winds turning NW, remaining blustery. Throughout, gusts up around 25 kts possible. Can`t rule out near gale force gusts. SMALL CRAFT HEADLINES in effect for the later-half of Tuesday going into Tuesday night as seas build up around 5 to 7 feet on the outer waters. Outlook /Wednesday through Friday/... Wednesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain showers. Wednesday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of snow showers. Areas of visibility 1 to 3 nm. Thanksgiving Day: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of snow showers. Visibility 1 to 3 nm. Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Local visibility 1 to 3 nm. 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...Winter Weather Advisory from 10 PM this evening to 5 PM EST Tuesday for MAZ002>004-008-009-026. RI...None. MARINE...Small Craft Advisory from 4 PM Tuesday to 4 AM EST Wednesday for ANZ232>234. Small Craft Advisory from 1 PM Tuesday to 1 AM EST Wednesday for ANZ231-251. Small Craft Advisory from 4 PM Tuesday to 6 AM EST Wednesday for ANZ235-237. Small Craft Advisory from 10 AM Tuesday to 6 AM EST Wednesday for ANZ250. Small Craft Advisory from 1 PM Tuesday to 6 AM EST Wednesday for ANZ254>256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Nocera/Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell CLIMATE...Sipprell

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