Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 PM EST Mon Nov 25 2024 Valid 12Z Thu Nov 28 2024 - 12Z Mon Dec 02 2024 ...Unsettled weather for Thanksgiving to be followed by first significant Arctic air blast and lake effect snow event... ...Overview... For Thanksgiving Day into the weekend, the large scale weather pattern will undergo a considerable transition, with a northern and southern stream shortwave energy combining into a broad, large scale trough over much of the central to eastern United States. The low pressure system track and sensible weather on the leading edge of this pattern change remains uncertain though guidance has trended closer toward a common scenario today. A wet, unsettled Thanksgiving is likely for many across the East with a threat of wintry precipitation and heavy snow for the interior Northeast. However, in the wake of this passing system, the weather will turn sharply colder with an intrusion of Arctic air from the northern Rockies and northern Plains through the Eastern seaboard. This will usher some of the coldest air of the season so far and will bring a favorable setup for significant lake effect snow downwind of the Great Lakes. ...Guidance/Predictability Assessment... The greatest model uncertainly and impacts to sensible weather lie with the beginning of the forecast period - Thanksgiving Day - with the large scale pattern undergoing a significant change. While all of the available guidance does show shortwave energy within the developing trough over the central and east, the position/strength of the feature remains somewhat uncertain. While over the past few days there were large timing and spatial differences, at least today the spatial spread has lessened and seems to be converging on a common scenario. The latest GFS has trended closer to the ECMWF/CMC solution, bringing the low pressure system across the Ohio Valley into the Mid-Atlantic (compared to a southern solution advertised over the past few days). It's still on the southern side of the model spread, while the latest ECMWF remains sharper/stronger with the shortwave energy and therefore wraps precipitation further north/interior into the Northeast. So while there is greater confidence on a wet/rainy period for the Mid- Atlantic, the northern extent (and possible snow accumulations) remains a bit less uncertain but bears watching for potentially impactful holiday winter weather for some interior Northeast locations. If the ECMWF- like scenario were to verify, a wet/heavy snow would occur with some potential for significant accumulations. The WPC forecast tried to strike a balance between the most suppressed and the farthest northern solutions with a blend of the deterministic guidance early on resulted in a favorable solution and trended well with continuity. Beyond Thursday, the weather pattern becomes a bit more predictable with the broad troughing encompassing much of the central and eastern United States. A quick moving shortwave trough is likely to pass through the Plains, Midwest, and Ohio Valley Friday/Saturday, potentially bringing a streak of light precipitation. Temperature profiles would support mostly light snow, so a possible area to watch for impactful snow accumulations into the weekend for some areas. The various models do disagree on the timing/position of this feature, which appears to be within the normal spread for this far out on a subtle shortwave. The WPC forecast was based on a blend of the deterministic models for Days 3-5 followed by a 50 percent blend of the ECENS and GEFS means. Overall, the NBM QPF looked reasonable for Thursday then with the significant lake effect snow potential, QPF was nudged higher in the favored locations using some of the deterministic guidance. ...Weather/Hazards Highlights... For Thanksgiving Day, the East Coast weather system will bring widespread precipitation from the Southeast into the Northeast. Some of the rain could be moderate in intensity around the Carolinas but likely falls short of introducing an Excessive Rainfall Outlook at this time. For the Mid-Atlantic and the Northeast, a mostly cold rain is expected through for far interior locations, a mix of rain/snow is likely, with the greatest potential for heavy/wet snow for the higher elevations of the Adirondacks, Greens, Whites, and interior Maine. There remains some uncertainty on the strength and intensity of low pressure and some of this winter precipitation could be heavy for the interior Northeast locations, bringing some locations a White Thanksgiving. The Day 4 Winter Weather Outlook is currently advertising moderate chances (50 to 70 percent) for impactful snow (liquid equivalent of at least 0.25") and this system bears watching for potentially significant/impactful holiday winter weather. In the wake of this system, a very pronounced upper level trough will settle over the central and eastern United States. A significant Arctic air intrusion will bring some of the coldest air of the season so far to places from the northern Plains and northern Rockies eastward. The cold air will arrive first on Thanksgiving Day and Friday and spread eastward through the weekend. The combination of the very cold temperatures and windy conditions will bring dangerously cold wind chills to parts of the Northern Plains and Upper Midwest. High temperatures will be in the single digits to teens and overnight lows well below zero. Wind chill readings across much of North Dakota will be below negative 20F. This will pose an increased risk of hypothermia and frostbite on exposed skin. Heavy lake-effect snow is expected Friday through Sunday, developing east/southeast of the lakes and resulting in significant accumulations for some of the favored lake effect locations. Travel disruptions are likely especially on I-90 between Cleveland and Buffalo and I-81 north of Syracuse. While forecast accumulations will become clearer over the next few days, the latest Winter Weather Outlook is high (above 70 percent) in the favored spots, especially downwind of Lake Erie and Lake Ontario. Elsewhere, a quieter period is likely for the West precipitation- wise. Parts of California may see light to locally modest precipitation at times later this week into the weekend with some energy aloft, with some light amounts perhaps spilling into the Southwest. The same goes for the Pacific Northwest into the northern Rockies. Models are starting to show a general signal for a round of light to moderate snow across parts of the northern Plains on Friday into the Mid-Mississippi and Ohio Valleys Saturday. Over the weekend and early next week, return flow of moisture could start to come into the western Gulf Coast region for some shower activity. Tate/Taylor Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw