Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Wed Nov 27 2024 Valid 12Z Sat Nov 30 2024 - 12Z Wed Dec 04 2024 ...Arctic cold will spread across the northern Plains with below normal temperatures reaching most areas east of the Rockies... ...Multiple days of heavy lake effect snow this weekend into next week... ...Overview... A large scale upper trough is expected to encompass much of the central to eastern United States over the weekend, with some very gradual shift east as next week progresses. This pattern with west/northwest winds will become favorable for long duration lake effect snow. Some of the snowfall accumulations could be significant in favored locations. The trough aloft and cold surface high pressure stemming from the Arctic will cause temperatures to drop well below average across much of the country east of the Rockies. This will be the coldest air of the season so far, with temperatures falling into the negative teens and wind chills in the -20s in North Dakota Saturday morning. Meanwhile benign weather and more moderate temperatures are forecast for the West. ...Guidance/Predictability Assessment... The large scale pattern over the weekend is fortunately reasonably predictable with the broad troughing encompassing much of the central and eastern United States. Smaller scale features embedded within the mean flow could have sensible weather impacts though. This includes multiple shortwaves tracking through the western and southern sides of the trough that could provide forcing for light snow events and help determine where the frontal position will be. By Tuesday-Wednesday the models differ with the strength of a shortwave exiting into the western Atlantic and also with how much energy comes south from central Canada. The 12Z GFS for example was quite strong with the leading shortwave, and along with the 12Z CMC ended up showing two trough axes within the broader one by Wednesday. GEFS means showed hints of this as well. The 12Z ECMWF on the other hand was strong and deep with the Canadian energy, which dominated to show one deeper trough. The 18Z and newer 00Z GFS indicates one trough but shallower. Generally preferred a broad trough at this point to try to avoid picking out individual shortwave timing. Split flow will prevail across the eastern Pacific to western U.S., as northern stream ridging comes across the Pacific Northwest, while a couple rounds of southern stream troughing nudges toward California. Models are pretty agreeable on the northern stream ridge, other than some CMC runs that have shown shortwaves moving through the northern side of the ridge and suppressing it if not outright becoming out of phase. There are some timing differences with the southern stream troughing, both the first weak energy over the weekend and the next round of positively tilted troughing approaching by Tuesday or so, but nothing too egregious for a Pacific forecast in the medium range time frame, so a model/mean blend worked fine. With broad agreement on the large scale, the WPC forecast used a blend of the deterministic models early in the forecast period, and gradually reduced the deterministics while increasing the proportion of ensemble means to half by Day 7 to smooth out differences from the individual models. ...Weather/Hazards Highlights... As a pronounced upper trough settles over the central and eastern U.S., a significant Arctic air intrusion in the form of a cold surface high will spread from the northern Plains southeastward into the weekend. This event does not look to break many temperature records nor is it particularly early in the season for the first Arctic airmass. However, the cold will be a noticeable change in many areas, and for some across the northern Plains, potentially dangerous cold temperatures are expected. Overnight lows in the negative teens could combine with wind to bring wind chills down into the -20s, approaching -30F, in North Dakota in particular for the weekend. This will pose an increased risk of hypothermia and frostbite on exposed skin, so take appropriate preparedness actions. Highs are forecast to remain in the single digits and teens. Temperatures are likely to gradually moderate across the north-central U.S. and reach average values by midweek. Meanwhile the cold will spread farther southeast in moderated form, reaching most areas east of the Rockies across the Mississippi Valley, Ohio/Tennessee Valleys, and toward the Appalachians and Eastern Seaboard. A handful of record low max and min temperatures are possible across the Ohio Valley into the Appalachians, where temperatures are forecast to be around 15-20F below average and stay below freezing for highs in many areas. By Tuesday-Wednesday the coldest air will migrate over the East with temperatures generally 10-15 degrees below normal as the upper trough axis slowly shifts east. Heavy lake-effect snow is expected through much of the period, as westerly flow develops snow east of the lakes through Saturday, and winds may shift to a more northwesterly direction into early next week for a snow focus southeast of the lakes Sunday-Tuesday. Significant accumulations are possible for some of the favored lake effect locations. While specific accumulations will become clearer over the next few days, peak amounts are likely downwind of Lake Erie and Lake Ontario. Travel disruptions are likely especially on I-90 between Cleveland and Buffalo and I-81 north of Syracuse. Elsewhere, a quieter period is likely for the West precipitation- wise. Light to locally modest precipitation is possible at times in the Pacific Northwest into the northern Rockies. Expect temperatures in the Rockies westward to be near average late this week and warm to a few degrees above normal into early next week. Models continue to show a general signal for a round of light to moderate snow across parts of the north-central Plains to Mid- Mississippi and Ohio Valleys Saturday, though the axis and timing are somewhat uncertain depending on a shortwave aloft. Return flow of moisture could start to come into the western Gulf Coast region over the weekend for some shower activity, which may spread inland into the south-central U.S. by Monday and Tuesday. Tate 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