Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 PM EST Fri Nov 25 2022 Valid 12Z Mon Nov 28 2022 - 12Z Fri Dec 02 2022 ...Snow likely across higher elevations of the West into next week... ...Heavy Rain and severe weather possible across parts of the South and into the Ohio Valley Tuesday-Wednesday next week... ...Overview... Latest guidance agrees fairly well for the large scale pattern that should feature mean troughing over the West and an initial Bahamas/Cuba upper ridge ultimately building westward over the Gulf of Mexico. Within this pattern, features across the lower 48 should be progressive. Leading New England low pressure and trailing front should quickly depart after early Monday. Then western-central U.S. trough energy during the first half of the week should spread rain and mountain snow across the north-central West Coast into central Rockies, and then support deepening surface low pressure that tracks from the central Plains into the Great lakes Tuesday-Wednesday. The trailing cold front may focus heavy rainfall from the South into the Ohio Valley, with severe weather also possible over southern areas, while some snow will be possible north/west of the low track. Another system moving into the West during the latter half of the week may spread precipitation farther to the south and east across the region. The overall pattern will favor below normal temperatures over much of the West and parts of the High Plains, with Montana and nearby areas likely to see the most persistent and coldest anomalies. ...Guidance and Predictability Assessment... The first primary forecast issue involves the combination of trough energy reaching the West early in the week and a small upper low farther east near the Canadian border (with its own surface system), with these subtle details having a meaningful impact on the depth and track of central Plains through Great Lakes low pressure Tuesday-Wednesday. Latest GFS runs including the new 12Z version are on the weaker (and south) side while passing through the Great Lakes but eventually strengthen more over Canada. The rest of the deterministic models (especially the UKMET and ECMWF) show this system deepening quicker over the Great Lakes around next Wednesday. However, these important details are sufficiently small in scale to have low predictability several days out in time so an intermediate approach for depth and track appears reasonable at this time. This was achieved by an operational model composite as the starting point for the forecast into early day 5 Wednesday. The next trough entering the Northwest around Wednesday next week cotinues to show more variability in terms of track and timing. The latest CMC runs continue to be quicker with the initial trough and then another reinforcing shortwave later in the week. Meanwhile, the GFS and ECMWF suggest a little slower progression through the Northwest with more of a south component into possibly northern California next Friday. The ensemble means show this same varibility, withe the GEFS mean a little more progressive than the GFS, but the ECENS mean suggesting a southward shift like the ECMWF. Despite the differences, a general model composite blend for day 3-5 worked well, with more emphasis towards the ECMWF. By the end of next week, increased contribution from the means, along with the GFS and ECMWF. The CMC was removed after day 5 due to its issues with late week troughing in the West. Overall, this approach maintained good continuity with the previous shift as well. ...Weather/Hazard Highlights... Expect the western U.S. to see two primary precipitation events next week. The first should spread rain and mountain snow from parts of the Pacific Northwest and northern California through the Great Basin and central Rockies Monday-Tuesday, with highest totals over favored terrain from the Oregon Cascades into the central third of the Rockies. The next area of moisture should arrive into the Pacific Northwest on Tuesday and then expand steadily southeastward across the West through the rest of the week. Some precipitation may be on the locally moderate to heavy side over the Pacific Northwest Tuesday-Wednesday, with declining confidence in the coverage and intensity elsewhere later in the week due to uncertainty in upper trough details. Precipitation could reach as far south as central or southern California late week. Snow levels in the Northwest could lower to fairly low elevations toward the end of the week. During Tuesday-Wednesday, strengthening central Plains into Great Lakes low pressure as well as its trailing cold front plus a separate warm front lifting through Gulf of Mexico will likely bring precipitation to much of the eastern half of the lower 48. Widespread and potentially heavy rainfall is possible from the lower Mississippi Valley into the southern Appalachians and potentially some locations farther north as well. Sufficient moisture and instability in the warm sector could also lead to severe weather, with the Storm Prediction Center already highlighting the chance in their outlook for next Tuesday across parts of eastern Texas into the lower Mississippi Valley. There also remains potential for snow to the northwest of the surface low track with a possible focus over the Upper Midwest. Details on this snow potential will become better defined once guidance shows better clustering for track and depth. Finally, this storm may produce strong winds over some areas of the southwest and also across the Great Lakes. Much of the West and portions of the High Plains should see below normal temperatures for most of next week. Coldest anomalies will generally correspond to the two primary upper troughs affecting the region, especially on Tuesday and Friday when the greatest coverage of highs 20-30F below normal is likely over Montana and also farther south/southeast on Tuesday. Expect highs 10-20F below normal over the Interior West on Tuesday and the Northwest/Great Basin on Friday. Meanwhile, the south-central and east-central U.S. will likely see a brief period of above normal temperatures (up to plus 10-15F anomalies for highs and slightly warmer for morning lows) on Tuesday-Wednesday ahead of the system emerging from the West/Plains. Then the cold air behind the front will drop readings over most of these areas to below normal by next Thursday. Santorelli/Rausch 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 Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Tue-Wed, Nov 29-Nov 30. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Tue-Wed, Nov 29-Nov 30. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Wed-Thu, Nov 30-Dec 1. - Severe weather across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley, Tue, Nov 29. - Flooding occurring or imminent across portions of the Southeast. - High winds across portions of the Southern Rockies, the Southern Plains, and the Southwest, Tue, Nov 29. - High winds across portions of the Northeast and the Great Lakes, Wed, Nov 30. - Much below normal temperatures across portions of the Pacific Northwest, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Fri, Dec 2. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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