Extended Forecast Discussion NWS Weather Prediction Center College Park MD 330 PM EST Thu Dec 23 2021 Valid 12Z Sun Dec 26 2021 - 12Z Thu Dec 30 2021 ...Potential for heavy mountain snow and light/moderate lower elevation Pacific Northwest snow continues through early next week across much of the West... ...Bitterly cold temperatures are forecast across portions of the Northwest to Northern Plains next week... ...Overview... A rather stagnant pattern is forecast for the medium range period led by a persistent and highly amplified northern Pacific ridge/upper high. Downstream, deep troughing is expected over the western U.S. as a likely closed mid-upper low drifts eastward over southwestern and south-central Canada, while shortwaves eject from the upper trough and are carried along in fast west-southwesterly mean flow, which will support a series of low pressure systems developing and tracking over the central and northeastern U.S. Meanwhile, another upper high moving from the Gulf of Mexico early week into the Bahamas by midweek will spread ridging and above normal temperatures to the southeastern quadrant of the country south of the storm track. ...Guidance/Predictability Assessment... Model guidance remains in good agreement with the large-scale pattern described above, especially through the middle of next week. Less certain aspects of the forecast include the rounds of shortwave energy propagating eastward from the main trough. There are some variations in the 00/06Z model guidance with a Northeast shortwave already by early Sunday, with the 00Z GFS among the strongest with the energy, which also brings its associated surface low closer to the Northeast coast compared to the consensus that is farther east in the Atlantic. That solution was not favored and the early part of the medium range forecast was based on a blend favoring the 06Z GFS and 00Z ECMWF, along with smaller proportions of the 00Z UKMET and CMC. Good model agreement with this blend also persisted with frontal timing in the central U.S. for early week, despite some variability in location of surface lows along the front. By Tuesday through Thursday, most guidance indicates a mid-upper low closing off over southwestern/south-central Canada as a piece of the polar vortex likely drops southward, and this was indicated by the latest WPC forecast. Shortwaves spinning around the low have less certainty with strength and timing once again. There is a good bit of run-to-run variability in model guidance, but for the 00Z/06Z cycle, the 06Z GFS appeared to be the greatest outlier in deterministic guidance, essentially dividing the upper low by early Thursday with one closed low along the OR/CA border. Thus by the latter part of the forecast period, transitioned to using a proportion of the 00Z GFS rather than the 06Z while still favoring the 00Z ECMWF, and ended the forecast period with 50% GEFS and EC ensemble means combined with 50% deterministic models to temper the individual forecast differences. ...Weather/Hazards Highlights... Rounds of precipitation are forecast for the West as shortwave energy rounding the persistent upper trough periodically enhances moisture inflow. The highest snow totals are likely for the typical terrain enhanced areas of the Cascades, Sierra Nevada, and portions of the Rockies, while coastal Oregon and California could see locally heavy precipitation as well. Snow levels could lower enough under the cold upper trough to bring snow to cities like Seattle and Portland continuing from the short range period into Sunday, while other parts of the West could also see lowering snow levels as the upper trough axis nudges inland. Generally progressive low pressure systems will spread periodic precipitation across the north-central and northeastern U.S. through next week. Snow chances should be confined to the Midwest to Great Lakes and interior Northeast, with a round of heavier snow possible for the Upper Midwest Sunday into Monday. Additional chances for wintry weather are possible later next week as well, but precipitation type, coverage, and amounts are still uncertain. The associated trailing cold front slowly sinking southeastward could provide a focus for enhanced rainfall across the Ohio Valley around Tuesday-Wednesday and toward the Tennessee Valley and Southern Appalachians by Thursday of next week, but with uncertainty remaining regarding the amounts and axis of the heaviest rain. Mild to warm weather is expected across the south-central U.S. northeastward through next week. The southern Great Plains will be at the center of the greatest warm temperature anomalies on Sunday (20-35F above normal), with record daily high minimum and maximum temperatures likely. Anomalous to record warmth may press slightly eastward as the week progresses, with temperatures 20+ degrees above average expected to spread into parts of the Mississippi and Tennessee Valleys. Highs in the 70s will be widespread in the Gulf Coast states, with some temperatures exceeding 80F. Meanwhile, frigid temperatures are likely in parts of the northwestern and north-central U.S., spreading from Montana Sunday into more of the Northwest and Northern Plains through midweek. With temperatures 20-40 degrees below average, even high temperatures could remain below 0F for Montana and North Dakota around Tuesday and Wednesday. Temperatures are also expected to trend colder in the rest of the western U.S., with expanding coverage of highs 10-25F below normal next Monday-Thursday. Increasing numbers of locations along and just inland from the West Coast could see daily records for low maxima Sunday through Wednesday. 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 Hazards: - Heavy precipitation across portions of California and the Pacific Northwest, Sun, Dec 26. - Heavy snow across portions of California, the Great Lakes, the Pacific Northwest, the Upper Mississippi Valley, and the Northern Plains, Sun, Dec 26. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Sun-Mon, Dec 26-Dec 27. - Flooding occurring or imminent across portions of the Pacific Northwest. - High winds across portions of the Southern Rockies, the Central Rockies, the Southern Plains, the Central Plains, and the Southwest, Sun, Dec 26 and Tue, Dec28. - Much below normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Mon-Thu, Dec 27-Dec 30. - Much below normal temperatures across portions of the Northern Plains and the Northern Rockies, Sun-Thu, Dec 26-Dec 30. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Tue-Thu, Dec 28-Dec 30. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml