Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM EST Fri Dec 24 2021 Valid 12Z Mon Dec 27 2021 - 12Z Fri Dec 31 2021 ...Potential for heavy mountain snow in the West continues through early next week... ...Bitterly cold temperatures are forecast across portions of the Northwest to Northern Plains next week... ...Overview... A rather stagnant pattern is forecast for at least the early part of the medium range period led by a persistent and highly amplified northern Pacific ridge/upper high. Downstream, troughing is expected over the western U.S. as a mid-upper low drifts eastward over west-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. By the latter half of the week, model guidance is indicating the potential for a southern stream upper low dropping through the eastern Pacific just west of the California coast. ...Guidance/Predictability Assessment... Model guidance shows relatively good agreement with the large scale pattern of West troughing and Southeast ridging through much of next week, but with some differences in some smaller scale details, particularly pertaining to a couple of shortwaves rotating out of the western U.S. trough. There is good consensus with an initial shortwave and surface low moving across the Great Lakes Monday, and model agreement is increasing somewhat for another shortwave and low there by early Wednesday, with a somewhat faster moving trailing cold front compared to previous forecasts. This is in conjunction with a model trend toward less energy dropping south with a less consolidated/not closed upper low over southwestern/south-central Canada compared to guidance from a day or so ago. This aspect of the forecast remains uncertain given the run-to-run variability. The other main area of model differences in the 00/06Z cycle is with the possible southern stream low developing as energy drops south along the eastern Pacific and West Coast. Generally, model guidance has trended toward the existence of this feature, though with differences in the timing of its closing off and with its track. The 00Z GFS seemed to be an exception at this point in showing a weak and fast trough Thursday-Friday, with its 06Z and 12Z runs as well as runs from yesterday indicating a closed low. For the WPC blend, the 00Z/06Z guidance presented enough agreement for days 3-4 to warrant a purely deterministic model blend between the 06Z GFS, 00Z ECMWF, and 00Z CMC. After that, the ensemble means were introduced and increased as the period progressed to help mitigate the smaller scale detail differences, which could take several days to resolve still. Favored the 00Z EC ensemble mean due to its better representation of a southern stream trough/low off of California compared to the GEFS means. This approach provided good continuity with the previous WPC forecast overall. ...Weather/Hazards Highlights... Precipitation is likely to continue in the West as the medium range period begins Monday as shortwave energy rounds the persistent upper trough and enhances moisture inflow. Multiple feet of snow are forecast for the Sierra Nevada continuing from Sunday into Monday, while other terrain areas of California could also see local enhancement. Additional precipitation should spread into the Intermountain West as well especially on Monday and Tuesday, with the Mogollon Rim and portions of the Rockies with possibly heavy totals. Southern California could see more precipitation from midweek onward as an upper low may form offshore, directing another round of moisture into the area. Generally progressive low pressure systems will spread periodic precipitation across parts of the central and eastern U.S. through next week. The best snow chances should be confined to the Great Lakes and interior Northeast, with rain to the south. 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/Southeast by Thursday-Friday of next week, but with uncertainty remaining regarding the amounts and axis of the heaviest rain. Bitterly cold temperatures are likely in parts of the northwestern and north-central U.S., with daytime highs (and a lesser extent overnight lows) 20 to nearly 40 degrees below average. Some areas of North Dakota and Montana may stay below 0F for more than 48 hours Monday-Wednesday before moderating somewhat by Thursday. Cold temperatures combined with gusty winds could result in dangerously cold wind chills, especially for parts of the northern High Plains where wind chill values could dip well below 0F, even where temperatures are above zero. 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 week. Increasing numbers of locations along and just inland from the West Coast could see daily records for low maximum and minimum temperatures Monday through Wednesday. Meanwhile, mild to warm weather is expected across the south-central U.S. and northeastward through next week. The southern Plains to Mississippi/Tennessee Valleys will be at the center of the greatest warm temperature anomalies, especially Monday-Wednesday where many places could be 20 to 30 degrees above normal. Daily record high minimum and maximum temperatures are likely. Anomalous to record warmth should press slightly eastward as the week progresses, with well above average temperatures expected to spread into parts of the Ohio Valley parts of the East later next week. 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 rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, the Ohio/Tennessee Valley, Wed-Fri, Dec 29-Dec 31. - Heavy rain across portions of the Great Lakes, the Middle Mississippi Valley, the Central Appalachians, and the Ohio Valley, Tue-Wed, Dec 28-Dec 29. - Heavy snow across portions of the Central Great Basin and California, Mon, Dec 27. - Flooding occurring or imminent across portions of the Pacific Northwest. - Much below normal temperatures across portions of the Pacific Northwest, the Northern Rockies/Great Basin/Plains, Mon-Fri, Dec 27-Dec 31. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Tue-Wed, Dec 28-Dec 29. - Heavy precipitation across portions of the Alaska Panhandle, Wed, Dec 29. - Heavy snow across portions of mainland Alaska, Tue, Dec 28. - Much below normal temperatures across portions of the Alaska Panhandle, Mon-Tue, Dec 27-Dec 28. 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