Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1250 AM EST Sat Dec 25 2021 Valid 12Z Tue Dec 28 2021 - 12Z Sat Jan 01 2022 ...Bitterly cold temperatures are forecast across portions of the Northwest to Northern Plains early next week... ...Overview... The upper pattern remains relatively unchanged throughout much 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, and shortwaves eject from the upper trough and are carried along in fast west-southwesterly mean flow. This should support a series of low pressure systems tracking over the central and northeastern U.S.. Meanwhile, another upper high over the Bahamas will allow for ridging and above normal temperatures across the southeastern quadrant of the country to the 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 off the California coast. ...Guidance/Predictability Assessment... The latest models and ensembles continue to show good agreement with the large scale pattern through much of the period, but with some lingering uncertainties in the smaller scale details. A series of shortwaves rotating out of the Western U.S. trough towards the Great Lakes/Northeast have come into a little bit better agreement but there are some timing/placement differences will may take until the short range to resolve. This would impact the exact axis of heavy rainfall across the Ohio Valley mid next week. The other notable area of concern is regarding the next shortwave dropping down the West Coast, with potential for a closed low off the California coast by next Thursday. The 18z/yesterday GFS followed suit with its predecessor runs in being weaker and much faster than the better consensus It brings the energy into the Southwest by early Friday while the rest of the deterministic models (and ensemble means) show a well developed closed low well off of Southern California at that time. For the WPC blend, the available guidance presented enough agreement for days 3-4 to warrant a purely deterministic model blend for days 3-4/Tuesday-Wednesday. After that, the ensemble means were introduced and gradually increased as the period progressed to help mitigate the smaller scale detail differences. The GFS was not included in the blend after day 4, due to the issues with the closed low off southern California. This approach also provided good continuity with the previous WPC forecast overall. ...Weather/Hazards Highlights... Precipitation across the West should be waning by the start of the period on Tuesday, although some modest snow accumulations may continue in the higher terrain, especially for parts of Arizona and the Colorado Rockies. Activity should quiet down on Wednesday, but another round of heavy mountain snows are possible for portions of the Pacific Northwest/northern Rockies ahead of the shortwave dropping down the West Coast on Thursday. The closed low development off the California coast may help direct increased moisture into parts of southern California as well later next week. 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 will be ongoing by Tuesday 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 Tuesday-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 for many. Temperatures are also expected to trend colder in the rest of the western U.S., with expanding coverage of highs 10-25F below normal early to mid next week. Increasing numbers of locations along and just inland from the West Coast could see daily records for low maximum and minimum temperatures Tuesday and Wednesday. Meanwhile, mild to warm weather is expected across the south-central U.S. and northeastward through next week where many places from the southern Plains to the Ohio Valley/Mid-Atlantic could be 20 to 30 degrees above normal Tuesday into Wednesday. Daily record high minimum and maximum temperatures are likely. As the pattern deamplifies just slightly over the country later next week, temperatures across both the West and East should moderate, but still maintain a general cool in the West, warm in the East, just less extreme. Santorelli 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, 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