Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM 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 for much of next week... ...Overview... As a highly amplified ridge/upper high persists over the northern Pacific through much of next week, troughing is generally expected downstream across the western U.S. during the period while upper lows meander over Canada. Some encroachment of the troughing into the north-central and northeastern tier of the U.S. is also likely as shortwaves round the trough, which will also cause a series of low pressure systems to track across the central U.S. to Northeast. Meanwhile, another upper high over the Caribbean to 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 especially by the latter part of the week. Relatively minor model variations were seen with a series of shortwaves rotating out of the Western U.S. trough towards the Great Lakes/Northeast and the associated surface lows Tuesday-Wednesday, but some strength/placement differences may take until the short range to resolve, and could impact the frontal placement and exact axis of heavy rainfall across the Ohio Valley midweek. Another notable area of concern is regarding the next shortwave dropping down the West Coast, with a closed low likely to develop off the California coast by late Wednesday or Thursday. Deterministic models from 00/06Z and the incoming 12Z cycle still support the idea of this development, but with differences particularly with the timing of its formation, eastern turn toward the coast, and absorption back into the mean flow by late week. The 00Z ECMWF was a bit of an early outlier in closing off the low by early Wednesday, while the 06Z GFS was a quicker outlier with the turn eastward into the California coast compared to other guidance/consensus and its previous (and newer 12Z) runs. The early part of the medium range forecast was based on a blend of purely deterministic models--the 00Z ECMWF/CMC/UKMET and the 00Z/06Z GFS. As the period progressed, eliminated the 06Z GFS from the blend and also introduced and gradually increased the EC and GEFS ensemble means to help mitigate the smaller scale detail differences. This provided good continuity with the previous WPC forecast. ...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 in the West on Wednesday, but another round of heavy mountain snows are possible for portions of the Pacific Northwest/northern Rockies particularly on Thursday ahead of the shortwave dropping down the West Coast. The closed low development off the California coast may help direct increased moisture into parts of southern California and possibly the Southwest 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, possibly even into the latter half of the week, despite temperatures moderating slightly as the week progresses. Cold temperatures combined with gusty winds could result in dangerously cold wind chills, especially for northern parts of the 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 half of the 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 pattern of cool in the West/north-central U.S. and warm in the south-central/eastern U.S., just less extreme. Tate/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