Extended Forecast Discussion NWS Weather Prediction Center College Park MD 140 AM EST Sun Dec 26 2021 Valid 12Z Wed Dec 29 2021 - 12Z Sun Jan 02 2022 ...Bitterly cold temperatures to continue through much of the period across portions of the Northern Plains... ...Overview... An amplified ridge/upper high should remain anchored over the northern Pacific into next weekend, resulting in amplified and renewed troughing downstream across the western U.S. including a southern stream cutoff upper low which drops down the California coast later this week. Northern stream shortwaves through the Western trough and rounding the base of a large vortex across central Canada, will allow for a series of low pressure systems to track from the Central U.S. to the Northeast. Meanwhile, another upper high over the Caribbean will allow for ridging across the southeastern quadrant of the country to the south of the storm track. The guidance indicates the anomalously strong high over the northern Pacific may finally begin to weaken next weekend which should allow for some ridging to move into the Northwest and shift the general trough axis eastward. ...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. 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 Wednesday-Friday. Models are coming into a little bit better agreement on a closed cutoff low of the California coast Thursday-Friday. The 18z GFS is a bit farther west of the better consensus while the UKMET is the quickest to bring it into the Southwest next weekend. At this point, prefer a middle ground solution closer to the CMC/ECMWF and ensemble means. As the southern stream low/energy works into the Southwest, ridging should once again build into the Northwest as the anomalous north Pacific ridge finally weakens. The models show some uncertainties regarding timing and strength of the ridge by days 6-7 related to questions upstream with troughing dropping down from Alaska. The early part of the medium range forecast was based on a blend of purely deterministic models, minus the UKMET which was much too quick with the southern stream energy into the Southwest. After day 4, the ensemble means were introduced into the blend and gradually increased to help mitigate the late period differences. This provided good continuity through day 6 with yesterdays WPC forecast. ...Weather/Hazards Highlights... Precipitation across the West should be rather quiet compared to the short range period, although some modest snow accumulations may continue in the higher terrain, especially for parts of Arizona and the Colorado Rockies on Wednesday. By Thursday though, another round of heavy mountain snows are possible for portions of the Pacific Northwest/northern Rockies particularly 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 the southwest as well Wednesday-Friday, with snow likely in the higher terrain. A couple of generally progressive cold fronts will move through the central and eastern U.S., allowing for periods of widespread and moderate precipitation across the Tennessee Valley and Appalachians. The first round on Wednesday into Thursday may bring a threat for locally heavy to excessive rainfall, particularly for parts of the southern Appalachians, but with lingering uncertainty regarding amounts and axis of heavies rain. Precipitation chances increase again in roughly the same region by the weekend as the next system makes its way eastward. Some light snow may fall across northern areas from the Great Lakes into the Northeast with the second front. Bitterly cold temperatures should continue through at least Saturday in parts of especially the northern Plains with daytime highs (and a lesser extent overnight lows) 20 to 30 degrees below average. Some areas of northern North Dakota and Montana may stay below 0F even into next weekend, despite temperatures moderating slightly as the week progresses. The addition of 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 be below normal across the rest of the western U.S., with the coldest anomalies on Wednesday where daytime highs could still be 10-20F below normal. Some records are still possible into Wednesday across parts of the West. 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 15-25 degrees above normal on 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. A potential pattern shift may finally bring values back closer to normal by late next weekend. 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