Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM 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 through much of the week, eventually getting suppressed next weekend by potent upper-level energy dropping south from high latitudes. Downstream, this pattern will result in general troughing (and chilly weather) across the western U.S., including a southern stream likely closed low that 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 across the central and eastern CONUS. The mean trough and a cold front ahead of it are forecast to finally press eastward into the central U.S. next weekend as the pattern changes in the wake of the northern Pacific high weakening. ...Guidance/Predictability Assessment... The latest models and ensembles continue to show good agreement with the large scale pattern described above through much of the period, but with some typical uncertainties in the smaller scale details. Relatively minor model variations were seen with a series of shortwaves and the associated surface lows tracking across the central U.S. to Northeast Wednesday-Friday. For the most part, 00/06Z model guidance continues to indicate fairly good consensus for the shortwave dropping southward near the West Coast Wednesday and developing into a closed/cutoff low at least for some period of time Thursday-Friday (though timing of the cutoff differs somewhat from model to model). The greater concerns are for the track and timing of the low--the 00Z UKMET remained a fast outlier with the eastward turn of the upper low and its absorption back into the main flow compared to consensus, with the 00Z GFS slightly slow. Preferred a blend of the more agreeable 06Z GFS and 00Z ECMWF and CMC with this feature. By the weekend, guidance all has the trough axis pushing into the central U.S. as ridging finally builds into the West, though with some timing and amplitude differences that are well within normal spread for a days 6-7 forecast. The WPC forecast for the early part of the medium range period was based on a blend of the 06Z GFS and 00Z ECMWF/UKMET/CMC guidance, quickly phasing out the UKMET for the reasons above. Since deterministic guidance was in fairly good agreement through the forecast period, was able to keep the blend for the 500 mb heights/surface pressures about 70% deterministic models through the period, with the other 30% GEFS and EC ensemble means. Favored the GEFS mean slightly more given it showed more indication of the potential for a closed low offshore of California late this week. ...Weather/Hazards Highlights... A couple of generally progressive cold fronts will move through the central and eastern U.S., allowing for periods of widespread rain and thunderstorms across the Tennessee Valley and Appalachians into the Southeast. The first round on Wednesday into Thursday may bring a threat for locally heavy to excessive rainfall for the Tennessee Valley and Southern Appalachians and potential severe weather across the Deep South, but with lingering uncertainty regarding placement and rainfall amounts. Precipitation chances increase again in roughly the same region by the weekend as the next cold front makes its way eastward. This second system may bring wintry weather across northern areas from the Midwest/Great Lakes into the Northeast over the weekend. After a break from heavy precipitation across much of the West on Wednesday (with the exception of southern California), another round of heavy mountain snows is possible for portions of the Pacific Northwest/northern Rockies ahead of the shortwave dropping down the West Coast. As this feature develops into the likely closed low, it will direct increased moisture into parts of southern California and into the Four Corners region through Friday or Saturday, with snow likely in the higher terrain. Bitterly cold temperatures should continue through at least Saturday particularly for the northern Plains with daytime highs (and a lesser extent overnight lows) 20 to 35 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 by about 5-15 degrees across the rest of the western U.S., with record cool highs still possible Wednesday. Temperatures could cool further over the Intermountain West by Saturday as a high pressure system builds. 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 through Saturday. Daily record high minimum and maximum temperatures are likely. A potential pattern shift may finally bring values back closer to normal, possibly below average in the south-central U.S., by Sunday. 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 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