Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 AM EST Thu Dec 23 2021 Valid 12Z Sun Dec 26 2021 - 12Z Thu Dec 30 2021 ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles continue to indicate a highly amplified Pacific-into-Alaska upper ridge to persist into the middle of next week and support a deep upper trough that drifts east from the West Coast through the Interior West, anchored by a piece of the polar vortex that dips toward southern Canada. The trough will maintain a lengthy period of chilly and wet/snowy conditions over The West with the highest precipitation amount from the Oregon Cascades down through the Sierra Nevada. Individual shortwaves ejecting from the upper trough and carried along in fast west-southwesterly mean flow will support a series of waves/frontal systems to develop over the central Plains and then track quickly toward the Great Lakes. Predictability for system details has been and probably will continue to be low. The polar vortex extension in Canada will likely support intrusion of arctic into the northern Plains. This will be contrasted with well above normal readings over the Deep South into the eastern U.S. to the south of the general storm track. The WPC medium-range forecasts were composed based on a blend of 40% from the 18Z GFS/GEFS, 40% from the 12Z ECMWF/EC mean, and 20% from the 12Z CMC/CMC mean. An even blend of the deterministic and ensemble solutions was used for Days 3 to 5 where model spread was at manageable levels. By Day 6, the deterministic solutions begin to diverge while the corresponding ensemble means remain compatible with one another. A higher percentage of the ensemble means was therefore incorporated for Days 6 and 7. ...Weather/Hazards Highlights... Shortwave troughs/energy rounding a persistent upper trough that drifts inland from the West Coast Saturday through Monday will periodically enhance the flow of moisture into the West. Areas of focused/terrain-enhanced precipitation will extend from the coast into the Rockies. The best potential for episodes of moderate to heavy precipitation and highest precipitation totals will extend from the Oregon Cascades and coastal ranges down through central California and the Sierra Nevada. At lower elevations of OR/CA, the continued rainfall will increase the soil saturation and sensitivity to local runoff. Meaningful precipitation will also be likely across the highest terrain of the Great Basin and the central/northern Rockies. Colder air filtering into the Pacific Northwest will bring snow levels for Seattle and Portland to sea level by Saturday while other parts of the West will also see lowering snow levels as the upper trough axis nudges inland. Generally fast-moving waves will periodically spread precipitation across the northern tier states and over parts of the East. The majority of wintry weather should be confined to areas from the northern Plains through the Great Lakes with some further changes for New England. The first round of wintry weather should be from the holiday weekend into Monday. A second round should follow on Tuesday but model solution diverges regarding the associated low pressure system tracking toward the Great Lakes. Precipitation coverage and amounts remain more uncertain than desired so continue to monitor forecasts as details gain better clarity. Farther south, there continues to be meaningful model signal for an axis of enhanced rainfall across the Ohio Valley near/north of a meandering front for early next week. The southern Great Plains will be at the center of the greatest warm temperature anomalies this weekend with decent coverage of max/min readings 20-30F or so above normal. Such temperatures will at record levels across much of Texas on Sunday. This record setting warmth will shift east Monday/Tuesday with the most likely max temperature records for the central Gulf coast with temperature anomalies of plus 20F or greater extending northeast from there across the Ohio/Tennessee Valleys. Anomalies should become somewhat more suppressed and less extreme by next Wednesday as the mean frontal boundary east of the Rockies pushes a little farther south. Meanwhile the core of coldest northern tier air will be over Montana and North Dakota through Monday with with frigid sub-zero temperatures which are 20-40+F below normal. The leading edge of this cold airmass will make a gradual southeastward progress across the central Plains early next week. Western U.S. temperatures will trend colder with the Northwest seeing expanding coverage of highs 10-25F below normal next Monday-Wednesday. Increasing numbers of locations along and just inland from the West Coast could see daily records for low maxima Sunday through Tuesday. Kong/Jackson 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