Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EST Wed Dec 22 2021 Valid 12Z Sat Dec 25 2021 - 12Z Wed Dec 29 2021 ...Pattern Overview and Guidance/Predictability Assessment... A very amplified Pacific-into-Alaska upper ridge persists into the middle of next week and supports a deep upper trough that drifts east from the West Coast through the Interior West, anchored by a mean low over northwestern Canada. The trough will maintain a lengthy period of chilly and wet/snowy conditions over The West with the highest five-day liquid equivalents likely from the OR Cascades 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 the east from the CO Rockies, bringing snow to the northern tier and rain for the central and southern sections of the CONUS. Predictability for system details has been and probably will continue to be low. Associated fronts will create a pronounced temperature contrast, between well above normal readings over the central and southern sections east o the Rockies and much below normal temperatures slowly expanding over the northern Plains. The forecast over The East Saturday night through Sunday remains fairly uncertain as a wave shifts east from the Great Lakes to off the Northeast. The 06Z and 12Z GFS remain the strongest with this system and was thus limited in the model blend for Days 3/4. Behind this leading system, the next well-defined shortwave digging into the mean trough should extend nearly the length of the West Coast by Saturday evening and eject east over the Rockies through Sunday, bringing a system into the Upper Midwest/Great Lakes Monday. The 06Z/12Z GFS runs remain somewhat weaker and more suppressed than most other guidance. Then the third wave tracks east from the Rockies Monday night potentially reaching the Great Lakes Tuesday night. Uncertainty reigns with the following wave from the Gulf of Alaska Tuesday/Wednesday with the 06Z/12Z GFS tracking it off the West Coast, the 00Z CMC stalling it near Vancouver Island and the 00Z ECMWF over the interior Northwest. The 00Z UKMET remained slower with the initial wave on Day 3, but was not much different than the 00Z CMC there, so small percentages of both were used on Days 3/4 before getting removed in favor of more 06Z GEFS and 00Z ECENS means. The Days 6/7 forecast is rather uncertain with the trough expanding over the west from two amplifying shortwaves, so the timing and strength of these features will come into better focus through the next few forecast cycles. ...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 five-day totals will extend from the OR Cascades and coastal ranges 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 eastward. 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. Precipitation coverage and amounts remain more uncertain than desired so continue to monitor forecasts as details gain better clarity. Depending on position of the mean frontal boundary over the East and wave details, rainfall could trend heavier over the Ohio/Tennessee Valleys into the Appalachians by Tuesday-Wednesday. 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 break daily high max records across much of Texas Sunday. This record setting warm air shifts 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 temperatures will likely be 20-40F 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 look see daily records for low maxima Sunday through Tuesday. 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 Hazards: - Heavy precipitation across portions of California and the Pacific Northwest, Sun, Dec 26. - Heavy snow across portions of California and the Pacific Northwest, Sat-Mon, Dec 25-Dec 27. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Sat-Tue, Dec 25-Dec 28. - Heavy snow across portions of the Great Lakes and the Upper Mississippi Valley, Sun, Dec 26. - Flooding occurring or imminent across portions of the Pacific Northwest. - High winds across portions of the Southern Rockies, the Central Rockies, the Southern Plains, and the Central Plains, Sun, Dec 26. - Much below normal temperatures across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Mon-Wed, Dec 27-Dec 29. - Much below normal temperatures across portions of the Northern Plains and the Northern Rockies, Sat-Wed, Dec 25-Dec 29. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Tue-Wed, Dec 28-Dec 29. 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