Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Dec 22 2021 Valid 12Z Sat Dec 25 2021 - 12Z Wed Dec 29 2021 ...Pattern Overview and Guidance/Predictability Assessment... Today's guidance continues to show a very amplified Pacific into Alaska upper ridge supporting a deep upper trough over the West Coast/Interior West, anchored by a mean low over western Canada. The trough will maintain a lengthy period of chilly and wet/snowy conditions over West with the highest five-day liquid equivalents likely to be between the Cascades and 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 of the Rockies, bringing snow to northern areas and rain farther south. Predictability for system details has been and probably will continue to be a lot lower than that for the large scale mean pattern. Associated fronts will separate a pronounced temperature contrast, between well above normal readings over and east of the southern half or so of the Plains and much below normal temperatures settling over the far northern Plains. The forecast over the East during Christmas weekend is more uncertain than typically would be the case for the days 3-4 time frame. Starting with yesterday's 00Z ECMWF, guidance has recently changed some details for West Coast shortwave evolution around Thursday-Friday. This has led to a dampening of a wave reaching the Great Lakes by the start of the period while enhancing a trailing wave to the southwest. Still there significant differences for the second wave even in the new 00Z guidance, ranging between the GFS that remains quite strong like its 18Z run (and faster 12Z run) and the weak/suppressed CMC. Behind this leading system, the next well-defined shortwave digging into the mean trough should be near the West Coast early Christmas Day/Saturday and eject through/beyond the West thereafter, bringing a system into the Upper Midwest/Great Lakes around Monday. Latest GFS runs are somewhat weaker and more suppressed than most other guidance while the 00Z GEFS mean offers a slight nudge in the consensus direction. Then another feature trails by about two days, bringing a wave into the East by next Wednesday, while additional energy (with a little more spread for shape, and possibly supporting a surface system) should arrive near the West Coast around midweek as well. The 12Z UKMET was out of sync with an average of other guidance over the East early in the period so the first part of the forecast consisted of a blend of other models and a small 18Z GEFS/12Z ECMWF mean component. Then ensemble weight gradually increased to 30-50 percent late in the period. The end result reflected varying ideas of continuity and guidance trends while staying on the conservative side where guidance spread was the greatest. ...Weather/Hazards Highlights... The forecast remains little changed, with shortwave troughs/energy rounding a persistent upper trough aligned near or inland from the West Coast periodically enhancing 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 Cascades and coastal areas of the Pacific Northwest into northern California and the Sierra Nevada. At lower elevations 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 to sea level by Saturday night while other parts of the West will also see declining 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 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 half of the Great Plains will be at the center of the greatest warm temperature anomalies during the period with decent coverage of max/min readings 20-30F or so above normal. Such temperatures look to challenge daily high max records across much of Texas from the weekend into early next week. At times plus 20F or greater anomalies may extend as far east/northeast as parts of the Ohio/Tennessee Valleys ahead of each frontal system. 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 where temperatures will likely be 20-40F below normal. The leading edge of this airmass will make very gradual southeastward progress during the period. Western U.S. temperatures will trend colder with the Northwest seeing expanding coverage of highs 10-25F below normal next Monday-Wednesday. Some locations in the West could see daily records for cold highs. Rausch 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