Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Tue Dec 21 2021 Valid 12Z Fri Dec 24 2021 - 12Z Tue Dec 28 2021 ...Pattern Overview and Guidance/Predictability Assessment... Guidance continues to show an impressively strong and persistent ridge over and north of the East-Central Pacific helping to hold an amplified upper trough in place over or slightly inland from the West Coast. This upper trough will support an extended period of precipitation and mostly below normal high temperatures over a broad portion of the West. The trough's shape will vary from day to day depending on individual shortwaves digging into the trough, and features ejecting from the trough will periodically support waves/systems to the east of the Rockies. Associated fronts will separate a pronounced temperature contrast, between well above normal readings over and east of the southern two-thirds of the Plains and much below normal temperatures becoming established over the far northern Plains. The primary forecast problems involve the specifics of individual shortwaves within the western trough and then over eastern U.S. how a conflict of large scale pattern teleconnections gets resolved with respect to ejecting features--namely the Pacific/western U.S. pattern favoring above normal upper heights over the East versus the ridging/positive height anomalies northeast of Hudson Bay favoring moderately below normal eastern U.S. heights. As for the first system of interest, there has been decent agreement in principle that fast-moving shortwave energy emerging over the Rockies/Plains early Friday will support a Plains-Midwest through Northeast system Friday and Christmas Day/Saturday. Guidance has had some difficulty in resolving the details due in part to the blocky nature of flow over eastern Canada. New 00Z runs had showed some added improvement in clustering for specifics, but the ECMWF has changed considerably with shortwave details near the West Coast--ultimately leading to a major continuity change farther east. Attention then turns to the next bundle of energy amplifying near the West Coast around the end of the week and separate shortwave energy lingering offshore to the south (lingering energy from an upper low now over the eastern Pacific). The northern stream shortwave has reasonable agreement but there are important detail differences with the southern energy--leading to significant issues for the precipitation forecast over the southwestern U.S. The combination of these features should progress across the West and then support one or more surface lows/fronts from the Plains eastward Sunday onward. By Monday-Tuesday individual model runs and ensemble members increasingly diverge for details at the surface and aloft east of the Rockies, perhaps reflecting the competing teleconnection tendencies over the East. These differences, as well as decreasing confidence for details within the western trough late in the period, favor trending the initial 12Z/18Z operational model blend used early in the forecast toward at least two-thirds weight of the ensemble means (18Z GEFS and 12Z ECens/CMCens) by day 7 Tuesday. ...Weather/Hazards Highlights... Shortwaves digging into the persistent upper trough aligned near the West Coast will periodically enhance the flow of moisture into the West, with areas of focused/terrain-enhanced precipitation extending from the coast into the Rockies. Expect episodes of moderate to heavy precipitation from the Cascades and coastal areas of the Pacific Northwest into California, which will increase the soil saturation and sensitivity to local runoff. However during the extended time frame uncertainty on timing and interactions of shortwaves will temper confidence in resolving the exact location and intensity of lower elevation rainfall with respect to local runoff/flooding issues. Highest precipitation/liquid equivalents should extend from the Oregon Cascades into northwest California and the Sierra Nevada. Meaningful precipitation will also be likely across the highest terrain of the Great Basin and the central/northern Rockies. Areas from southern California through Arizona and the southern Great Basin/south-central Rockies may also see a period of enhanced precipitation late this week and/or into the weekend but specifics are very sensitive to upper flow details for which models do not yet agree. Colder air filtering into the Northwest may lead to very low snow levels by next Sunday-Tuesday while other parts of the West may also see declining snow levels as the upper trough axis nudges eastward. An area of precipitation will accompany a system crossing the northeastern quadrant of the lower 48 Friday into Christmas Day/Saturday. The best potential for wintry weather will extend from the Upper Great Lakes into New England. Another area of precipitation may spread across northern tier areas and parts of the East Sunday onward. Details of this precip area are still ambiguous though once again any wintry weather should be confined to northern latitudes. The southern half of the Plains will be at the center of the greatest warm temperature anomalies during the period with decent coverage of max/min readings 20-30F above normal. Such temperatures may challenge daily records. Some of this warmth will extend farther northeast/east as determined by system/front progression and position. Anomalies should become somewhat more suppressed and less extreme by next Tuesday. Meanwhile temperatures of 20-40F below normal will settle over the extreme northern Plains (primarily Montana and North Dakota) from the weekend onward. Some gradual broadening of the western upper trough should bring a gradual cooling trend to the region with some highs 10F or more below normal. 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