Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 PM EST Tue Dec 21 2021 Valid 12Z Fri Dec 24 2021 - 12Z Tue Dec 28 2021 ...Pattern Overview and Guidance/Predictability Assessment... An impressively strong and persistent ridge over the East-Central North Pacific and into Alaska will maintain amplified troughing along or a little inland from the West Coast and a deep cyclonic gyre over Canada's NW Territories Thursday through this weekend and possibly well into or through next week. This troughing and deep low 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 with good confidence on a potent reinforcing shortwave trough pushing down the coast Friday night before swinging inland into CA Saturday with the next notable wave likely Tuesday/Tuesday night. Shortwave troughs and impulses ejecting east from the trough will periodically track east across the Rockies and the northern tier of the CONUS. 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 their track and intensity over the northeastern U.S. with a conflict of large scale pattern teleconnections -- the Pacific/western U.S. pattern favors above normal upper heights over the East while the ridging/positive height anomalies northeast of Hudson Bay favoring moderately below normal eastern U.S. heights. Overall the 00Z CMC is notably different than the GFS/ECM/UKMET (namely with a more southern Canadian cold core low/gyre), so it was not included in the model blend. Uncertainty still is noted with timing and strength of two waves shifting east across the Great Lakes Friday night and Saturday night. The 06Z GFS was much more potent with the second wave than any other 00Z guidance and the 12Z GFS has backed off to a more open and thus faster solution (where the feature quickly shifts offshore over the Mid-Atlantic Saturday night) which is in better agreement with the latest consensus. The amplifying trough off the West Coast Friday night rounds a lobe of low pressure that anchors over the Salish Sea Friday through Sunday per the latest non-CMC guidance consensus. This trough then swings east across The West and then support one or more surface lows/fronts from the Plains eastward Sunday onward. However, the details in this trough development/progression in deterministic guidance is different enough to warrant some inclusion of 06Z GEFS and 00Z ECENS means even for Day 3. 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 00Z/06Z operational/ensemble model blend used through Day 5 to lean more toward the ECENS and GEFS means for Days 6/7. ...Weather/Hazards Highlights... Shortwave troughs/energy rounding a 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. The 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. 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. It is looking likely that two areas of precipitation in the northeastern quadrant of the CONUS will accompany a pair of shortwave troughs Saturday/Sunday with the first pushing off New England Saturday/Christmas morning and the next crossing from the central Plains Saturday morning and the northern Mid-Atlantic Saturday night. Wintry weather is likely on the north side of both these waves from the Upper Great Lakes through New England. The next wave looks to take a more northern route, tracking from the central Plains to the Lake Superior and into Canada Sunday/Monday. 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 above normal. Such temperatures look to challenge daily high max records across much of Texas Saturday through Monday and maybe Tuesday. This warmth will expand east with the northern extent determined by system/frontal progression over the northern tier. Anomalies should become somewhat more suppressed and less extreme by next Tuesday as the mean troughing continues to broaden and shift eastward toward the Rockies. Meanwhile temperatures of 20 to 40F below normal will push across MT/ND this weekend, expanding to SD/MN by Monday with closer to normal winter temperatures reaching the central Plains and Upper Great Lakes by 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 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