Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Dec 06 2021 Valid 12Z Thu Dec 09 2021 - 12Z Mon Dec 13 2021 ...Considerable mountain snows are likely across the Cascades, Sierra Nevada, Intermountain West, and Rockies Thursday into Friday... ...Overview... As the medium range period begins Thursday, a broad mean upper trough causing quasi-zonal flow across much of the country will be in place, supporting an active weather pattern with embedded shortwaves pushing low pressure/frontal systems across the country. The pattern is forecast to amplify late week as energy entering the western side of the mean trough develops a more pronounced upper trough over the West, with the trough likely to progress steadily onward into the central and eastern U.S. during the weekend. This system will bring the potential for meaningful precipitation and heavy mountain snows over the West and then organized precipitation spreading into the central and eastern U.S. by late week through the weekend along and ahead of a strong cold front. As that trough exits the U.S. around Monday, broad ridging should dominate the lower 48 as another eastern Pacific trough amplifies. ...Guidance/Predictability Assessment... The 12/18Z model cycle continues to be in reasonably good agreement with the overall large-scale pattern described above, while smaller-scale shortwave uncertainties continue to lower confidence in the details. Some slight model differences were seen with amplitude and track of the shortwave energy dropping through the West Thursday-Friday serving to deepen the trough, while there is also variety in track and timing of when a southern stream shortwave west of Baja California Thursday gets absorbed likely on Friday. The 12Z ECMWF appears to be a bit of an outlier with the latter, with a eastern position and faster absorption compared to other guidance and its previous runs, which is likely a cause of minor differences in the broad western trough Friday (slightly eastward extent and more of a neutral than positive tilt in the 12Z ECMWF compared to other guidance). Fortunately this does not cause any significant issues with the surface low pressure deepening in the lee of the Central Rockies Friday, and the bulk of model guidance is in overall good agreement with the trough (and the associated surface cold front ahead of it) tracking across the central U.S. Saturday and toward the East Sunday-Monday. Differences with the specifics of how energy will be distributed within the trough as well as frontal wave differences remain--though new 00Z guidance shows better clustering with a surface low in the Great Lakes early Saturday. By Sunday, the 12Z CMC separates the northern and southern streams and closes off a mid-upper low in the Lower Mississippi Valley, which is an outlier at this point but not outside the realm of possibilities. By early next week, troughing is forecast to develop in the eastern Pacific, but models show variations in timing of the trough pushing southward, which impacts the Pacific Northwest in particular. GFS runs and to some extent the GEFS means have been consistent in showing the troughing with more of a southward extent by Sunday, giving the Northwest southwesterly flow, while slower troughing in the ECMWF and CMC runs lead to zonal east-west flow there Sunday. The differences progress into Monday with how close troughing will be to the coast. Differences for these two major features are generally within the realm of guidance error for the late extended time frame (though understood to be very important for the character of precipitation over the Pacific Northwest next weekend). The WPC forecast was based on a blend of the 12/18Z deterministic guidance early in the period, and phased in the GEFS and EC ensemble means to about 60% of the blend by the end of the period to reduce smaller scale model differences. ...Weather/Hazards Highlights... As the trough deepens in the West, precipitation will spread from the Pacific Northwest and Northern Rockies southward to California into the Great Basin to Central Rockies Thursday and Friday. Favored terrain in Arizona could also see some enhanced activity. Generally expect higher elevation snow and lower elevation rain, with snow levels likely somewhat low as upper troughing amplifies. Parts of Southern California may see the first rainfall in over a month. Then as the western upper trough approaches/pushes into the Plains toward the end of the week, precipitation should track ahead of a low pressure system and a strong cold front. Some wintry weather could spread into the Northern Plains and Midwest/Great Lakes Friday in the cold air north and west of the low track. Then rainfall may become moderate to heavy over parts of the east-central U.S. especially by Saturday with the aid of strengthening Gulf inflow ahead of the strong cold front. Additionally, heavy precipitation chances are forecast to increase in the Pacific Northwest starting around Friday night or Saturday and continuing through the weekend as a potential atmospheric river event sets up, though uncertainty remains with the progression/southward extent of moisture, and will continue to be monitored. Ahead of the trough and cold front, mild to warm temperatures are forecast to be in place in the central U.S. Thursday and Friday, with highs currently forecast to rise into the 80s in portions of Texas and the Lower Mississippi Valley, which could approach or set daily records as the temperatures should be 15-30F above normal. These warm temperatures will push into the Ohio/Tennessee Valleys and Eastern Seaboard on Saturday, while temperatures moderate to near normal behind. The western U.S. can expect below average temperatures especially Friday and Saturday, with the Central Great Basin in particular seeing temperatures 10-20F below normal. Then, as ridging builds across the lower 48 by early next week, temperatures should return to 10-20F above normal across the northern two-thirds of the Plains by next Sunday and Monday. Tate 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