Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sat Dec 05 2020 Valid 12Z Tue Dec 08 2020 - 12Z Sat Dec 12 2020 ...Overview... An closed upper low is forecast to drop south and settle west of Baja California around Tue-Wed, cut off from northern stream flow overtop before getting kicked toward the Southwest/Southern Plains Thu-Fri as an uncertain upper trough digs upstream into the West. Troughing over the East is expected to de-amplify during the early part of the medium range period and lose influence. Fairly quiet weather is expected across the nation overall in terms of precipitation. ...Guidance Evaluation/Preferences... The initial pattern described above for the first part of the period has fairly good agreement amongst the model guidance, though with some minor differences in embedded shortwave energy moving through the weakening trough in the East. A 00Z/06Z multi-model mainly deterministic blend was able to be used through about midweek. After that, model spread quickly increases in both the northern and southern streams. 00Z/06Z GFS runs and the 00Z UKMET take the southern stream low back east toward the Southwest faster than the 00Z ECMWF/CMC Thu, but model variability becomes even greater from there due to big differences in northern stream energy moving through the Gulf of Alaska/northeastern Pacific, affecting the phasing of the streams. Run-to-run variability is also large. Thus by late week, mainly used the EC/GEFS ensemble means for today's WPC fronts/pressures forecasts. ...Sensible Weather and Hazards... The most notable precipitation is forecast for the Pacific Northwest, with some moderate amounts especially in the Coastal Range and Cascades around Tue, and rounds of light amounts through the week. Snow levels could begin relatively high but lower as the week progresses. Light amounts should spread to the Northern Rockies and more broadly across especially higher elevations of the West given the potential digging upper trough. Rain may develop for the Southwest and Southern Plains around Thu as the Baja upper low/trough ejects, and depending on the upper pattern's evolution and the potential to tap into Gulf of Mexico moisture, precipitation could spread across the east-central U.S. by late week. Precipitation location and amounts remain highly uncertain. Ridging will promote milder than normal temperatures by 15-25 degrees across the High Plains to the Upper Midwest/Corn Belt, moderating closer to normal by late week. Actual temperature readings could approach 60 degrees as far north as South Dakota on Tuesday. Colder than normal temperatures are favored for much of the East for Tue into Wed, before trending back toward more typical values and potentially a bit above normal. Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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