Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM EST Sun Dec 06 2020 Valid 12Z Wed Dec 09 2020 - 12Z Sun Dec 13 2020 ...Overview... The medium range period will begin Wed with a closed upper low west of Baja California, which is expected to move eastward and become absorbed as northern stream energy drops southeastward from the northern Pacific. As the phasing amplifies a trough across the Intermountain West into the central U.S., this should induce surface cyclogenesis, and the developing low pressure system is forecast to track northeastward across the Plains late week into the Great Lakes region over the weekend. At least some precipitation could spread across the central/eastern U.S. as this pattern promotes moisture inflow from the Gulf of Mexico. In the East, upper troughing is expected to quickly push offshore by midweek, with zonal flow/low amplitude ridging emerging ahead of the central U.S. trough. ...Model Guidance Evaluation... The large-scale pattern described above was generally well agreed upon by the deterministic and ensemble model guidance, especially considering the variability that models were showing in previous days. A multi-model deterministic blend of 00Z/06Z guidance was able to be used for the first part of the period, with reasonably good clustering with the upper trough and developing surface low in the High Plains. By Fri, there are some variations on the frontal system evolution, namely whether one deeper low (GFS solutions) or a couple of weaker frontal wave lows (00Z EC/CMC solutions) will prevail before lifting into the Midwest. These differences were relatively minor for the medium range though. Questions about the trough axis/tilt over the weekend also remain--GEFS and EC ensemble means tend to have a more positive tilt with troughing still entrenched over the Four Corners, while deterministic models vary in their trough axes and ensemble members have a fair amount of spread too. The variability partly depends on models' handling of shortwave energy moving through the western side of the trough. Generally this energy seems weaker this model cycle than in previous ones. Overall, preferred to lean toward the GEFS/EC ensemble means by the late period, but with some incorporation of the 06Z GFS/00Z ECMWF/00Z CMC deterministic runs especially to add depth to features. ...Sensible Weather and Potential Hazards... Multiple rounds of rain and higher elevation snow are possible in the Pacific Northwest through the period as a series of weakening fronts approach. The heaviest totals are likely later this week and into the weekend over the Coastal Range and Cascades as moist onshore flow increases. Light to moderate snows could spread across much of the Rockies underneath the cold upper trough. Farther east, precipitation is likely to develop across the central to eastern U.S. in association with the ejecting trough and developing low pressure/frontal system and Gulf moisture inflow ahead of it. Rain and possibly thunderstorms could spread across the warm sector from the Southwest/Southern Plains Thu, into the Mississippi and Ohio Valleys late week, and toward the East Sun. There is also some potential for wintry weather on the backside of the low across northern portions of the Plains and into the Upper Midwest/Great Lakes for Fri into the weekend. Snow amounts and location remain uncertain at this time, however. Mild temperatures remain likely around midweek for the north-central U.S., with high temperatures around 15 to 25 degrees above normal and actual values in the 50s and 60s there. Then as the upper trough takes hold, near to slightly below normal temperatures are expected for the Intermountain West, Rockies, and Plains. Warmer than average temperatures should push eastward ahead of the trough, spreading from the Mississippi Valley into the Ohio/Tennessee Valleys and toward the Eastern Seaboard late week. 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