Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1250 AM EST Tue Dec 8 2020 Valid 12Z Fri Dec 11 2020 - 12Z Tue Dec 15 2020 ...General Weather Pattern... A building upper level trough over the Rockies late in the week will induce surface cyclogenesis over the central Plains, and this low is forecast to intensify as it lifts northeastward towards the Great Lakes, and then a triple point low likely forms and becomes the dominant low over the interior Northeast by late Sunday and into Monday. Meanwhile, a well developed Pacific storm system is forecast to affect Washington and Oregon by Sunday and Monday with additional rain/mountain snow and gusty winds. The overall pattern is devoid of arctic air masses with most of the flow originating from the Pacific. ...Model Guidance Evaluation and Preferences... The deterministic guidance and the ensemble means are in decent agreement regarding the building trough over the southern Plains and the broad ridge axis across the eastern U.S. to begin the forecast period on Friday, with more uncertainty across southwestern Canada and the Pacific Northwest and northern Rockies with lead shortwave energy in the northern stream flow. By Sunday, amplitude differences become apparent in the south-central U.S. trough, with the GFS becoming considerably faster than the slower and more amplified 12Z ECMWF solution. By Monday night, the 12Z CMC is much more aggressive in building a second upper trough across the Rockies and then over the Plains on Tuesday, and appears to be an outlier solution when compared to the ensembles. Below average forecast confidence exists for the latter half of the medium range, with basically a compromise of the EC/NAEFS means for Monday and Tuesday. Some 18Z GFS and 12Z ECMWF was utilized through the weekend, albeit in lower percentages than usual, along with some previous WPC continuity. ...Weather/Threats Summary... Temperatures are expected to be 5-15 degrees above normal ahead of the developing Midwest storm system through Saturday, initially over the Ohio Valley Friday and then to the East Coast states by this weekend. Cooler air behind the system will only yield temperatures near to slightly below normal across the north-central U.S. by the weekend. Rainfall associated with this low and cold front will extend from the lower Mississippi River valley to the Northeast U.S., but limited Gulf moisture return and the overall progressive nature of the system are expected to prevent amounts from becoming excessive. Some snow and mixed precipitation is likely on the northwestern side of the storm from the Upper Midwest to the northern Great Lakes. The Pacific Northwest will see more appreciable precipitation with continued onshore flow and especially as a warm front lifts northward along the coast late Friday and into Saturday. There will likely be another increase in low elevation rain and mountain snow for the beginning of the week as the main cold front moves inland, along with gusty winds. Amounts should be modest to perhaps locally heavy overall, as multi-day aerial average totals could reach into the 2-4 inch range, with more in the mountains and less in the valleys. Hamrick 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 Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Sat-Sun, Dec 12-Dec 13. - Heavy snow across portions of the Central/Northern Great Basin and the Central/Northern Rockies, Sun-Mon, Dec 13-Dec 14. 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