Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EST Wed Dec 02 2020 Valid 12Z Sat Dec 05 2020 - 12Z Wed Dec 09 2020 ...Guidance Evaluation/Preferences... Complicated flow pattern will generally be guided by a positive height anomaly in western Canada (upper ridging into the northwestern corner of the CONUS) which favors troughing or an upper low in northwestern Mexico and longwave positively-tilted troughing from the Plains to the Northeast. Through the 12Z/18Z guidance, the models continued to struggle with the several players within the longwave pattern. This mostly broke down to two camps: the 12Z ECMWF/ECMWF ensemble mean and everyone else. Given that the ECMWF/ECMWF ensemble mean made a rather significant change from even just before the medium range period in the East (stronger northern stream trough resulting in a quicker southern stream interaction and movement to the east), opted to rely mostly on the GFS/Canadian/UKMET cluster with the older 00Z/01 ECMWF ensemble mean, which was overall closer to continuity as well. GFS was not preferred in the West/Southwest as it did not close off an upper low as predicted by several runs of the guidance. 12Z Canadian ended up as the closest deterministic model toward the preference until next Tue or so, when the GEFS mean represented a best start amid ongoing uncertainty in the details. Quick look at the 00Z guidance shows a bit less spread but still lots to be worked out. ...Sensible Weather and Hazards... Lead system in the East will lift through the I-95 corridor with mostly rain and some inland/elevation snow. This could be locally modest near the surface low track and to its northwest side, though that path remains uncertain. Some lingering light rain/snow is possible on the backside of the system as it departs, aided by the eastern Great Lakes. Southern portion of the front will bring some rain to central and southern Florida, generally on the light side but potentially some heavier amounts. Pacific systems mostly offshore to start will bring some light rain and mountain snow to Washington (mainly Sat-Mon). A cold front is forecast to come into western Washington on Tuesday with a more widespread area of rain/snow especially for the coastal ranges and Cascades as well as northern Idaho and northwestern Montana. Temperatures will be near to below normal across the southern tier west of the Rockies into the Northeast under a general west to northwest flow. Milder than normal temperatures are expected for the northern tier (High Plains to the Upper Midwest). Daytime temperatures may be 10-20 degrees above normal over some areas, especially the Dakotas. Actual readings may climb into the 50s to near 60. Fracasso 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