Extended Forecast Discussion NWS Weather Prediction Center College Park MD 521 PM EST Mon Nov 30 2020 Valid 12Z Thu Dec 03 2020 - 12Z Mon Dec 07 2020 ...Active pattern for the East into next weekend... 17Z Update: A rex block pattern is expected to be in place across the western U.S. by the end of this week and into Saturday as an upper low becomes pinched off from the positively tilted trough over the Plains. This upper low will likely retrograde towards the Desert Southwest, with an upper level high developing over the Pacific Northwest, thus keeping the active storm track well north of the Canadian border. A developing coastal low still remains likely for the weekend as the broad upper trough over the eastern U.S. remains in place. The WPC fronts and pressures forecast was primarily derived from a CMC/ECMWF/NAEFS/EC mean blend for much of the medium range period, along with some previous WPC continuity. The GFS was weighted less given a more progressive solution off the East Coast, and the CMC becomes stronger with an upper low dropping across the Great Lakes by Sunday and loses ensemble support, so it was not used for days 6 and 7. It is also worth noting that the EC ensemble members are stronger with the upper low over the Desert Southwest compared to the CMC and GFS members, so this feature could trend stronger in future updates. /Hamrick ...Guidance Evaluation/Preferences... Teleconnections from a strong positive anomaly at 500mb over Saskatchewan through the period favors a closed low to its south near the Southwestern US/Mexico border and positively tilted trough from extending northeastward to southeastern Canada. Upper ridging is forecast to remain over the Pacific Northwest. The models/ensembles have largely shown this synoptic orientation but continue to differ on timing of the embedded systems. Broad consensus/blend approach has served well to limit cycle-to-cycle and day-to-day changes, and this was utilized again with the 12Z/18Z guidance. The GEFS mean and ECMWF ensemble mean were fairly close through the period (GEFS a little quicker) and the GFS/ECMWF/Canadian showed adequate clustering with the means through about Saturday. Uncertainty rapidly increases next Sun/Mon in the East owing to how the upper trough evolves (or remains closed off to the south) and how subsequent/reinforcing troughing may promote another wound-up system in the Northeast next Sun-Mon. Opted to be conservative due to the uncertainty and blended the ensemble means for the last two days of the period. ...Sensible Weather and Hazards... Overall, the South and East will be the wetter areas near and east of the upper trough while the remainder of the country will be relatively dry. The West and Northern Tier will see generally above average temperatures while below average readings will be mostly across the Southern Tier. The main system of interest will be a low pressure area forming in the Gulf of Mexico and sweeping east and northeast up the fall line through the East. Moderate to locally heavy rain is expected again over the northwest Gulf Coast early on, perhaps focused right along the coast. Some snow could be possible on the northern side of the precipitation shield. As the system moves up the East coast, another round of rain is likely for areas along and east of the Appalachians with some snow on the west side. Fracasso Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sun-Mon, Dec 6-Dec 7. - High winds across portions of the Alaska Panhandle and mainland Alaska, Sun-Mon, Dec 6-Dec 7. 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