Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 AM EST Fri Nov 13 2020 Valid 12Z Mon Nov 16 2020 - 12Z Fri Nov 20 2020 ...Overview... The dominant focus for significant precipitation next week will be over parts of the West, ahead of an eastern Pacific upper trough whose axis should finally move ashore by next Fri. A downstream ridge will progress from the West into the Plains early-mid week and then flatten/broaden somewhat due to energy likely to eject from the East Pacific mean trough, while a fairly amplified trough will cross the eastern U.S. during the first half of the week and then depart into the Atlantic. ...Guidance Evaluation/Preferences... The most contentious aspect of the forecast involves a developing storm system which guidance brings offshore the Pacific Northwest by late Mon/early Tue. In response to differences in evolution for shortwave energy within the East Pacific mean trough aloft, models have varied significantly among deep/concentrated versus weaker/elongated depictions as well as for longitude of the track. Multi-run continuity through the 12Z/18Z runs offered the GEFS/ECMWF means as the most stable solution for timing/track, with some concepts of the ECMWF and GFS being the closest to the means in principle. The 12Z CMC was weak/slow and the UKMET was deeper but also slow. New 00Z runs still show important differences but as a whole seem be somewhat closer together than in the prior cycle. Going forward in time the GFS/ECMWF and their means remained fairly well clustered for the shortwave energy that ejects inland to support a southern Canada-northern U.S. wave/frontal system Wed-Fri. Clustering was better than average for the remaining East Pacific upper trough to reach the West by next Fri, though latest GFS runs have been on the deep/amplified side of the spread for the trough at that time. Within the early-mid week eastern trough aloft there has been some spread and variability for details and again the 12Z UKMET/CMC noticeably differed from other models and means in some respects. Thus the updated forecast blend for the overall forecast domain employed the 12Z ECMWF/18Z GFS and their means along with a little input from the old 00Z/12 ECMWF to tone down some less confident details in the 12Z run. Total ensemble weight was about one-third from Mon into Wed and then steadily increased to 70 percent by day 7 Fri. ...Sensible weather and hazards... Expect the flow of moisture ahead of the East Pacific mean trough aloft to support highest five-day precipitation totals across favored terrain from the coastal/Cascade ranges of Washington/Oregon into northern California. The deepening storm forecast to track off the Pacific Northwest coast around late Mon into Tue may enhance the moisture flow as well as produce a brief period of strong winds. Some moisture will extend into the northern Rockies through the week and perhaps into the Northern Plains by next Fri. West Coast rain and high elevation snow should trend lighter by Thu-Fri as the upper trough axis approaches/reaches the area. Elsewhere some areas of precipitation will be possible to the lee of the upper-eastern Great Lakes during the first half of the week while some areas of mostly light precipitation may accompany the frontal system traversing the northern tier U.S. into New England mid-late week. A large portion of the western and central U.S. will see above to well above normal temperatures during the period. Greatest anomalies of plus 10-20F or slightly greater are most likely over the north-central through southern High Plains and over/near the Southwest. Best potential for some daily record highs should be over the Southwest. Expect the upper trough arriving from the Pacific and leading surface front to bring highs down to near or slightly below normal values by next Fri over the West Coast states and western Great Basin. Meanwhile some of the Plains warmth should extend into the Midwest/Ohio Valley late in the week, reversing an early-mid week episode of chilly weather over the East. Within this chilly period the best potential for double-digit anomalies may be on Wed when parts of the Mid-Atlantic/Northeast could see highs up to 10-15F below normal. Rausch Additional 3-7 Day Hazards 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