Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Sat Nov 14 2020 Valid 12Z Tue Nov 17 2020 - 12Z Sat Nov 21 2020 ...Overview... The start of the period early Tue should feature a fairly amplified pattern aloft, with an East Pacific mean trough/Rockies-Plains ridge/eastern U.S. trough. The mean pattern should flatten over the following couple days as leading energy ejects from the Pacific trough and flows across the northern U.S./southern Canada. By late week and next weekend, guidance trends/divergence over the past day or so for residual energy within and/or upstream flow feeding into the East Pacific mean trough have increased the uncertainty regarding the timing and character of trough energy that may reach the West in that time frame--with effects on downstream flow details as well. The forecast pattern should promote highest precipitation totals over northern California and the Pacific Northwest with lesser amounts extending into the Rockies. ...Guidance Evaluation/Preferences... Guidance has been steadily improving its clustering for the strong storm forecast to track northward off the Pacific Northwest coast toward Vancouver Island during the day Tue. By the 12Z/18Z cycles the CMC was the only holdout, continuing its weak/elongated tendency from prior runs. The new 00Z CMC is getting closer but is still a bit slow with its consolidation and timing, however the new 00Z ECMWF has adjusted slower. Going forward into Wed-Thu there remains decent agreement with respect to energy from this system ejecting inland and supporting a southern Canada/northern tier U.S. low and frontal system. Given the non-CMC preference for the initial East Pacific system and reasonable consensus elsewhere, the updated forecast emphasized the 18Z GFS and 12Z ECMWF/UKMET during the early part of the period. By late week into the weekend, latest models and ensemble members rapidly diverge for shortwave details across the eastern Pacific and into the western U.S. These differences affect flow farther east as well, with models/members ranging from a broad trough to broad ridge aloft over the northeastern quadrant of the lower 48 by day 7 Sat. Over the past day or so GEFS mean runs and the generally more amplified GFS have been fairly steady with the arrival of Pacific trough energy. On the other hand the ECMWF has trended significantly slower while the ECMWF mean has adjusted slower to a lesser degree. The widening solution spread late in the period favors a conservative approach to incorporating trends and increasing emphasis on the ensemble means. Thus the forecast blend reached 50-70 percent total ensemble mean weight by days 6-7. Also the operational ECMWF input went more to the old 00Z run since its incoming upper trough was closer to the 12Z ECMWF mean. The blend yielded a modestly slower adjustment over the West versus continuity due to partial account for ECMWF/CMC-based guidance trends. ...Sensible Weather and Hazards... The heaviest rain and high elevation snow over the West will extend from northern California through the Pacific Northwest and to a lesser extent into the northern Rockies--especially Tue-Wed in the moist flow ahead of the strong Tue Pacific storm/trailing front followed by residual troughing aloft. Generally expect precipitation to trend lighter with time over the West Coast states while moisture spreads farther inland, eventually reaching into the central Rockies. However note that rapidly increasing uncertainty with flow details aloft lowers confidence regarding coverage and intensity of precip over the West by Fri-Sat. This uncertainty extends into the central U.S. next Sat with differences aloft leading to a wide range of precip potential between significant and modest totals. At the very least there is decent agreement on the idea that the front extending from the system traversing southern Canada/northern U.S. Wed onward should settle over the Plains/Mississippi Valley for a time as upper flow flattens. This front would provide a focus for developing activity depending on timing and strength of upstream shortwave energy. Some rain/snow may accompany the anchoring system over parts of the Upper Great Lakes/Northeast Thu-Fri, and before then the eastern Lakes may see lingering lake effect snow. Easterly low level flow may bring periods of mostly light rain to the Florida Peninsula early and late in the period with a drier period in-between. Pattern evolution will lead to a northwest-to-southeast erosion of above to well above normal temperatures initially over much of the western and central U.S. Expect the warmest anomalies to be over the north-central/central portions of the Plains on Wed with highs potentially exceeding 20F above normal at some locations. Double-digit warm anomalies may be confined to only the far southern High Plains by Sat. Best potential for daily records will be over the Southwest around midweek while isolated records may be possible over the High Plains. Incoming cooler air should bring highs down to near or slightly below normal over the West. The coolest day over the East should be Wed when highs may be 10-15F below normal from the southern Mid-Atlantic into the Northeast. After midweek a modified form of the Plains warmth will spread eastward to bring some double-digit anomalies into the Midwest and then by Sat plus 5-10F anomalies for highs in the East. 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