Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Tue Nov 17 2020 Valid 12Z Fri Nov 20 2020 - 12Z Tue Nov 24 2020 ...Overview and Guidance Evaluation... From the multi-day mean perspective guidance shows broadly cyclonic flow with fairly brisk progression. Individual model and ensemble member solutions indicate above average difficulty in resolving important embedded shortwave details that will affect sensible weather over some areas. The first feature of note will be a shortwave moving into the West late this week and continuing across the country thereafter, interacting with a leading surface front initially extending from the northeastern U.S. through the central Plains into the Four Corners states. Compared to yesterday the guidance is showing increased spread regarding individual impulses within the overall trough, with differences in phasing or flow separation leading to some very different ideas for low pressure evolution over the eastern half of the lower 48 from late weekend into next week. For the purposes of the deterministic forecast, prefer to lean away from the more separated solutions aloft with stronger/slower surface low pressure like the 12Z ECMWF/CMC given the progressive pattern--but with the understanding that some potential for that scenario does exist. The new 00Z ECMWF has trended away from the 12Z run's separated/slow evolution. Upstream the ECMWF-based guidance has been trending toward other faster and less amplified solutions for leading energy forecast to approach/reach the West by around Sun. This reflects greater influence of strong North Pacific flow upstream, which should ultimately develop a broad trough aloft over the northeastern Pacific by the end of the period next Tue, and suggests that the positive height anomaly expected to be near 40N 170W may not be strong enough to support a more amplified eastern Pacific/West Coast trough aloft recommended by teleconnections. The incoming energy around Sun will provide additional detail uncertainties as it continues across the western/central U.S. early next week. During the first half of the period an operational model blend among the 12Z models and 18Z GFS provided an intermediate starting point for significant features given lower predictability smaller scale detail issues aloft. Then the forecast rapidly adjusted toward primary emphasis on the ensemble means, with the 18Z GFS providing the only operational model input after early Mon. ...Sensible Weather and Hazards... From late this week through the weekend one area of significant precipitation will start out as snow and lower elevation rain over/near the central Rockies and quickly expand across portions of the central U.S. as the upper shortwave progressing from the West into the Plains interacts with a northeast-southwest front and likely encourages some degree of surface wave development. Areas from the central Plains through Midwest into central Great Lakes continue to see the best potential for highest totals from this event. Meaningful rain may also extend to the south of the central Plains/Midwest. The northern periphery of the moisture shield may contain some snow, most likely from the central/north-central High Plains into the Upper Great Lakes if any does fall. Precipitation coverage and intensity become increasingly uncertain over the central/eastern U.S. by early next week given a variety of solutions for upper shortwave details and associated surface evolution--not just the leading system of interest but also for upstream energy emerging from the West toward the end of the period. Over the Northwest, for late this week expect most rain and higher elevation snow to be confined to the northern Pacific Northwest/extreme northern Rockies and be of light-moderate intensity. Then the northern half of the West Coast into the northern Rockies will likely see a trend toward heavier precipitation from Sun through Tue as strong Pacific flow aloft brings a surge of moisture into the region, with highest totals focused over favored windward terrain. Late this week areas from Arizona and the southern Rockies northeastward will see well above normal temperatures in the warm sector ahead of the initial northeast-southwest front, with a decent area of plus 10-20F anomalies along with min temps possibly exceeding 20F above normal over parts of the central Plains. System/front progression will bring plus 5-15F anomalies into the East around Sun-Mon. Trailing cooler air will offer near to moderately below normal anomalies spreading from the West into the southern half of the Plains Fri-Mon before moderating farther east thereafter. The West will see temperatures rebound to near or slightly above normal by next Mon-Tue. The northern Plains should be moderately above normal most of the period, with a late warming trend possibly bring some highs at least 10F above normal by next Tue. 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