Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 PM EST Wed Nov 18 2020 Valid 12Z Sat Nov 21 2020 - 12Z Wed Nov 25 2020 ...Overview and Guidance Evaluation... The consensus among the latest model guidance continues to favor multiple shortwave troughs moving through the West/Central U.S. which setups a mean trough through the extended period. Model spread begins fairly early (by Sunday) and increases through the mid/late periods. The largest differences are in the upstream flow with the waves coming onshore the Pacific Northwest/West Coast and these translates downstream as the week progresses. These notable spreads affect the timing, latitude, and strength of the disturbances coming onshore; which decreases forecast confidence and certainty, in addition, to the location and axis of QPF with these features. Given the uncertainty, multiple pieces of guidance show an area over Southern/Central Plains to the Lower Great Lakes with scattered to widespread precipitation early in the week- with embedded higher qpf areas in eastern Kansas on Saturday and near the Ozarks region of Missouri on Sunday. An additional area of scattered to widespread precipitation will be over the Middle Mississippi/Tennessee/Ohio Valleys by Wednesday. After a brief lull, additional moderate rain will overspread the favored windward slopes of the western Washington/Oregon with a secondary max over the Northern Rockies. The 00Z ECMWF continued to closely resemble the evolution of the ensemble means. The preferred blend was mainly comprised of the 00Z ECWMF/06 GFS with some weighting the 00Z CMC/UKMET initially, however transitioned to mainly ECWMF/GFS and their means by early/mid week with nearly 50-60% of the ensemble means by the end of the forecast. ...Sensible Weather and Hazards... A swath of organized precipitation is expected from the Central/South-Central Plains into parts of the central and/or southeastern Great Lakes region, as shortwave energy likely encourages surface wave development through the Mid-Mississippi Valley and eastern Great Lakes. The majority of the precipitation is expected to fall as rain; however, along the northern periphery of the moisture shield snow may be possible. The southward trend for the axis of highest rainfall has continued. Upstream shortwave energy emerging from the West may produce another area of central/eastern U.S. precipitation next Monday-Wednesday but with low confidence in specifics for timing/coverage/intensity. The general forecast for the Pacific Northwest is consistent regarding the trend toward an increasingly wet pattern as mean troughing aloft over the northeastern Pacific favors onshore flow and the approach/arrival of surface fronts. Favored upslope areas will have higher amounts, falling mainly as rain in the lower elevations with snow likely in the mountains, especially for the Coastal and Cascade Ranges with a secondary max over the Northern Rockies. Lighter precipitation may reach as far south as northern California. Temperatures during the period will vary in accordance with system progression. Readings of 5-15F above normal at the start of the period on Sat from the southern Rockies into the southern two-thirds of the East will get pushed to the south and east ahead of the front anchored by the developing wave forecast to track into southeastern Canada by early Mon. Behind the front moderately cool air over the West will settle into the central/southern Plains through Mon. Less of this cool air will reach the East where readings should only fall to within a few degrees on either side of normal after Mon. The Plains should see a warming trend by next Tue-Wed with the northern half of the region seeing the best chance for some plus 10-15F anomalies for highs primarily on Wed. Campbell/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