Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Wed Nov 18 2020 Valid 12Z Sat Nov 21 2020 - 12Z Wed Nov 25 2020 ...Overview and Guidance Evaluation... Latest guidance continues to show fairly progressive and broadly cyclonic mean flow aloft. This flow will contain a series of shortwaves, some of which will be of sufficiently small scale and/or involve stream interaction uncertainty to lower confidence in important specifics several days out in time. While still not fully resolved, one area of significant precipitation will likely extend from the central/south-central Plains into the Lower Great Lakes this weekend as a developing wave tracks northeast from the Plains/Middle Mississippi Valley. The other event that appears to have reasonably good predictability is an increase of precipitation over the Pacific Northwest/northern Rockies during the first half of next week as stronger flow reaches that region. Shortwave energy reaching the West around late weekend may produce another area of central/eastern U.S. precipitation next week as it continues eastward but with great sensitivity to low-confidence details. For the leading central/eastern U.S. evolution from the weekend into early next week, guidance seems to be in the process of trying to converge toward a moderately progressive and phased solution aloft in contrast to latest CMC runs which still want to show some flow separation within the overall shortwave--thus holding back a trailing surface wave not depicted in consensus. Recent GFS/GEFS runs have been gradually trending slower with the surface wave, in the direction of the ECMWF mean that has been stable over the past couple days (north of Lake Ontario as of 12Z Mon). There are still unresolved differences for specifics of the southern part of the shortwave with effects on progression/southward extent of the front trailing from the aforementioned surface wave. The next upper shortwave that reaches the West around Sun and continues eastward thereafter also has meaningful timing and detail differences. Multi-day trends favor the progressive majority cluster rather than the sharper and slower UKMET as the energy comes into the West. Farther eastward the guidance has been very inconsistent regarding the strength and timing of the energy, favoring a conservative blend approach until better clustering develops. Guidance agrees fairly well that strong upstream flow will reach the Northwest after Mon but by midweek there are meaningful differences on the exact shape of this flow as well as for details downstream. At that time the 12Z ECMWF more closely resembles the general ideas of the ensemble means than recent GFS/CMC runs. An operational model blend worked well to depict consensus ideas for about the first half of the period. Then the forecast quickly incorporated 50-60 percent total weight of the 18Z GEFS/12Z ECMWF means while reducing GFS input due to the less favorable comparison to the means late. ...Sensible Weather and Hazards... During the weekend expect an area of organized precipitation from the central/south-central Plains into parts of the central and/or southeastern Great Lakes, as shortwave energy likely encourages surface wave development through the Middle Mississippi Valley and eastern Great Lakes. Most precip should be in the form of rain, some of which may be locally moderate to heavy, while some snow may exist in the northern periphery of the moisture shield. Recent trends have nudged the axis of highest rainfall totals a bit southward and further adjustments may be possible due to dependence on subtle details aloft. Upstream shortwave energy emerging from the West may produce another area of central/eastern U.S. precipitation next Mon-Wed but with low confidence in specifics for timing/coverage/intensity. The general forecast for the 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. Expect highest totals to be over favored terrain across the Pacific Northwest with a secondary maximum over the northern Rockies, while lighter precipitation may reach as far south as northern California. Lower levels will see mostly rain and mid-highest elevations of the Coastal ranges, Cascades, and northern Rockies should see snow. 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. 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