Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Sun Nov 15 2020 Valid 12Z Wed Nov 18 2020 - 12Z Sun Nov 22 2020 ...Overview... During Wed-Fri the mean flow aloft over much of the lower 48 will trend southwesterly as Mexico/Gulf of Mexico ridging expands into the Southeast while initial West Coast trough energy ejects inland and additional Pacific shortwave energy approaches the West Coast. This latter trough energy should move into the West late week into the weekend and extend from the northern Plains through the Four Corners states by early Sun. With time expect precipitation emphasis to shift from the northern half of the West Coast/northern Rockies into the central Rockies and farther east into portions of the central U.S., as moisture interacts with a front that stalls over the central Plains/Midwest. An upstream northern Pacific trough may extend far enough east/southeast to have some influence on the Pacific Northwest by next Sun. ...Guidance Evaluation/Preferences... Compared to some previous cycles, the 12Z/18Z models provided much improved agreement with respect to the general evolution of upper troughing expected to reach near the West Coast around early Fri and continue eastward thereafter. The most pronounced adjustment was in the ECMWF whose 12Z run trended much faster than its prior couple runs--toward the ensemble means and GFS. There are still uncertainties as this mid-late period trough will likely contain separate pieces of energy from different streams. Some changes in the new 00Z ECMWF illustrate this issue. Other 00Z model runs maintain better than average agreement in principle for a trough axis from the northern Plains into far northwestern Mexico by early Sun, albeit with some detail differences before reaching that point. Farther eastward there is decent continuity/consensus with Canadian low pressure that will bring a front across approximately the northern half of the central/eastern U.S. and extend into parts of the West. The portion of the front over the West may move only slowly around late week as it awaits the arrival of the eastern Pacific trough energy and then sink farther southeast as the upper trough continues eastward. The Plains into eastern U.S. part of the front should eventually stall as it becomes parallel to mean flow aloft. Improved agreement regarding the upper trough coming through the West Fri into the weekend has narrowed the spread for the potential position of the front over the Plains. The final Pacific trough coming into the picture off the West Coast by next Sun shows typical spread given the distant time frame. Representative of historical biases, latest GFS runs are a bit on the leading side of the envelope while the 12Z ECMWF was a little on the slower side. An intermediate solution appears reasonable for this feature. A 12Z model blend along with the 18Z GFS represented consensus well on day 3 Wed. With some UKMET details straying from most other solutions thereafter, the remainder of the forecast incorporated the 18Z GFS/12Z ECMWF and lesser weight of the 12Z CMC along with a gradually increasing contribution from the GEFS/ECMWF means. Still the operational runs were close enough to the means in principle to allow for 60 percent total weight versus 40 percent means by day 7 Sun. ...Sensible Weather and Hazards... Expect some locally enhanced rain and high elevation snow to continue into at least Wed over favored terrain from the central/northern West Coast into the northern Rockies, followed by a lighter trend for precipitation. Trailing shortwave energy will bring rain/higher elevation snow into parts of the Great Basin/central Rockies and vicinity from Fri into the weekend, with highest totals expected over the central Rockies. Also during the weekend moisture extending into the central U.S. should interact with a stalled wavy surface front extending from the central Plains through the northern Ohio Valley/Lower Great Lakes, leading to increasing coverage and intensity of precipitation. Some snow may be possible in the extreme northern periphery of the precip shield. Ahead of the more progressive northern part of the frontal system, an area of mostly light precip may brush the far northern Great Lakes and portions of the Northeast late in the week. Easterly low level flow may promote periods of mostly light rain over Florida. The western and central U.S. will see a cooling trend over the course of the period as near to modestly below normal temperatures replace initially above to much above normal readings. Early in the period, the northern Plains into central High Plains should see the most extreme warm anomalies on Wed with some areas seeing highs 20-25F above normal while a broader area of plus 10F or greater anomalies extends from the Plains into the Southwest. Best potential for some daily records will extend from the Southwest into the southern Rockies/High Plains mid-late week. The eastern half of the country will see the reverse trend. Locations from the Mid-Atlantic into the northeast should see highs 10-15F or so below normal on Wed, followed by above normal temperatures spreading over a majority of the East. Best potential for double-digit warm anomalies will be over the Midwest into Ohio Valley/Great Lakes in the warm sector ahead of the front that approaches/settles over the region. 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