Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 PM 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. Recent model runs show increasing likelihood of a low pressure wave forming along the front and tracking toward the Great Lakes by next weekend. Meanwhile, 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... This morning's model guidance shows above normal agreement on the overall synoptic pattern evolution during much of the medium-range period across the U.S. A low pressure system exiting the Canadian Rockies will continue to track across southern Canada during the latter part of this week as the associated trailing cold front becomes nearly stationary across the central Plains to the Midwest and lower Great Lakes by next weekend. By next Sunday, differences in if and where or when a wave of low pressure will develop on the front begin to emerge among the global models. The 00Z ECMWF was first to introduce a low pressure wave developing on the front and then tracking toward the Great Lakes by next Sunday. The 00Z CMC showed a less-defined indication of this wave while the 06Z GFS only showed a slight indication. The 12Z GFS then followed suit with a well-defined wave on the front by next Sunday in better agreement with the 12Z ECMWF. The 12Z CMC was the least enthusiastic regarding this low pressure wave. Meanwhile, the next Pacific trough coming into the picture off the West Coast by next Sun shows typical model spread given the distant time frame. Representative of historical biases, latest GFS runs remain on the fast side of the envelope while the 12Z ECMWF was on the slow side. An intermediate solution appears reasonable for handling this feature. An even blend of the 00Z ECMWF and the 06Z GFS together with increasing weights from their ensemble means were used to derived the WPC pressure and wind grids as forecast hour increases. The blend yielded excellent agreement with the previous WPC forecast package. ...Sensible Weather and Hazards... Expect some locally enhanced rain and high elevation snow to continue with a very gradual lessening trend into late this week over favored terrain from the central/northern West Coast to the northern Rockies. 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. Over Florida, easterly low level flow may promote periods of mostly light rain. The western and central U.S. will see a gradual 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. Kong/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