Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Sat Feb 20 2021 Valid 12Z Tue Feb 23 2021 - 12Z Sat Feb 27 2021 ...Overview and Guidance Evaluation... A relatively progressive upper flow pattern will be in place for the beginning of the week as the eastern U.S. trough departs across the western Atlantic. This will be followed by zonal flow across much of the central and eastern U.S. ahead of an amplifying western U.S. trough by Wednesday. A second trough builds in for the Pacific Northwest by the end of the week. The 00Z model guidance suite is overall in above average agreement on the synoptic scale pattern through Wednesday, with a split upper level flow pattern likely developing. By Thursday and beyond, the CMC becomes a little more amplified and progressive with the northern stream trough building in across the north-central U.S. Similar to last night, the 00Z GFS parallel is strong with the second trough/closed low approaching the Pacific Northwest by Thursday night, but is competitive elsewhere across the continental U.S. For the fronts and pressures forecast, the 18Z GFS parallel was preferred over the operational 18Z GFS given some timing differences with the pattern change across the West, with the parallel version closer in line with the model consensus. The GEFS mean did not appear amplified enough with the southern stream trough over the southeastern U.S., so it was not favored for this forecast. A blend of the 12Z ECMWF/18Z parallel GFS/12Z CMC was used through early Thursday, and then less weighting towards the CMC by next weekend and more weighting towards the GFS parallel/ECMWF/EC mean. ...Weather/Threats Highlights... Things will be relatively uneventful across most regions of the nation compared to the extreme winter weather that has been endured by many locations this past week. The storm system crossing the Pacific Northwest and the Rockies is expected to produce heavy snow for the higher terrain of the central and northern Rockies by the middle of the week, and will likely affect much of the Interstate 25 corridor in Colorado. By the end of the week, a corridor of light to moderate rainfall will be possible across portions of the south-central U.S. and extending towards the southern Appalachians, with perhaps some snow on the northern edge of the precipitation. There is still a good deal of model spread regarding the placement of this QPF axis, so changes in the forecast going forward are expected. In the temperature department, the building trough and the passage of the cold front across the Intermountain West will herald a change to readings falling to 10-20 degrees below average for the western High Plains to the Rockies, along not nearly as impressive as the recent Arctic blast based on our current forecasts. Temperatures will likely be 10-20 degrees above average ahead of that same storm system across the northern Plains on Tuesday. By the end of the forecast period next Saturday, expect temperatures to be near to slightly below normal for most of the continental U.S. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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