Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Feb 20 2021 Valid 12Z Tue Feb 23 2021 - 12Z Sat Feb 27 2021 ...Overview and Guidance Evaluation... The medium range period is forecast to begin Tuesday with generally zonal upper-level flow, as a trough deamplifies across the eastern CONUS but as troughing begins to develop in the Northwest. The flow pattern should be relatively progressive through the week, as the Northwest trough deepens and tracks eastward, and then additional troughing and energy come into the West for the latter part of the week. Model guidance is in generally good agreement with the longwave pattern through the medium range period. For Tuesday/Wednesday, a deterministic model blend was able to be used with the initial trough dropping southward and with an emerging surface low over the south-central CONUS. Some differences arise with the depth, position, and degree of separation from the main flow with a shortwave making its way through the broad trough around the Four Corners region Thursday. ECMWF runs have tended to be on the stronger and more separated side with this shortwave, with the 00Z UKMET fairly strong as well, and the 00Z/06Z GFS on the weaker side, and this does have implications for the location of the mid-upper trough axis in the East Fri. Tended a little toward the stronger side of the guidance envelope for this feature. Then, overall there was good clustering with ensemble guidance with troughing across the western half of the U.S. by Friday into Saturday. Deterministic 00Z/06Z model guidance was more disagreeable with that trough axis and the eastern extent of the trough, so decided to heavily utilize the 00Z ECENS/06Z GEFS means by the end of the period to minimize those differences. ...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 West could be reasonably active in terms of precipitation, with heavy snow possible in higher elevations of the Cascades and Rockies. Snow could spread across the Interstate 25 corridor in Colorado as well around midweek, but model differences remain with the extent of lower elevation snow. 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 north-central U.S. on Tuesday, spreading to the Northeast midweek. By the end of the forecast period next Saturday, expect temperatures to be near to slightly below normal for most of the CONUS. Tate/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