Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 PM EST Sun Feb 21 2021 Valid 12Z Wed Feb 24 2021 - 12Z Sun Feb 28 2021 ...Overview and Guidance Evaluation... The medium range period will begin with a low amplitude mid-upper trough with a hint of split northern/southern stream flow moving fairly quickly from the West Wednesday toward the East by Friday. Then, additional energy comes into the West for late week into the weekend, deepening a trough there and allowing Gulf of Mexico moisture return to central/eastern parts of the CONUS. Model guidance is in generally good agreement with the overall/longwave pattern, especially for the first part of the medium range period, but with some differences in the specifics. A multi-model deterministic blend of guidance was able to be used for Wednesday/Thursday. Model differences begin to arise with shortwave energy and the initial trough around Thursday night and Friday, with how quickly it moves across the East and how it phases with the large-scale flow. The best clustering seemed to be around the 00Z ECMWF, with the 00Z UKMET having around the same positioning albeit weaker, and the parallel (v.16) 06Z GFS was similar as well. Weighting the aforementioned models seemed like a good compromise, as the 00Z/06Z GFS runs were more progressive and the 00Z CMC was much slower with the shortwave. Then by the weekend, while there is good agreement for renewed upper troughing for the western half of the U.S., and additional energy dropping southward into the trough and sharpening it up by Sunday, detail differences led to more incorporation of the 06Z GEFS mean and 00Z EC mean by this time frame. ...Weather/Threats Highlights... The West can expect rounds of precipitation through the end of the week, with higher elevations of the Cascades and Northern Rockies/Bitterroots likely to see the most snow. Snow is also currently forecast for the Rockies and into the Front Range of Colorado around Wednesday, but with some uncertainty as to how far east into lower elevations it spreads. Farther east, moisture is expected to increase for the southeastern quadrant of the country, and a couple of rounds of moderate to potentially heavy rainfall are possible for the Lower Mississippi Valley into the Tennessee/Lower Ohio Valleys and Southeast. The axis of the heavier rainfall as well as total amounts remain in question at this point. Lighter precipitation farther north across the Midwest could be in the form of snow. In terms of temperatures, the initial trough should bring below normal temperatures by 10-20 degrees across the Rockies and into the Southern High Plains Wednesday into Thursday. Cooler weather could then spread across the East Friday after near to above average temperatures there around midweek. For the weekend, below temperatures are forecast for portions of the western and central U.S., with near to slightly above average temperatures across the East. Tate 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