Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EST Thu Feb 25 2021 Valid 12Z Sun Feb 28 2021 - 12Z Thu Mar 4 2021 ***Heavy rain threat continues from the southern Plains to the Tennessee Valley this weekend into early next week*** ...Overview and Guidance Evaluation... The forecast period begins on Sunday with a positively tilted upper level trough over the western U.S. that is forecast to track across the southern Plains and result in a potential heavy rain threat across portions of the south-central U.S. for the beginning of next week. There will likely be a relatively well defined northern and southern stream flow, with the northern stream being more progressive. A second trough will likely reach the West Coast region by midweek and then affect the Rockies by Thursday. The deterministic models and ensembles are struggling with both the timing and evolution of the next trough approaching the West Coast from the Gulf of Alaska/North Pacific region as early as Sunday night. The GFS and to a lesser extent the GEFS mean are considerably slower and develop a closed upper level low that lingers well offshore, whilst the CMC, ECMWF, and EC mean are much farther east and over the western U.S. by Tuesday. These differences have implications for the southern tier states for the middle to end of the week, and this leads to below average forecast confidence. Model agreement is better for the central to eastern U.S., although notewrothy differences in QPF remain regarding the latitudinal placement of the heavy rainfall axis. The WPC medium-range fronts and pressures was derived based on the consensus of the 12Z ECMWF/12Z EC mean, some of the GFS parallel since it was closer to the favored solution, and smaller contribution from the 12Z CMC/CMC mean with more of the ensemble means on day 6-7 to account for the forecast uncertainty. ...Weather/Threats Highlights... The upper level trough across the southern Rockies and then the southern Plains will allow Gulf moisture to lift northward as multiple waves of low pressure develop along a slow-moving front. This setup will favor a few rounds of locally heavy rainfall late this weekend into early next week from the Arklatex to the southern Appalachians. The latest guidance shows the possibility of general rainfall totaling 1 to 3 inches during the Sunday to Tuesday time period. High temperatures will likely be about 5 to 15 degrees above average across the eastern U.S. on Sunday ahead of the front, and overnight lows will likely be even more above average across portions of the Deep South owing to southerly flow from the Gulf of Mexico and widespread cloud cover. These readings should return closer to late February/early March averages as the frontal boundary settles south of the Gulf Coast by the middle of the week. Meanwhile, readings should be slightly above average for much of the central and northern Plains for much of this forecast period. 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