Extended Forecast Discussion NWS Weather Prediction Center College Park MD 146 AM EST Sat Mar 13 2021 Valid 12Z Tue Mar 16 2021 - 12Z Sat Mar 20 2021 ...Bouts of heavy rainfall possible across the southeastern quadrant of the CONUS next week... ...Overview... An upper low over the Four Corners region Tuesday will track eastward through the week, promoting a chance of locally heavy rainfall over parts of the South/Tennessee Valley later in the week. In the Pacific, a reloading upper low will send weakening systems into the West with the brunt of the precipitation into British Columbia. Temperatures will be near normal for much of the lower 48 with any warm-up ahead of a cold front replaced be cooler air in its wake. ...Model Guidance/Predictability Assessment... The 12Z/18Z guidance suite was overall clustered well enough such that a consensus blend approach served well to start the forecast period. The recent GFS runs differed from the ECMWF/Canadian/UKMET (and parallel GFS) over the Pac NW early in the period with how lingering upper troughing exits the region, but otherwise was acceptable. By later in the week, the GFS became quicker to take the trough and surface front eastward but the Canadian was much slower. The 12Z ECMWF was a middle ground solution near the ensemble mean consensus (ECMWF ensemble mean even slower than the ECMWF HRES) and was a reasonable solution. Increased ensemble weighting by the start of next weekend in favor of the ECMWF ensemble mean which was a bit more defined and consistent than the GEFS mean as teleconnections support at least modest ridging along and east of the Rockies with Pacific troughing largely offshore. ...Weather/Threats Highlights... Lead system will bring light to modest rain/snow to the central Rockies and into the Plains Tuesday, with rainfall expanding over the Southeast along a warm front. To the north, some light snow is possible through the Corn Belt and into higher elevations of the central Appalachians and into the Northeast later in the week as the area of low pressure heads toward the East Coast. Rainfall could be heavy in some locations in the warm air mass near and inland from the Gulf Coast Tue-Thu. Farther west, approaching Pacific system will spread low elevation rain and higher elevation snow to the West Coast (NorCal and northern Sierra into coastal OR/WA) especially Wednesday onward. Temperatures will be mild over the Southeast Tue-Wed with widespread 70s just south of the warm front. Colder than normal temperatures will follow the cold front out of the Rockies Tuesday, where highs will be 5-15 degrees below normal. A gradual warm-up is forecast for the interior West into the High Plains later in the period as upper ridging crests over the Rockies. Fracasso 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