Extended Forecast Discussion NWS Weather Prediction Center College Park MD 233 PM EDT Tue Oct 05 2021 Valid 12Z Fri Oct 08 2021 - 12Z Tue Oct 12 2021 ...Pattern Overview and Guidance Evaluation/preferences... The stagnant upper low over the east-central U.S. during the short range should be diminishing by Friday, with general troughing lingering along the East Coast through the weekend and upper ridging trying to build back in early next week. To the west, amplified troughing will drive a cold front through the Central U.S. this weekend, eventually weakening across the Midwest. The next system should dig into the Western states late this weekend, amplifying into a fairly deep system by the day 6-7 period. The latest guidance generally agrees on a deep system, but disagree on the details. Some of the models show the potential for a closed low to develop over the Rockies/Southwest region, but there remains plenty of run-to-run variability and uncertainty. The WPC blend used a nearly equal blend of the 00z ECMWF, CMC, and 06z GFS for days 3-4. Began incorporating minor amounts of the ensemble means (ECENS/GEFS) to try and mitigate details differences that arise both with respect to the initial troughing across the Central/Midwest states and then with the late period trough over the West. By day 7, the blend is a 50 percent ensemble mean and 50 percent GFS/ECMWF combo. This approach maintains good continuity with the previous WPC shift. ...Weather/Hazards highlights... Enhanced rainfall may continue into the medium range period over parts of the southern/central Appalachians, with a continued runoff/flood threat as heavy rainfall is also expected during the short range period over this same region. By Friday, the focus should shift to the coast where a frontal boundary and area of low pressure, with low tropical development probabilities, could bring localized heavy rainfall to portions of the Outer Banks/Southeast Coast Saturday-Sunday. Meanwhile, the first front lifting through the north-central U.S. could bring moderate to locally heavy rainfall to parts of the northern Plains and Upper Mississippi Valley this weekend. Temperatures ahead of the front across the Central U.S. into parts of the Northeast should be somewhat warm, with daytime highs averaging 5-15F above normal. Out West, moderate to heavy rainfall and mountain snow is possible across parts of the Pacific Northwest on Sunday, with moisture shifting inland along the attendant cold front. Deep troughing will bring much below normal temperatures behind the cold front with daytime highs as much as 20-25 degrees below normal across much of the interior West, especially by next Monday and Tuesday. This would support lowering snow levels in the mountains with potential for decent snow accumulations in the highest terrain of the northern Rockies. Santorelli 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