Extended Forecast Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Thu Aug 06 2020 Valid 12Z Sun Aug 09 2020 - 12Z Thu Aug 13 2020 ...Overview and Guidance/Predictability Assessment... The latest cycles of most models and ensemble means maintain good continuity and agreement for the large scale pattern. Upper ridging will persist from the Atlantic across the Florida Peninsula through southern portions of the Plains/Rockies. Around the northern periphery of this ridge expect an upper trough anchored by a closed or nearly closed low over Canada to progress across the northern tier of the lower 48. This trough will push a cold front steadily from the Northern Plains into the Northeast/Mid-Atlantic over the course of the period. Upstream flow will amplify into a trough that settles near the West Coast by midweek, with a leading cold front pushing into the West during Tue-Thu. This trough should eventually lead to inland ejection of at least some of the weak low/trough energy initially just offshore California. In response to the feature reaching the West, expect the trailing part of the central/eastern U.S. front to stall and then return north as a warm front. The WPC sea-level pressure and 500 mb height fields were derived using a blend of the 00Z ECMWF/EC mean and 06Z GFS/GEFS for Days 3-5, with higher percentage from the ensemble means together with a small percentage of the 00Z NAEFS for Days 6-7. Some model differences regarding the amplitude, tilt and speed of the West Coast trough emerge by Day 7. But even then, the discrepancy appears quite reasonably for a Day 7 forecast. ...Weather/Hazard Highlights... Multiple episodes of showers and thunderstorms will be likely across much of the area extending from the northern half of the Plains through most the East. The most prominent focus for this convection will be the cold front that is forecast to traverse the Northern Plains followed by the Northeast/Mid-Atlantic during the period, with stalling of the trailing part of the front leading to a continued potential for rainfall back into the central U.S. at times. On Sun there will also be a weakening warm front crossing the Mississippi Valley and another front over parts of the Upper Midwest/Upper Great Lakes. The Southeast/Florida will see diurnal convection of varying intensity, with a weakening front over the Southeast providing an added focus early in the period. Within the broad area of shower/thunderstorm potential there will likely be areas of heavy rainfall but it will take additional time to refine the regions that may receive the highest totals. The Storm Prediction Center is monitoring the potential for severe convection across parts of the northern tier. Check future outlooks as important details are better resolved. Above normal temperatures will spread from the Plains to the Mid-Atlantic/Northeast ahead of the cold front progressing eastward from the Northern Plains. Anomalies may reach as high as plus 10-15F over some areas from the Midwest into the Northeast, especially for morning lows. The southern High Plains into southern Arizona should be consistently above normal by 5-10F through the period under ridging aloft while portions of the Southeast may see highs several degrees above normal Sun-Mon. The cool airmass over the northern Rockies/Plains on Sun will moderate thereafter as it continues eastward. The Northwest will see a cooling trend by Wed-Thu as upper troughing reaches the area. Rausch/Kong Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Severe weather across portions of the Northern/Central Plains, the Upper/Middle Mississippi Valley, and the Great Lakes, Sun, Aug 9. - Flooding occurring or imminent across portions of the Middle Mississippi Valley. 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