Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM 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 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 possibly return north as a warm front. The upper trough crossing the northern tier U.S. has displayed somewhat better clustering over the past couple days versus earlier forecast, but there is still some run-to-run variability along with detail spread. The primary adjustment by consensus in the 12Z/18Z cycles is to be more amplified with the upper trough at some valid times--leading to a farther south and east extent of the leading cold front. This represents a solution closer to some runs from 24-36 hours ago. Primary differences for the developing West Coast trough during the latter half of the period emerge by day 7 Thu. Guidance indicates moderate progression of the upper low that reaches western Canada and a portion of the trough immediately to the south, which would be consistent with the established pattern that has displayed some degree of progression. However solutions diverge over how much troughing remains near the West Coast--whether from the initial trough or upstream energy that provides reinforcement. The 12Z ECMWF/ECMWF mean are on the weaker side of the spread with the lingering trough while the 18Z GFS/GEFS mean are noticeably deeper. CMC/CMC mean runs lean to the deeper side but the new 00Z GFS has backed away some for the depth. Even relative to the slightly farther east upstream positive height anomaly of the 12Z ECMWF/ECMWF mean, teleconnections favor somewhat deeper troughing over the Northwest than what their forecasts show. This favors nudging the forecast blend a little more in the 18Z GFS/GEFS mean direction, but uncertainty with specific shortwaves at nearly a week out in time tempers confidence. At least for the time being the new 00Z ECMWF provides support for this idea, adjusting much stronger with energy that drops into the Pacific Northwest toward the end of the period. A composite of the 18Z GFS and 12Z ECMWF/UKMET/CMC provided a reasonable starting point for the forecast early in the period. Then the blend transitioned toward 40-60 percent total ensemble weight by days 6-7 Wed-Thu while eliminating the 12Z CMC due to that run becoming excessively slow/amplified with eastern troughing aloft. The blend tilted 60 percent toward the 18Z GFS/GEFS mean relative to the 12Z ECMWF/ECMWF mean by day 7 Thu to reflect West Coast preferences at that time. ...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 travels from the Northern Plains to 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 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 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