Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Tue Aug 04 2020 Valid 12Z Fri Aug 07 2020 - 12Z Tue Aug 11 2020 ...Overview and Guidance/Predictability Assessment... Upper ridging is expected to prevail across much of the southern tier of the U.S. while in the northern stream, a few shortwave troughs move through the somewhat progressive flow pattern. The embedded shortwave troughs will bring a series of fronts from the Northwest to the Upper Great Lakes regions. Upper troughing is likely to persist across the Pacific Northwest. A shortwave trough passes through the Northeast states early in the period (Fri-Sat) before giving way to some zonal flow or slight ridging aloft. As with the previous forecast cycle, the largest model differences are with respect to the evolving shortwave trough in the northern U.S. / southern Canada stream. The GFS and to some degree the GEFS continue to be the slower yet more amplified solution in contrast to the flatter and more progressive ECMWF/CMC solution. However, the 12Z GFS did trend toward the slower and more agreeable 00Z ECMWF solution. This adds confidence to using more of the ECMWF/ECENS in the model blend choices. Even with the issues that develop by the latter half of the period, there is actually decent agreement in principle for an upstream trough to reach the Pacific Northwest around the end of the period next Tue. The model blend for this cycle started with the 00Z ECMWF/UKMET/CMC and 06Z GFS with the highest weights given to the the 00Z ECMWF and CMC, which showed very good agreement. This approach then transitioned to more inclusion of the 00Z ECENS and 06Z GEFS by day 6/7 with decreasing confidence in the deterministic model guidance. ...Weather/Hazard Highlights... An active northern tier characterized by several shortwaves and storm systems through the period will be the primary focus for showers and thunderstorms across the Northern Plains through the Midwest and Great Lakes/Ohio Valley. While there is some potential for locally heavy rainfall, confidence and predictability is low in pinpointing specific areas at this time. The weak upper trough crossing the East late this week and weekend along with a leading wavy surface front will produce rain of varying intensity, most likely over the Mid-Atlantic/Southeast. Areas near the East Coast receiving heavy rain from Isaias will be sensitive to any additional rainfall from the late week/weekend precipitation. Temperature wise, significant departures from early to mid August normals are not expected, with only 5-10F anomalies forecast. Minimum temperatures will be typically above normal from the Plains eastward while highs average 5-10F above normal. There is still uncertainty over the pace of a warming trend over/near the northern Rockies though. Above normal temperatures (up to plus 5-10F anomalies) over the southern High Plains will expand back to Arizona with time as upper ridging adjusts a bit westward. Farther north an area of warmth will progress from the Northern Plains into the Midwest and eventually Great Lakes/Northeast, with most anomalies also in the plus 5-10F range and a few pockets locally warmer. Rausch/Taylor 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: - Flooding possible across portions of the Mid-Atlantic, the Northeast, the Central Appalachians, and the Northern Plains. - Flooding occurring or imminent across portions of the Middle Mississippi Valley and the Northern Plains. - Flooding likely across portions of the Northern Plains. 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