Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Tue Aug 04 2020 Valid 12Z Fri Aug 07 2020 - 12Z Tue Aug 11 2020 ...Overview and Guidance/Predictability Assessment... Guidance continues to forecast a fairly typical summertime pattern from late this week into next week. Upper ridging will prevail over much of the southern tier U.S. with the primary high center likely to return to the southern Rockies after starting the period over the Plains. Progressive North Pacific into western North American flow will extend farther eastward across the northern U.S./southern Canada with time. Embedded shortwaves will bring a series of fronts to areas from the Northwest to the Upper Great Lakes. A weak upper trough evolving near the California coast may close off a low that retrogrades a bit by the latter half of the period. Meanwhile a weak trough will cross the East between Fri and the weekend. Models and ensembles are still having difficulty in resolving the shortwave details within the North Pacific through northern U.S./southern Canada stream. Broadly speaking GFS/GEFS mean runs have tended to be on the slower and more amplified side of the spread with troughing over the Northwest/northern Rockies during the weekend. The 12Z GFS provided one exception to that tendency, more closely resembling the flatter/progressive cluster represented by the ECMWF. However while the ECMWF has been consistently fast, the 12Z run begins to outpace most other guidance/previous runs late in the period. The new 00Z ECMWF has trended noticeably slower and more amplified than the 12Z run--leading to some reduction of spread. 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. Preferring to maintain a blend approach to resolve the uncertainty with flow across the northern U.S./southern Canada and associated frontal specifics, the updated forecast starts with an operational blend (18Z GFS and 12Z ECMWF/UKMET/CMC) early in the period and then transitions to a model/mean mix that incorporates half to two-thirds total weight of the 18Z GEFS/12Z ECMWF means by days 6-7 Mon-Tue. By mid-late period the ECMWF input includes both the 12Z and prior 00Z run due to timing differences that arise. Elsewhere the blend represents continuity/consensus for the upper trough/low near the California coast, upper ridge over the southern U.S., the warm front that advances across the Plains/Mississippi Valley, and the wavy front over parts of the Mid-Atlantic/Southeast. ...Weather/Hazard Highlights... The combination of northern tier fronts along with a warm front farther south advancing eastward across the Plains/Mississippi Valley will provide a focus for episodes of showers and thunderstorms from the Northern Plains through the Midwest and Great Lakes/Ohio Valley, possibly extending somewhat farther south/southeast as well. Some of this activity may be heavy but confidence is typically low in pinpointing specific areas favored for highest convective rainfall totals several days in advance. 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. Most areas should see single-digit temperature anomalies during the period. Areas from the West Coast states into the northern Rockies will see mostly below normal temperatures on Fri followed by a warming trend to slightly below/moderately above normal readings as separate upper troughs weaken or depart. There is still uncertainty over the pace of the 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 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