Extended Forecast Discussion NWS Weather Prediction Center College Park MD 331 PM EDT Mon Aug 17 2020 Valid 12Z Thu Aug 20 2020 - 12Z Mon Aug 24 2020 ...Exceptional late summer heat wave continues for much of The West into next week... ...Overview and Weather/Hazard Highlights... Strong upper ridging over the Southwest will persist and even re-strengthen this weekend into next week as a Gulf of Alaska low ejects east, topping the northern part of the ridge from the Southwest high as it crosses the Canadian Rockies this weekend. Troughing lingers over the lower Ohio, Tennessee, and lower Mississippi River Valleys. This will result in continued well above normal temperatures in areas west of the Rockies, with some record high values reached or exceeded each day throughout the Great Basin southward to the Colorado River Valley/Southwest especially through this week. Some isolated showers/storms will dot the landscape over the Rockies each day. A stalled frontal boundary near the Gulf Coast and through the Southeast will be a focus for periods of showers and thunderstorms, especially across Florida. To the north, transient rainfall is forecast across the Upper Midwest/Great Lakes later this the weekend along frontal boundaries associated with weak shortwave troughs coming off the western ridge and the stronger remnants of the Gulf of Alaska low that crosses the Canadian Prairies Sunday into Monday. This wave looks to approach the Great Lakes Monday. Cooler than normal temperatures will be prevalent over the Southeast by only a few degrees. ...Guidance/Predictability Assessment... Rather good agreement among latest deterministic guidance was used for the WPC blend for days 3-5, anchored by the 00Z ECMWF, 06Z GFS, and 00Z UKMET which offered the good agreement and consistency from the previous forecast cycle. The 00Z CMC was close with most features but was still stronger with the Gulf of Alaska low remnants as they cross the Canadian Prairies and was much farther offshore with a secondary low off the BC coast this weekend. The 12Z GFS is now flatter and quicker with the Gulf of Alaska low remnants over the Canadian Prairies Sun/Mon and continues the trend of the GFS being much slower/farther north with Genevieve off the southern Baja Peninsula. Trended to a nearly even blend of the 06Z GFS/GEFS and 00Z ECMWF/ECMWF ensemble mean for days 6/7 forecast which strengthen the western upper high. It should be noted that the 06Z GEFS is stronger on Day 7 with the high (with a 594dm closed isoheight over UT/AZ) which is greater than the 06Z or 12Z GFS. Jackson 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: - Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Fri-Sun, Aug 21-Aug 23. - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Thu-Fri, Aug 20-Aug 21. - Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains. - Excessive heat across portions of the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat-Mon, Aug 22-Aug 24. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Thu-Mon, Aug 20-Aug 24. 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