Extended Forecast Discussion NWS Weather Prediction Center College Park MD 311 PM EDT Tue Aug 18 2020 Valid 12Z Fri Aug 21 2020 - 12Z Tue Aug 25 2020 ...Hot temperatures to continue 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 while a trough over the east-central U.S. slowly weakens. The ridge will promote continued well above normal temperatures in areas west of the Rockies and to some degree the central/southern Rockies as well. Expect some record high values each day throughout the Great Basin southward to the Colorado River Valley/Southwest into the weekend. Some isolated showers/storms will dot the landscape over the Rockies each day but the larger focus for rainfall will be in the Gulf of Mexico and adjacent areas. A stalled front ahead of the east-central U.S. upper trough will dissipate this weekend but its moisture will remain in the northeast Gulf and in less abundant form over the eastern states. Areas from the northern Gulf into Florida have the best chance of an inch or so of rain each day depending on thunderstorm propagation/mobility. Tropical features described below may also produce enhanced rainfall over some areas near the Gulf. To the north, at least a couple northern tier U.S.-southern Canada waves/fronts should bring transient rainfall to areas between the Northern Plains and Northeast. Temperatures outside the West will be near to above normal except for a few degrees cooler than normal over the Southeast, especially late this week into the weekend. ...Tropical Features... In the East Pacific, the latest National Hurricane Center forecast expects Hurricane Genevieve to weaken as the system turns northwestward into colder waters. With the upper high to its northeast, then east, some reflection of Genevieve's remnants may extend toward the northern California coast but without retaining much moisture. In the Atlantic basin, NHC outlooks continue to advertise a high likelihood of development from a broad area of low pressure initially near 40W longitude. Per today's 17Z NHC/WPC tropical coordination call and the most common signal among latest model/ensemble guidance, anticipate this feature to move west-northwest just north of Cuba and into the eastern Gulf of Mexico early next week. Note that the full solution envelope becomes quite broad by next Mon-Tue, ranging between the Bahamas (or even a bit to the east) and the Gulf. In addition a feature originating from the western Caribbean may emerge into the western Gulf with the full array of guidance also quite diverse with its specifics. Please consult latest NHC products for further information. ...Guidance/Predictability Assessment... An operational model consensus provided a reasonable starting point for about the first half of the period. Then guidance diverged regarding the ejection of the upper low/trough initially over the Northeast Pacific along with strength of the upper high over the West. Latest GFS runs and 00Z ECMWF have trended faster with the Pacific ejection and weaker with the western ridge. Individual ensemble members vary but overall their means maintain slower ejection and greater persistence of the upper ridge. The 00Z CMC offered a compromise. Preference late in the period was to wait for some degree of continuity in the GFS/ECMWF runs before adjusting the forecast significantly. Therefore the blend most closely reflected the 06Z GEFS/00Z ECMWF means, 00Z CMC, and WPC continuity. Most 12Z models seem to maintain the idea of recent GFS/ECMWF trends. Uncertain specifics of this evolution and downstream shortwave questions will affect wave/front details across the northern U.S. and southern Canada. Guidance has been variable thus far and noticeable continuity changes from previous cycle even in a consensus blend highlight the relatively lower confidence in this aspect of the forecast. Rausch/Fracasso 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, the Lower Mississippi Valley, and the Southern Plains, Mon-Tue, Aug 24-Aug 25. - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Fri, Aug 21. - Heavy rain across portions of the Southeast, Fri-Tue, Aug 21-Aug 25. - Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Fri-Tue, Aug 21-Aug 25. - Excessive heat across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, the Northern Great Basin, and the Southwest, Fri-Mon, Aug 21-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