Extended Forecast Discussion NWS Weather Prediction Center College Park MD 401 PM EDT Thu Aug 13 2020 Valid 12Z Sun Aug 16 2020 - 12Z Thu Aug 20 2020 ...Heat Wave with Record Temperatures for the dry West... ...Guidance/Predictability Assessment... For the medium range period (Sunday - Thursday), there is relatively good agreement in the deterministic and ensemble guidance for a persistent western U.S. ridge and eastern U.S. trough. Days 3-5 were well clustered on this idea and a near equal weighting of the 00Z ECMWF, 06Z GFS, 00Z UKMET, and 00Z CMC were utilized. Beyond that time frame, increasing uncertainty to the strength and blocking of the western U.S. ridge, particularly how it extends northward into the Intermountain West and Pacific NW (flatter GFS versus more amplified ECMWF) led to more inclusion of the ECENS and GEFS means at this time. ...Weather/Hazard Highlights... A closed upper high centered over the southern Great Basin along with upper ridging from the southern Plains through the West will continue to inhibit monsoonal flow. Much above normal temperatures will build north with the ridge over the Intermountain West and West Coast to the Pacific Northwest. Heat wave temperatures will range 5-15 degrees above normal, upwards to 15-20 degrees above normal in the Northwest Sun-Tue. Record high max and min temperatures will persist over the Desert Southwest with temperatures up to 115-120 degrees. Additional record highs will spreads over the rest of the West in tandem with upper trough amplification off the West Coast. This will foster a continued dry pattern with frontal relief expected only for the Pacific Northwest late in the forecast period. Meanwhile, amplified upper troughing settles over the east-central U.S.. Widespread showers and thunderstorms will gradually push through the Eastern Seaboard early next week as a wavy lead front/modest coastal low and a temperature moderating main front digs robustly down through the central and eastern U.S.. This may produce some local runoff issues, especially early period for the recently wet Mid-Atlantic. A wavy trailing front will settle through the Southeast and Gulf of Mexico back to the Plains/Rockies and focus additional bouts of convection, some strong to severe, into mid-later next week as impulses dig to the lee of the western ridge. Schichtel/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: - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Tue-Wed, Aug 18-Aug 19. - Heavy rain across portions of the Mid-Atlantic, Sun, Aug 16. - Flooding possible across portions of the Upper Mississippi Valley. - Excessive heat across portions of California, the Lower Mississippi Valley, the Pacific Northwest, the Northern Great Basin, and the Southern Plains, Sun, Aug 16. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Sun-Thu, Aug 16-Aug 20. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Sun-Mon, Aug 16-Aug 17. - Much above normal temperatures across portions of the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, and the Pacific Northwest, Sun-Tue, Aug 16-Aug 18. - Much above normal temperatures across portions of the Central Rockies and the Central Great Basin, Sun-Wed, Aug 16-Aug 19. - Heavy rain across portions of the Alaska Panhandle, Sun-Mon, Aug 16-Aug 17. 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