Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Fri Aug 23 2019 Valid 12Z Mon Aug 26 2019 - 12Z Fri Aug 30 2019 ...Overview and Guidance/Predictability Assessment... Broad cyclonic flow aloft is expected across the northern tier of the nation for much of the upcoming week as an upper level low develops over southern Canada and then Hudson Bay. This is expected to result in a broad upper level trough over the northern Plains and extending to the Great Lakes with a Canadian surface high in place. Upper level ridging extending from Texas to California is expected to continue through midweek, and also across the western Atlantic Ocean. A heavily ECMWF-based blend was used as a forecast starting point through much of the forecast period, with a minority component of the CMC also used during days 3-4 (Mon-Tue). The GFS was much weaker/closer to the coast relative to the ECMWF with an area of low pressure off the Carolinas on Mon. In this case the ECMWF was more closely aligned with the ensemble consensus. Farther north, models showed significant timing differences even on day 3 with northern stream energy crossing the Gulf of Alaska which eventually dives southeast into the broad upper trough across the north central U.S. The GFS was substantially faster with this energy and as a result the GFS was discarded and not used in the forecast blend for this update. By days 5-7 (Wed-Fri) models began to show some timing differences with upper shortwave energy in the broader trough across the north central U.S., and with the progression of a couple additional cold fronts from the central into the eastern U.S. Given these differences, ensemble means (ECENS and GEFS) were used much more heavily later in the forecast period. ...Weather Highlights/Threats... Low pressure east of the Carolinas may produce areas of heavy rainfall near or just offshore early Monday before the system quickly moves northeast away from the coast. Meanwhile, a hint of fall is expected across the north-central U.S. next week with high temperatures generally 5 to 15 degrees below normal, which equates to high temperatures in the 60s and 70s F for much of this region. It should continue feeling like the middle of summer from Texas to the Great Basin and the Desert Southwest with temperatures 5 to 15 degrees above normal, with the greatest anomalies expected early next week across western Texas and New Mexico. High temperatures could exceed 110 degrees across the lower elevations of southern Arizona and southeast California on Monday and Tuesday. Areas of moderate to locally heavy rainfall will be possible ahead of the cold front crossing the Upper Midwest and Great Lakes region early next week. For midweek and beyond, model uncertainty reduces confidence in placement of heaviest rainfall areas. However, there is the potential for a corridor of enhanced rainfall across the Central Plains later in the week near the trailing end of the front. Hamrick/Ryan 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