Extended Forecast Discussion NWS Weather Prediction Center College Park MD 236 AM 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 into next week as troughing to its east slowly weakens. This will result in continued well above normal temperatures in areas west of the Rockies, with some record high values probably/likely 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 frontal boundary will dissipate this weekend but its moisture will remain in the northeast Gulf. Areas of the north central Gulf eastward to Florida have the best chance of an inch or so of rain each day depending on thunderstorm propagation/mobility. To the north, transient rainfall is forecast across the Upper Midwest/Great Lakes around a Canadian system moving eastward. Yet another system over southern Canada Sun-Tue will spread some showers over the Northern/High Plains eastward next week. Temperatures outside the West will be near to above normal but a few degrees cooler than normal over the Southeast. ...Tropical Features... In the East Pacific, Hurricane Genevieve is forecast to weaken as it turns northwestward into colder waters per the latest National Hurricane Center forecast. With the upper high to its northeast, then east, its remnants may get tugged back toward the northern California coast but it may not retain much moisture. In the Atlantic, a vigorous tropical wave is forecast to develop into a tropical cyclone per the 06Z NHC outlook (90% chance of development through five days). Per the 17Z NHC/WPC tropical coordination call yesterday and the latest 12Z/18Z ensemble guidance, this feature is forecast to move WNW toward northern Cuba and the southern Bahamas/South Florida early next week. Ensemble spread remains quite large and a curve northward before the Bahamas is possible. Please consult the NHC for the latest information. ...Guidance/Predictability Assessment... Deterministic consensus proved to be an adequate starting point for the day 3-5 period, utilizing the latest 12Z/18Z guidance. Best agreement was with the upper high with less agreement over southern Canada with the smaller-scale features. By next week, ensemble consensus moves lower heights into the Pac NW as shortwaves move through the larger Gulf of Alaska circulation, but confidence is lower than average. GFS runs were generally quicker than the ECMWF, with the Canadian in the middle and the preferred solution. This also matched fairly well over South Florida with the potential tropical system. Majority ensemble weighting between the 18Z GEFS and 12Z ECMWF ensemble mean tempered some of the features by next Tue, but was seen as prudent given the uncertainty. 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 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