Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Tue Aug 27 2019 Valid 12Z Fri Aug 30 2019 - 12Z Tue Sep 3 2019 ...Expected Weather Pattern... Broad subtropical upper level ridge is expected to persist across the southern tier of the U.S. with the core of the upper high expected across the Intermountain West. The deep trough centered over the Great Lakes and extending northward to Hudson Bay is expected to lift out some with a slightly more zonal flow pattern compared to the short range forecast period. The Pacific upper level flow approaches the Pacific Northwest by early next week and the ridge axis then expands eastward across the High Plains. In addition, Tropical Storm Dorian will likely have an affect on parts of the Southeast U.S. by Sunday and into Monday. ...Model Evaluation... By the beginning of the forecast period Friday morning, the CMC is stronger with a shortwave trough crossing south-central Canada and the Great Lakes region compared to the model consensus, whilst the UKMET is stronger with the ridge axis across the western Atlantic and faster with the arrival of the Pacific closed low. Otherwise, there is very good agreement between the ensemble means and the deterministic GFS and ECMWF through Sunday night. Beyond that time, more noteworthy differences emerge regarding the trough near the Pacific Northwest and its eventual progression inland. The 12Z GFS appeared to compare slightly better with the ECMWF and the ensemble means with this feature, and thus it was weighted slightly more than the 18Z GFS, which is quicker than the model consensus. The starting point for the forecast through Day 5 was a blend of the 12Z/18Z GFS, ECMWF, EC mean, and some previous WPC continuity. More use of the ensemble means was incorporated for days 6 and 7, while still using some of the deterministic GFS and ECMWF. ...Sensible Weather... Multiple thunderstorm complexes are likely to develop near and to the north of a stationary front over parts of Nebraska, Kansas, and Oklahoma on Friday and into early Saturday. Although the axis of heaviest rainfall remains uncertain at this juncture, the potential exists for several inches of rainfall across parts of this region, and the WPC forecast has also trended slightly higher over the past couple of days. Flooding may become an issue if training convection develops near the front, and also strong to severe storms. Numerous showers and storms are also expected for the Florida Peninsula and extending northward to southern Alabama to southern Georgia. Tropical Storm Dorian is expected to cross the Bahamas on Saturday and potentially impact Florida on Sunday, bringing copious rainfall and gusty winds. The National Hurricane Center has additional information pertaining to this event. In the temperature department, a high quality airmass will provide a hint of fall across much of the north-central U.S. and the Great Lakes region as a Canadian surface high governs the weather pattern through the weekend, before a moderating trend commences with readings returning to slightly above average levels by early next week. Expect readings to be on the order of 5 to 15 degrees below late August averages, with the greatest departures from normal extending from the Dakotas to Kansas. The opposite should hold true across much of the western third of the U.S., with high temperatures running about 5 to 15 degrees above average. Highs in the lower 110s are possible across the lower elevations of the Desert Southwest through Saturday, with the potential for excessive heat criteria to be met. D. Hamrick 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