Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 PM EDT Tue Aug 27 2019 Valid 12Z Fri Aug 30 2019 - 12Z Tue Sep 03 2019 ...Tropical Storm Dorian forecast to affect Florida and parts of the Southeast U.S. by this weekend... ...Expected Weather Pattern... A 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. Amplified troughing centered over the Great Lakes and extending northward to Hudson Bay is expected to lift out some with a slightly more zonal flow pattern into the medium range. A Pacific shortwave/upper low approaches the Pacific Northwest by early next week pushing the ridge axis eastward into the High Plains. In addition, Dorian will likely have impacts on Florida and parts of the Southeast U.S. by this weekend. ...Model Evaluation... There remains very good agreement between deterministic models and ensemble means on the overall synoptic scale flow in the medium range period through day 4. Beyond day 4, a few noteworthy differences emerge regarding the shortwave near the Pacific Northwest and its eventual progression inland. The 00z UKMET (which drops out after day 5) is much faster to pull the shortwave inland, with the 00z ECMWF also slightly faster than the rest of the model consensus days 6 and 7 but still within reason. This cycle of the WPC fronts/pressure progs used a majority blend of the deterministic runs (06z GFS/00z ECMWF/00z CMC) with previous WPC continuity for days 3 and 4. Increasing weights of the ensemble means (GEFS/ECENS) was used day 5-7, with smaller contributions from the deterministic GFS/ECMWF and previous WPC continuity. ...Sensible Weather... Multiple thunderstorm complexes are likely to develop near and to the north of a stationary front over parts of the Central Plains on Friday and into early Saturday. Although the exact axis of heaviest rainfall remains uncertain at this point, the potential exists for several inches of rainfall across parts of this region. Flooding may become an issue if training convection develops near the front, especially within stronger thunderstorms. The latest track from the National Hurricane Center has Tropical Storm Dorian crossing the northern Bahama islands on Saturday with potential landfall along the central Florida coast by Sunday. The threat for heavy rainfall and gusty winds across Florida by this weekend is increasing. Heavy rainfall may also extend into parts of the Gulf Coast and Southeast later this weekend and into early next week depending on the exact track of Dorian, which remains highly uncertain at this time. Please refer to the latest advisories from the National Hurricane Center for additional information regarding Dorian. 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 closer to 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 10 degrees above average. Highs in the lower 110s are possible across the lower elevations of the Desert Southwest through Saturday. Santorelli/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 - Heavy rain across portions of the Central and Southern Plains, Fri-Sat, Aug 30-Aug 31. - Heavy rain across portions of the Southeast, Sun-Tue, Sep 1-Sep 3. - Flooding possible across portions of the Central Plains. - Flooding occurring or imminent across portions of the Central Plains and the Northern Plains. - Excessive heat across portions of Southeastern California and the Southwest, Fri-Sat, Aug 30-Aug 31. - High winds across portions of northeastern/coastal Florida, Sun, Sep 1. 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