Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Wed Aug 28 2019 Valid 12Z Sat Aug 31 2019 - 12Z Wed Sep 4 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 through Monday. 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 over the High Plains. In addition, Dorian will likely have impacts on Florida and parts of the Southeast U.S. by late this weekend and into Monday. ...Model Evaluation... The models indicate good synoptic scale agreement at the beginning of the forecast period on Saturday. However, differences become apparent by Sunday across the northeast Pacific in regards to the upper level low approaching the coast, with the CMC becoming more progressive and then phasing with a northern stream trough, becoming much more amplified than the model consensus by early next week across the north central U.S. With respect to the forecast track of Tropical Storm Dorian, the ECMWF, EC mean, and CMC appear to be clustered well Monday evening, while the GFS deviates and lifts the storm much farther up the East Coast and the UKMET delaying the storm's arrival. Towards the end of the forecast period midweek, forecast confidence decreases to below average levels given the big differences noted in the northern stream flow across southern Canada and how that affects surface frontal timing and low placement. The GFS is depicting a surface low Saskatchewan whilst the CMC and ECMWF are depicting more of a surface high pressure ridge over this same general area Wednesday morning. As a result of these differences, the EC mean and to a lesser extent the GEFS mean and 12Z ECMWF were used as a starting point towards the end of the forecast period. ...Sensible Weather... Tropical Storm Dorian is expected to make weather headlines by Sunday and into early next week across Florida and coastal portions of the Southeast. The official NHC forecast track depicts the storm tracking in the general direction of the Florida Peninsula by late Sunday, and this will increase the impacts from wind, heavy rains, and high surf. Model differences are still significant, so there will likely be further refinements to the forecast in future updates. The National Hurricane Center has additional information pertaining to 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 this weekend as a Canadian surface high governs the weather pattern, before a moderating trend commences with readings returning to near or slightly above average levels by early next week. Expect readings to be on the order of 5 to 15 degrees below late August averages on Saturday, 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. Warmer weather will likely return to much of the East Coast by midweek with highs slightly above early September averages. 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