Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Wed Aug 28 2019 Valid 12Z Sat Aug 31 2019 - 12Z Wed Sep 04 2019 ...Tropical Storm Dorian forecast to affect Florida and parts of the Southeast U.S. by this weekend/early next week... ...Expected Weather Pattern... A broad subtropical upper level ridge will likely persist across the southern tier of the U.S. with the core of the upper high settling over the Four Corners region during the weekend and early next week, then drifting into the Plains. A modest upper trough will progress from the Midwest through the Northeast early-mid period. Another trough may amplify into eastern Canada/northeastern quadrant of the CONUS by midweek in response to an upstream amplified pattern highlighted by a strong Bering Sea storm around Sun-Mon. Meanwhile a Pacific shortwave/upper low will approach the Pacific Northwest by early next week and help to nudge the Four Corners ridge eastward. The Southeast/Florida will need to monitor Dorian for potential impacts by late this weekend and into next week. ...Model Evaluation... The two features with the most notable spread/continuity changes are Dorian near the southeastern coast and the upper low/trough drifting toward the Pacific Northwest. For Dorian the past couple GFS runs have adjusted significantly to the right (though the 06Z run has trended slower with northward progression than the 00Z run Mon onward) while other guidance maintains varying degrees of a farther south track than GFS runs. Complicating factors for Dorian's forecast include specifics of the upper trough tracking out of the Midwest and then a general weakness aloft which most guidance suggests will develop over the southeastern states. For the upper low/trough nearing the Pacific Northwest, the primary trend since yesterday is toward slower progression in the ECMWF/GFS. The slower trend shows up in GEFS mean while the ECMWF mean has been fairly stable over the past 24-36 hours. In contrast the CMC has a very progressive/phased solution and individual ensemble members vary considerably as well. Thus it would not be surprising to see further adjustments in future runs. Toward the end of the period it appears best to put primary emphasis on the ensemble means as the ECMWF/GFS diverge, the 00-06Z GFS keeping the upper low along/offshore the northern California coast and the 00Z ECMWF ejecting it inland over the Northwest. The updated forecast started with a blend of operational guidance (majority 06Z GFS/00Z ECMWF) early in the period, started incorporating some input from the 06Z GEFS/00Z ECMWF means by day 5 Mon, and then reached 80 percent means by day 7 Wed. This blend also accounted for spread observed for features affecting southern Canada and into northern U.S. east of the Rockies. ...Sensible Weather... The 1500 UTC National Hurricane Center advisory for Tropical Storm Dorian shows the system strengthening to hurricane status by this evening/tonight. Dorian will likely make weather headlines by late weekend 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 into early Monday and this will increase the impacts from wind, heavy rains, and high surf. Model differences are still significant so expect further refinements to the forecast in future updates. Consult latest products from NHC for additional information pertaining to Dorian. Elsewhere, a band of moisture to the north of a mid-latitude front may bring rainfall from parts of the Midwest into New England during the first half of the period. Low pressure crossing the extreme northern Plains may be accompanied by some rain early next week. Scattered convection will be possible over parts of the southern half of the West with specifics becoming sensitive to uncertain details of flow aloft along the West Coast by the middle of next week. In the temperature department the northern-central Plains into Northeast should see below normal readings during the weekend followed by a moderating trend. Some locations from the north-central Plains into southwest Great Lakes may see minus 10-15F anomalies for highs on Sat. Expect another push of cool air to reach the northern-central Plains next Tue-Wed. On the other hand much of the western third of the U.S. will see max/min temperatures running about 5-10F degrees above average with some localized pockets of plus 10-15F anomalies. Warmer weather will likely return to areas near the East Coast by midweek with highs slightly above early September averages. Rausch/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