Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Thu Aug 29 2019 Valid 12Z Sun Sep 01 2019 - 12Z Thu Sep 05 2019 ...Hurricane Dorian forecast to affect Florida and parts of the Southeast U.S. by this weekend/early next week... ...Expected Weather Pattern... The large scale pattern will feature an upper level ridge generally centered over the Four Corners region while broad troughing should prevail over much of southern Canada and northern tier U.S. Resolving individual shortwaves within the northern stream flow, including a trough/upper low initially off the Pacific Northwest coast/Vancouver Island, continues to provide a challenge for guidance. This shortwave uncertainty affects the forecast of a surface wave which a majority of guidance shows tracking across the northern Plains early next week and then continuing across the Upper Great Lakes into eastern Canada. Meanwhile an upper ridge initially over the western Atlantic should direct Hurricane Dorian on a track toward Florida early next week, with a subsequent path most likely into the Southeast. ...Model Evaluation... Broadly speaking, except for the start of the period the 06Z GEFS/00Z ECMWF means provide the best overall template for the forecast as most operational models offer varying differences/continuity changes. From fairly early on the 00-06Z GFS are more aggressive to bring height falls across the northeastern Pacific/Gulf of Alaska versus most other guidance. This causes the GFS to be quicker than consensus to eject the trough/upper low initially off the Pacific Northwest coast. Regarding this feature the GEFS means are slower than the operational runs and in particular the 06Z GEFS mean has nudged a bit slower than its 00Z run to line up well with the ECMWF mean. As it is there is a somewhat faster trend versus yesterday--in contrast to the slower trend noted from 24 hours prior. Meanwhile the 00Z ECMWF is broader than most other guidance (including the means) with the southern Canada shortwave that provides the main upper support for the surface system expected to track across the northern tier U.S. into eastern Canada. For Dorian varying degrees of compromise between the 00Z ECMWF and 06Z GFS provided the best approximation of the latest National Hurricane center track, though the ECMWF strays eastward late in the period. Based on forecast considerations the updated forecast started with an operational model blend on day 3 Sun, trended to half models/means by day 5 Tue and mostly means thereafter. ...Sensible Weather... The main weather story will be Hurricane Dorian and its eventual track toward/into Florida and the Southeast U.S. from late weekend into next week. Expect Dorian to bring heavy rain and strong winds, along with storm surge near the coast. Consult National Hurricane Center products for the latest information regarding Dorian. Away from Dorian rainfall coverage should be fairly limited over the lower 48. Exceptions will be over the Northeast which may see a period of rain early in the week along with the northern tier states which should see an episode of focused rainfall with the system crossing the region. The Southwest/Four Corners states may see isolated to scattered rain but with generally light amounts. Expect highs to be 5-15F above normal over the Interior West/Rockies Sun-Mon followed by a slight cooling trend. The Northwest should see plus 5-10F anomalies for highs by Tue-Thu. Much of the West will likely see morning lows 5-15F above normal through the period. Areas from the northern-central Plains into the Northeast will see modestly below normal temperatures on Sun followed by a warming trend. Another front will bring readings back down to 5-10F below normal over the northern Plains on Tue with cool air then spreading as far south as the central Plains and east into the Great Lakes/Northeast. 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 Hazards: - Heavy rain across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Mon, Sep 2. - Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Thu, Sep 5. - Heavy rain across portions of the Southeast and the Mid-Atlantic, Sun-Thu, Sep 1-Sep 5. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Sun, Sep 1. - Flooding possible across portions of the Central Plains and the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, the Southern Plains, and the Northern Plains. - Excessive heat across portions of California and the Southwest, Sun, Sep 1. - High winds across portions of the Southeast, Sun-Thu, Sep 1-Sep 5. - High significant wave heights for coastal portions of the Southeast, Sun-Tue, Sep 1-Sep 3. 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