Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM EDT Mon Sep 02 2019 Valid 12Z Thu Sep 05 2019 - 12Z Mon Sep 09 2019 ...Hurricane Dorian forecast to affect parts of the Southeast and possibly coastal New England this week/weekend... ...16z Update... No major changes were needed for the day shift medium range forecast compared to the night shift. The track of Hurricane Dorian remains similar to previous forecasts, and the 00Z and 06Z GFS deterministic runs slowed down a bit, which made them more in line with the National Hurricane Center track and thus usable through day 4/Fri. Some uncertainty remains on how far west Dorian may go close to the Eastern Seaboard, but the main surface low spread in the 00Z EC ensemble is elongated northeast to southwest just offshore of the Eastern Seaboard on day 4-5, suggesting uncertainties in timing but not too much in the placement of the track. By day 5/Sat, 00Z and 06Z GFS runs track Dorian farther west toward Maine, but the deterministic 00Z EC is farther east than consensus by day 6/Sun. Thus by day 5 onward, leaned toward the GEFS and ECMWF means. This worked well elsewhere in the CONUS too, where troughs in the eastern Pacific/northwest U.S. had considerable run-to-run variability in the deterministic runs on day 6-7. Please see the previous discussion for details about sensible weather. Tate ...Overview... Upper ridging initially over the Four Corners/Southern Plains and western Atlantic will weaken over the weekend as Hurricane Dorian lifts north-northeastward near/along the Carolina coasts Thursday per the National Hurricane Center. Troughing in the northern stream will tug and then pull Dorian northeastward Friday as additional height falls push through Ontario. Dorian is forecast to continue into Canada just east of New England Saturday. Troughing off the west coast will slowly ease inland Thursday and be replenished upstream this weekend as a trough settles into the northern Great Basin. ...Guidance Evaluation and Preferences... Consensus position around the 12Z ECMWF/UKMET formed a starting point for Dorian Thursday along the NC/SC coasts per NHC. By late Friday, it is expected to quickly move away from the Outer Banks but may still pass just off Cape Cod (near 40N/70W benchmark) on its way toward Nova Scotia and the North Atlantic. Please consult the forecasts from the National Hurricane Center for the latest information on Dorian. On Thursday, a cold front off the Northeast/mid-Atlantic coast will hang up atop Dorian and eventually become enveloped with the system as it progresses through extratropical transition this weekend. Southern portion of the front may linger over the Southeast and provide a focus for some isolated showers/storms. Another cold front will push through the Great Lakes late Friday into Saturday and then through the Northeast/mid-Atlantic by Sunday. A blend of the 12Z ECMWF with the GEFS/ECMWF ensemble means sufficed as a starting point (GFS runs were quicker with Dorian ahead of NHC). In the west, additional troughing will approach WA/OR Saturday with typical uncertainty in the orientation/depth/evolution. The ECMWF/Canadian were more in line with the ensembles than the 12Z/18Z GFS runs but upstream uncertainty was higher than average in split-flow. ...Sensible Weather... Dorian will bring strong winds, heavy rain/flooding, and storm surge to parts of the Southeast late this week. Effects in New England, if any, are uncertain due to the dependence on northern stream flow and inherent timing uncertainty at this lead time. A cold front will bring cooler temperatures (5-10F below average) to the Mid-Atlantic and Northeast Thu/Fri. A stronger cold front will bring fall-like temperatures (10-20F below average) to the High Plains/Upper Midwest this weekend as many places rise only into the 60s. In the West, above average temperatures are favored Thursday ahead of the lead upper trough with a cooling trend thereafter. The South from Texas eastward will generally see above average temperatures. Away from Dorian, rainfall coverage should be fairly limited over the lower 48, with the exception being in the Pacific Northwest as the upper trough moves into WA/OR and then into the central Plains over the weekend as a cold front sinks southward. As the second upper trough settles into the Great Basin, rainfall may expand along the spine of the Rockies. Fracasso 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 coastal regions of the Southeast and the lower Mid-Atlantic, Thu-Fri, Sep 5-Sep 6. - Heavy rain across coastal regions of the Northeast, Fri, Sep 6. - Flooding possible across portions of the Southeast and the lower Mid-Atlantic. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, and the Northern Plains. - High winds across eastern portions of the Southeast and the Mid-Atlantic, Thu-Fri, Sep 5-Sep 6. - High winds across coastal regions of the Northeast, Fri, Sep 6. - Much above normal temperatures across portions of the Lower Mississippi Valley and the Southeast, Fri-Sun, Sep 6-Sep 8. - High significant wave heights for coastal portions of the Southeast, the Mid-Atlantic, and the Northeast, Thu-Fri, Sep 5-Sep 6. 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