Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Tue Sep 03 2019 Valid 12Z Fri Sep 06 2019 - 12Z Tue Sep 10 2019 ...Hurricane Dorian to possibly affect coastal New England on its track away from the US... ...Record heat likely for the Lower Mississippi Valley Friday into this weekend... ...16Z Update... It remains the case that Dorian may cause some impacts for coastal New England late in the week. Model spread on the track of Dorian remains reasonably low, but the timing in the 00Z ECMWF deterministic run and mean were slower than the NHC track. Elsewhere, some model spread remains regarding troughs approaching and progressing through the western U.S. Did lean away from the 00Z UKMET by day 5/Sun as it is more energetic than other guidance. The general pattern of troughing in the West and ridging shifting from the Rockies on day 5 to Mississippi Valley on day 6 does appear generally well-handled by the models. However, by day 7/Tue, operational models diverge with the details of embedded troughs in the western/central U.S., so leaned much more heavily on the 00Z GEFS/EC means by that point. For more details, please see the previous discussion below. Tate ...Overview... Upper ridging initially over the Four Corners will slide southeastward through the Lower Mississippi Valley and Southeast this weekend and into Florida by next week. This will be in response to Dorian's exit just offshore the New England coast as troughing settles into the West and then the Plains Sun-Tue. ...Guidance Evaluation and Preferences... Mix of the 12Z/18Z GFS and 12Z ECMWF (a bit slower) offered a good starting point for Dorian as it passes into Nova Scotia late Saturday. 00Z GFS was nearly aligned with the 03Z NHC forecast as well. Please consult the forecasts from the National Hurricane Center for the latest information on Dorian. Fronts hung up around Dorian will eventually become enveloped with the system as it progresses through extratropical transition. A 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 18Z GFS/12Z ECMWF with the GEFS/ECMWF ensemble means sufficed as a starting point with relatively good agreement. In the west, successive troughing will move into the Pac NW through the period, favored along 120W. 18Z GEFS mean was much closer (deeper/slower) to the 12Z ECMWF ensemble mean by the weekend (Canadian ensembles have been much quicker with the overall pattern as of late) and that blend with their operational components was used to iron out some timing differences there and downstream with low pressure moving through the High Plains Mon-Tue. ...Sensible Weather... Though Dorian is forecast to stay just offshore Cape Cod, the interaction with the frontal boundary to the north may allow for heavier rain to overspread southeastern Massachusetts and the Islands late this week. A cold front will bring cooler temperatures (5-10F below average) to the Mid-Atlantic and Northeast Friday in the wake of Dorian. 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 upper 50 and 60s. The Southern tier of the CONUS from Texas eastward will generally see well above average temperatures near 100F that are forecast to tie or break record highs. The heat will stay through the weekend into next week as temperatures stay in the middle 90s from Texas to Georgia. Rainfall will spread out of the northern Rockies into the Plains this weekend as an upper trough moves through Montana. This will be reinvigorated as the second upper trough and surface front move out of Montana into the Plains early next week. 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