Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EDT Wed Sep 04 2019 Valid 12Z Sat Sep 07 2019 - 12Z Wed Sep 11 2019 ...Record heat to continue for the Lower Mississippi Valley/Gulf Coast this weekend... ...Overview... Upper ridging will remain anchored over the Lower Mississippi Valley and Southeast this weekend and into next week. The West will see a lead trough reach the West Coast and then eject through the Great Basin this weekend. Right behind this system another trough should reach at least as far east as the West Coast. With rising heights over the Great Lakes/Northeast as Dorian and upstream eastern Canada shortwave energy depart, low pressure emerging from the West should track eastward along the US/Canadian border. ...Guidance Evaluation and Preferences... From day 3 Sat into early day 5 Mon a blend emphasizing the 06Z GFS and 00Z ECMWF/UKMET represents consensus well for significant features. The 00Z CMC compares quite poorly to other models/means for western U.S. details. Differences that arise over the North Pacific during this time frame lead to widening spread over the eastern Pacific/western U.S. by days 6-7 Tue-Wed. A robust guidance cluster continues to suggest that the GFS in particular may become too progressive over this area. The 06Z run is at least less extreme than the 00Z run. Latest GEFS means are somewhat faster with the western trough than the 00Z ECMWF/CMC means but all the means are closer together with the depiction of the upstream trough near 150W by early day 7 Wed. Meanwhile the operational ECMWF/CMC end up aligning the western U.S. trough between the slow ECMWF mean and faster GEFS mean. Interestingly the ECMWF's 00Z run is back to its run from 36 hours ago after a couple runs had a slow closed low. Overall maintaining about a 70 percent tilt toward the ECMWF/ECMWF mean versus the 06Z GFS/GEFS mean seems to provide the best balance among competing considerations of the fast side of the envelope likely being too fast (given western Canada ridging aloft) versus North Pacific flow possibly being progressive enough to nudge the West Coast trough a bit east of the exact ECMWF mean position. ...Sensible Weather... A potent 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 to Florida will generally see well above average temperatures in the mid-90s to around 100F that are forecast to tie or break record highs. The heat may temper only a bit next week as temperatures stay in the middle and upper 90s from Texas to Georgia. Much of the West will see below average temperatures under upper troughing, with lowest anomalies tending to be over north-central areas and the northern Rockies. Rainfall will dive through the northern Plains/Corn Belt this weekend with additional rainfall moving into the Northwest. As low pressure and associated fronts emerge from the West early next week, rain will advance in tandem through the northern tier states. Both areas of precipitation could contain some locally moderate or heavy totals. Questions remain over how much moisture will lift out of NW Mexico into the Southwest/Four Corners region next week. Differences remain but latest guidance discrepancies appear to be less pronounced than over the past day or so. Currently expect highest rainfall totals to be over southern Arizona and portions of New Mexico/extreme western Texas. On Sat Dorian will likely track to the east of New England, with coastal areas seeing a potential for a period of enhanced rain/wind. Rausch/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 portions of the Upper Mississippi Valley, the Great Lakes, the Northern Plains, and the Northern Rockies, Mon-Tue, Sep 9-Sep 10. - Heavy rain across portions of the Northeast, the Upper Mississippi Valley, and the Northern Plains, Sat-Sun, Sep 7-Sep 8. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Sun-Mon, Sep 8-Sep 9. - Heavy rain across portions of the Northeast, Sat, Sep 7. - Flooding possible across portions of the Southeast and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Northern Plains. - Flooding likely across portions of the Southeast. - Excessive heat across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Sat-Mon, Sep 7-Sep 9. - High winds across portions of the Northeast, Sat, Sep 7. - High winds across portions of the Northeast, Sat-Sun, Sep 7-Sep 8. - High significant wave heights for coastal portions of the Northeast, Sat, Sep 7. 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