Extended Forecast Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Thu Sep 01 2022 Valid 12Z Sun Sep 04 2022 - 12Z Thu Sep 08 2022 ...Prolonged and dangerous heat wave for portions of the Western U.S. through the holiday weekend and beyond... ...Heavy rain threat from southern parts of Texas through the Southeast and Southern Appalachians... ...Overview... A persistent upper high over the Great Basin and the associated ridge will bring hot, record-breaking, and potentially dangerous heat to portions of the western U.S. into the Rockies and northern High Plains. The Northwest in particular should see cooling temperatures starting around midweek next week as an upper trough edges closer with a cold front ahead of it. Farther east, another focus for significant weather will be found across southern portions of Texas and eastward as a stalling frontal boundary, a weak upper level disturbance, and plentiful Gulf moisture could bring multiple days of moderate to heavy rainfall and potential flash flooding. Additionally, a cold front slowly pressing southeastward could produce rain and thunderstorms across the eastern third of the nation early next week, with a particular rainfall focus likely over the southern Appalachians. ...Guidance Evaluation/Predictability Assessment... Model agreement as the period begins Sunday is fairly good, with the strong upper ridge in the West suppressed by a shortwave in the northern High Plains, and broader troughing centered in the east-central CONUS. Models differ somewhat regarding how strong the shortwave is through Monday-Tuesday, with GFS runs staying on the stronger side, but a multi-model deterministic blend from the available 00Z/06Z models was a reasonable compromise through days 3-4 of the forecast for that shortwave and the larger scale synoptic pattern. By day 5/Tuesday, most guidance shows the main trough axis shifting into the eastern U.S., but the main exception was the 00Z ECMWF. It indicates an upper low mainly stationary centered over Illinois even through day 7/Thursday. This was on the slower side of the guidance, including the ensemble means and most individual ensemble members, so preferred a somewhat more progressive solution. The currently incoming 12Z ECMWF is more progressive from the 00Z run as well. Then models vary somewhat with the timing and depth of troughing coming into the Pacific Northwest, and with potential for an eastern Pacific tropical system to develop and come near Baja California, and possibly eventually disrupting the flow over the U.S. as well. The latter part of the medium range period phased in the GEFS and EC ensemble means to about 60 percent by day 7 while lessening the proportion of the deterministic guidance, especially the 00Z EC, in the forecast blend. ...Weather/Hazards Highlights... An early September heat wave across the western U.S. is expected to produce daily high temperature anomalies of 10-20F each day from California northeastward into the Great Basin, northern Rockies, and northern High Plains. Temperatures will be well into the 100s with some 110s possible for California's Central Valley and into the Desert Southwest, while farther north 90+ degree temperatures will be widespread, nearing or exceeding 100F at some locations. Widespread daily high temperature records are possible, with some locations possibly reaching monthly records. Low temperatures are also forecast to be 10-20 degrees above normal and near record warm, adding to the heat stress, so the heat wave could become increasingly dangerous for vulnerable populations through the holiday weekend and into next week. Widespread High to Very High Heat Risk levels are expected along and near the Central Valley and Desert Regions of California through the first half of next week. Temperatures may moderate some by next Thursday, at least in the Northwest after a cold front moves through. A residual frontal boundary draped along the Gulf Coast interacting with favorable Gulf moisture transport and a weak upper level disturbance will help focus widespread showers and thunderstorms through the weekend into early next week. For Sunday, the focus for the heaviest rain potential remains across portions of southern to southeast Texas where the combination of intense rain rates and prolonged widespread moderate to heavy rain could total a few inches per the latest forecast guidance. Many areas are already running above normal precipitation departures recently, which could contribute to additional flash flooding. A Slight Risk in the Experimental Day 4 (12Z Sun - 12Z Mon) Excessive Rainfall Outlook remains in effect for these areas. By Sunday, the boundary becomes more diffuse and the signal for heavy rain is not as strong, but is still something to monitor over the coming days across southern Texas, with flooding issues possible particularly in areas that receive multiple rounds of heavy rainfall. Guidance also points to favorable moisture transport and lift across the southeastern U.S., and the Southern Appalachians in particular could be an area of focus for heavy rain given a front nearby and forcing aloft. The southeastern quadrant of the country is likely to remain cooler than normal at least in terms of highs due to the clouds and rain chances. Rain and storms are also possible farther north for the Ohio Valley, Mid-Atlantic, and Northeast near the front for the early part of next week. Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Sun, Sep 4. - Heavy rain across portions of the Southern Plains, Wed, Sep 7. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding likely across portions of the Lower Mississippi Valley. - Excessive heat across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Sun-Thu, Sep 4-Sep 8. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml