Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM 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... ...Heavy rain threat from Southern/Southeast Texas through the central Gulf Coast states... ...Overview... The anomalously strong upper ridge over the West will persist through the middle of next week, bringing hot, record-breaking, and potentially dangerous heat to portions of the western U.S. into the northern Rockies. The other focus for significant weather will be found across portions of Texas through the central Gulf Coast where interactions among 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. Across the northern tier of the U.S. into the Northeast, a few frontal passages are expected bringing chances of showers and thunderstorms ahead of it. By the end of the forecast period, the heat over the West will begin to subside and shift eastward as a stronger shortwave trough moves into the Pacific Northwest. ...Guidance Evaluation/Predictability Assessment... Fairly strong agreement and predictability with the large scale pattern over the CONUS through about day 5. The western U.S. upper ridge will remain centered over the Great Basin and a slow moving positively tilted trough over the central to eastern U.S.. By Day 5, the western ridge flattens a bit as deep troughing over the eastern Pacific begins to move eastward with the approach a stronger shortwave trough digging through. That energy is likely to reach the Pacific Northwest day 6 into day 7, though there remains some timing differences among the various deterministic guidance. The cluster analysis shows a relatively evenly-divided split among the global ensembles, suggesting a general blend/consensus approach was the best approach to the forecast today. The 01.01Z NBM was utilized for the sensible weather grids and a blend of the GFS/GEFS and ECMWF/EPS was used for the pressure fields/winds, maintaining good WPC continuity. ...Weather/Hazards Highlights... An early September heatwave across the western U.S. is expected to produce daily high temperature anomalies of 10-20F each day with the hottest day on track to be Monday /Labor Day/ for portions of California while the heat peaks Tuesday and Wednesday for the northern Rockies ahead of an approaching frontal passage. Readings well into the 90s with triple digit readings expected for the Desert Southwest into the California central valley. Widespread daily high temperature records are possible with some locations possibly reaching monthly records. Adding to the heat stress will be the very warm to near record overnight low temperatures as well. With little nighttime relief and the multiple day event expected, the heat wave could become increasingly dangerous for vulnerable populations through the holiday weekend. Widespread High to Very High Heat Risk levels are expected along and near the Central Valley and Desert Regions of California this weekend into early next week. 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 spell 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 isn't as strong though something to monitor over the coming days across southern Texas. Guidance also points to favorable moisture transport and lift over the southern Appalachians where rain totals could reach a few inches over the next week and this area will need to be monitored for potential heavy rain and flash flooding over the coming days too. Taylor/Fracasso 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 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