Extended Forecast Discussion NWS Weather Prediction Center College Park MD 432 PM EDT Wed Aug 31 2022 Valid 12Z Sat Sep 03 2022 - 12Z Wed Sep 07 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... A anomalously strong upper ridge positioned over the West will lead to a hot, record-breaking, and potentially dangerous heat wave right through this holiday weekend into next week for portions of the western U.S. and 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. Meanwhile, shortwave troughing will slip through the northern tier of the U.S. into the Great Lakes and Northeast bringing a couple frontal passages through the period and seasonable temperatures. Along and ahead of each front, showers and thunderstorms will be possible. ...Guidance Evaluation/Predictability Assessment... Upper pattern will feature a strong upper high over the Great Basin/Southwest to start the period (Sat) with weaker ridging over the western Atlantic. This will favor positively-tilted troughing from Texas to the Great Lakes. Through the weekend into next week, the upper high is forecast to flatten a bit as a shortwave or closed low pushes into the Pac NW and western Canada, with some disagreement on timing. 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 13Z 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 continuity from overnight. ...Weather/Hazards Highlights... The strong upper ridge for early September positioned over the Western U.S. will support temperatures 15-20F above normal each day with the hottest day likely setting up to be Monday / Labor Day / where highs in the mid to upper 90s will encompass much of the western areas, even spreading into the northern Plains, and triple digit readings are expected from the Desert Southwest through 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 Saturday, the best focus for heaviest rainfall looks to be from the central Gulf Coast westward to the Texas Gulf Coast and potentially further west to the TX Hill Country. Intense rain rates and prolonged widespread moderate to heavy rain could total up to several inches per the 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 Sat - 12Z Sun) Excessive Rainfall Outlook remains in effect for these areas. Going into Sunday, troughing over the Plains and the surface boundary retrograding westward into Texas further will bring additional rounds of heavy rain potentially to much of Southern/Southeast Texas. A Slight Risk in the Experimental Day 5 (12Z Sun-12Z Mon) Excessive Rainfall Outlook was maintained for these areas from overnight. While the specifics and exact locations of the heavy rainfall are still uncertain due to the convective/mesoscale nature of the system, the environmental ingredients and lift/focus from the synoptic features over what may become increasingly wet antecedent conditions could lead to instances of flash flooding this weekend. Fracasso/Taylor Hazards: - Heavy rain across portions of the Southern Plains, Sat-Wed, Sep 3-Sep 7. - Heavy rain across portions of the Southern Appalachians, Southeast, and the Lower Mississippi Valley, Sat-Sun, Sep 3-Sep 4. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Flooding likely across portions of the Lower Mississippi Valley. - Excessive heat across portions of the Northern Plains, the Northern Rockies, the Great Basin, California, and the Southwest, Sat-Wed, Sep 3-Sep 7. 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