Extended Forecast Discussion NWS Weather Prediction Center College Park MD 222 PM EDT Mon Sep 05 2022 Valid 12Z Thu Sep 08 2022 - 12Z Mon Sep 12 2022 ...Record setting heat wave across parts of the western U.S. and northern Plains through Thursday followed by gradual improvement... ...General Synopsis... The intense upper level ridge and the record setting heat wave associated with it is forecast to slowly erode and weaken by Thursday as shortwave energy moves into the Northwest, with somewhat of a pattern change going into the weekend. A broad/cut-off upper low/trough is forecast to be in place across the Deep South into this weekend, and this in combination with a weak frontal boundary, will tend to result in enhanced to potentially heavy/excessive rainfall along the Gulf Coast and into the Southeast. Tropical Storm Kay also bears watching as the track of this system to near/off the coast of California could result in an increase in rainfall across portions of the Southwest U.S. later in the forecast period. ...Guidance Evaluation/Predictability Assessment... The latest model guidance suite continues to show good synoptic scale agreement through day 5 on the overall pattern featuring a shortwave trough moving through through the Northwest into the northern U.S. and a cut off broad upper low nearly stagnant over the central Gulf coast. A general model compromise seemed to serve as a good starting point across the board for days 3 to 5. This weekend, and especially beyond, some larger scale differences begin to creep up, including some question on timing of the shortwave across the Upper Midwest and details surrounding its the evolution of a cut-off low taking shape over the Midwest/Ohio Valley early next week. Most of the deterministic model solutions have been consistent in showing this evolution, however the ensemble means only show a weaker shortwave, which suggests quite a bit of uncertainty still surrounds this feature. Out West, the next trough approaches by Sunday with some greater timing and strength differences. The 00z CMC continues to be noticeably stronger with this, so it was not favored in the late period blend for today. For days 6-7, the WPC blend favored nearly half of the ensemble means to help mitigate the differences, with some deterministic (more GFS and ECMWF) solutions for added system definition. This maintained good continuity with the previous shift as well. ...Weather/Hazards Highlights... Very hot temperatures by early September standards are forecast to continue into the workweek from California through the Great Basin into central/northern portions of the Rockies and High Plains. Daytime highs and overnight lows on the order of 10-25 degrees above average will be widespread through probably Thursday, and this will equate to highs of 105-110 degrees for the lower elevations of the Desert Southwest and even into California's Central Valley, while widespread 90s are expected across the Intermountain West and extending northeast across eastern Montana and the Dakotas. The magnitude and duration of the dangerous heat will both be contributors to potential safety concerns for sensitive groups. The heat should finally abate some across the Northwest by Thursday, followed by gradual improvement to close out the week farther south across the Intermountain West and especially across the northern Plains, where the change to cooler weather will likely be more noticeable. Meanwhile, slightly below normal highs and around normal lows are expected for portions of Texas into the Southeast owing to mostly cloudy skies and more widespread shower and thunderstorm coverage. In terms of rainfall potential, a meandering but weak frontal boundary is likely to stay draped along the Gulf Coast into southern Texas for much of the forecast period, which combined with deep Gulf moisture and a broad upper low, is forecast to produce scattered to at times widespread showers and thunderstorms. Given much above normal rainfall in some locations across the Southeast, heavy rainfall may increase the flood and flash flood threat, especially across portions of the Southeast on Friday where WPC has a slight risk highlighted on the experimental Day 5 ERO. By this weekend, a surge of increased moisture will likely arrive for the Desert Southwest in conjunction with Tropical Storm Kay with additional flooding concerns. There remain some track uncertainties which of course has implications for QPF across southern California and the Southwest. See those products issued by the National Hurricane Center for the latest information. Santorelli/Hamrick 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