Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Mon Sep 5 2022 Valid 12Z Thu Sep 8 2022 - 12Z Mon Sep 12 2022 ***Record setting heat wave across 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 as shortwave energy from the north impinges upon it, thus heralding a bit of a pattern change going into the weekend. A broad upper low/trough is forecast to be in place across the Deep South through Sunday, and this in combination with a weak frontal boundary, will tend to result in scattered to numerous showers and storms from Texas to the Southeast states. Tropical Storm Kay also bears watching as the eventual remnants of this storm could result in an increase in rainfall across portions of the Southwest U.S. later in the forecast period. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite initially has a good overall synoptic scale depiction of the overall pattern on Thursday and even continuing into Friday. The deterministic model guidance has come into better focus on the shortwave perturbations across the northern tier states compared to the past few days. However, the GFS remains more progressive with the trough across the northeastern U.S., whereas the ECMWF/CMC are in good agreement on a cut-off low developing over the Great Lakes by Sunday/Monday. The CMC becomes stronger with a shortwave trough approaching the Pacific Northwest by next Monday and has limited ensemble support. In terms of QPF, the NBM 4.1 along with some of the bias corrected model was used as a baseline. The WPC fronts/pressures forecast utilized a multi-model deterministic blend for the end of the week, gradually including and increasing the proportion of the ECMWF and GEFS means to about half by the end of the period as individual model differences increased. The forecast was fairly consistent with continuity, with one change being an increase in rainfall totals across the Southeast U.S., where a Slight Risk of excessive rainfall was introduced for the Day 5 period. ...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 such as the elderly, those working outside, and those without air conditioning. The good news going forward is that the heat should 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. By the end of the period, a surge of increased moisture will likely arrive for the Desert Southwest in conjunction with the eventual remnants of Tropical Storm Kay, based on recent projections from the National Hurricane Center. 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