Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sun Sep 4 2022 Valid 12Z Wed Sep 7 2022 - 12Z Sun Sep 11 2022 ***Record setting heat wave across the western U.S. and northern Plains through midweek followed by gradual improvement*** ...General Synopsis... The main weather headline going into the middle of the week will continue to be the anomalous upper level ridge that will be anchored across the western U.S. through about Thursday, and thus a continuance of the ongoing serious and record setting heat wave from the West Coast to the Northern Plains through Thursday, followed by a gradual cooling trend going into next weekend. Across the eastern U.S., a broad upper trough and weak closed low will likely be in place for the middle to end of the week across the Deep South and Tennessee River Valley, and moisture from a tropical disturbance may begin approaching the southwestern U.S. by the end of the week. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite initially has a good overall synoptic scale depiction of the overall pattern on Wednesday. Noteworthy timing differences are apparent by Thursday night with the GFS becoming more progressive with the trough across the northern Plains, and the CMC is on the slower side of the guidance. This trough will have implications on the break down of the huge ridge across the western U.S. late this week. Going into next weekend, the GFS is much stronger with the trough building across the Great Lakes, whereas the ECMWF/CMC are more indicative of a broad upper ridge here and the trough axis holding farther west. Forecast confidence remains below average for the Friday through Sunday time period with poor model run-to-run continuity over the past 24 hours, and future forecasts are subject to significant revisions once the models come into better alignment, particularly across the northern tier states. In terms of QPF, the NBM 4.1 along with some of the bias corrected model was used as a baseline. Taking these factors into account, the WPC fronts/pressures forecast was primarily derived from a multi-model consensus through early Thursday, followed by a gradual increase of the ensemble means whilst still keeping some of the ECMWF to account for increasing uncertainties by the end of the forecast 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 105-115 degrees for the lower elevations of the Desert Southwest and even into California's Central Valley, while widespread 90s and some low 100s are expected across the Intermountain West and extending northeast across eastern Montana and the Dakotas. Some all-time September high temperature records are within the realm of possibility, in addition to numerous daily record highs. 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. 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. There has been a trend for higher rainfall across the central/eastern Gulf Coast through the latter part of the week and into Saturday, while showers and thunderstorms could also increase in coverage across the Southeast and Southern Appalachians. By the end of the period, a surge of increased moisture may arrive in the Southwest in conjunction with a tropical system to the south that has a very good chance of developing further, 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