Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 PM EDT Fri Sep 02 2022 Valid 12Z Mon Sep 05 2022 - 12Z Fri Sep 09 2022 ...Prolonged heat wave across much of the West will last through the middle of next week... ...Overview... A strong upper level high will be anchored over the central Great Basin through much of next week, promoting continued excessive heat across much of the West and into the Northern Plains that could be record breaking. The heat wave is expected to gradually recede especially across the Northwest during the latter half of next week as an energetic trough comes in and suppresses the ridge/high. Meanwhile a broad upper trough with embedded energy will persist across the East, along with a couple of frontal boundaries that could enhance chances for rain and thunderstorms, particularly in the eastern U.S. on Monday and lingering in southern Texas through much of the period. Additional moisture may approach the southwestern U.S. by late next week streaming in from a possible tropical system to the south. ...Guidance Evaluation/Predictability Assessment... Good model agreement is present at the beginning of the period Monday with the 594+dm upper high at 500mb and the high remaining centered around the Central Great Basin through at least Wednesday. The details of its suppression by Thursday-Friday depend on the flow farther north, specifically a strong shortwave trough with a potential embedded closed low coming into the Northwest Wednesday-Friday. The 00Z/06Z model suite that was available for the WPC forecast happened to be pretty agreeable with its timing and that of an associated surface low tracking across southern Canada. Unfortunately the 12Z models have diverged somewhat, with the GFS much weaker with the shortwave, leading to a flatter pattern, while the ECMWF slowed down with the shortwave, so this is still an area of uncertainty. Meanwhile in the East, there is general model agreement for troughing. Embedded energy and the details of how the troughing is oriented at any given time is much less certain from model to model and run to run. The ECMWF and EC mean appeared most reasonable and consistent for the trough, with the GFS (especially the 00Z) likely too fast to move troughing offshore Wednesday onward. It looks like in the 12Z model suite there is better agreement for a closed low somewhere in the east-central U.S. Thursday-Friday. Thus the WPC forecast used a multi-model deterministic blend early in the period with a gradual transition toward more usage of the ensemble means, especially the ECens mean, by Thursday/Friday. This blend allowed for a stronger trough and surface low in the northwestern quadrant of the forecast domain Thursday-Friday, though as mentioned above this could change due to the newer guidance. Changes from the NBM and/or continuity include lowering QPF somewhat in parts of Texas given the NBM's higher bias than the bulk of the models, and following a more northward/more inland trend with QPF in the Ohio Valley to Northeast as the NBM and models agreed on this. ...Weather/Hazards Highlights... Considerably hot temperatures particularly by early September standards are forecast to continue this weekend through Labor Day and beyond from California through the Great Basin into central/northern portions of the Rockies and High Plains. High and low temperatures of 10-20F above average will be widespread through Wednesday, as actual temperatures of 110+ are forecast for the Desert Southwest and California's Central Valley while widespread 90s and some 100s are expected across the Intermountain West into the northern half of the Plains. Multiple daily records will be set, with some September monthly records possible as well. 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. Temperatures are expected to gradually moderate closer to normal Thursday and especially Friday, though could still be slightly warmer than average especially across California Friday. Meanwhile, slightly below normal highs and around normal lows are expected for portions of Texas into the Southeast given the cloudy and showery forecast. Parts of the Northeast could see below normal highs only reaching the 60s or low 70s on Labor Day behind a front, but should gradually warm to above normal through the week. A cold front pushing southward across the northeastern quadrant of the nation will promote showers and thunderstorms there along with potentially heavy rainfall and flooding on Monday and possibly into Tuesday, before clearing there as the front pushes south. Several rounds of rain and thunderstorms are possible for southern Texas with Gulf moisture present and a stationary front in its vicinity. The Southeast may see scattered thunderstorms for the first half of the week and these may increase in coverage by the latter part of the week, especially along the Gulf Coast and into the Southern Appalachians, but there are still some model differences with the details. By the end of the period, some increased moisture may arrive in the Southwest in conjunction with a tropical system to the south. Tate 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