Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Sep 03 2022 Valid 12Z Tue Sep 06 2022 - 12Z Sat Sep 10 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 and a weak closed low at times will persist across the East, along with a couple of frontal boundaries that could enhance chances for rain and thunderstorms, particularly in the Mid-Atlantic on Tuesday and spreading from southern Texas across the Gulf Coast and Southeast through the week. Additional moisture may approach the southwestern U.S. by the end of next week streaming in from tropical system that is likely to form to the south in the eastern Pacific and track near Baja California. ...Guidance Evaluation/Predictability Assessment... Good model agreement is present at the beginning of the period Tuesday with the 594+dm upper high at 500mb and the high remaining centered around the Central Great Basin through at least Wednesday, with more guidance than not now showing it persisting there into Thursday. The details of its suppression go awry by Thursday-Saturday as it depends on the flow farther north, specifically a shortwave trough coming into the Northwest probably Thursday, slower than a day or so ago. GFS runs and many GEFS members indicate this shortwave as weaker and faster than other guidance, as they split off some of the energy and have it spin in the Pacific separate from the main jet. The 00Z ECMWF and CMC appeared clustered well and were well aligned with the EC mean with its timing, while the UKMET was on the slower and stronger side. Thus the middle ground ECMWF/CMC/EC mean was favored for the timing of this shortwave, and for showing some QPF across the northwestern U.S. associated with this trough while GFS-based solutions were dry there. But this remains a quite uncertain feature given the model disagreements and the energy's origins near/over Siberia. The incoming 12Z models show additional divergence in solutions as the CMC is slower than its previous run while the ECMWF is faster. So additional forecast adjustments are likely with time. This particular forecast ended up slightly faster with the associated cold front Saturday in the north-central U.S. compared to the previous WPC forecast. Meanwhile in the East, there is general model agreement for upper troughing with an axis in the east-central U.S. and potentially some occasional ridging for Florida/the Carolina coast. The details of embedded energy in the trough are less certain, but models have come into better agreement with an upper low closing off in the Lower Mississippi Valley/Southeast on Thursday and lingering into the end of the week. Thus the WPC forecast used a multi-model deterministic blend early in the period, transitioning toward a blend favoring the ECMWF/EC mean/CMC along with a bit of the GEFS mean for the aforementioned reasons. This led to reasonable continuity with the previous forecast especially considering the below average model confidence by the end of the period. ...Weather/Hazards Highlights... Considerably hot temperatures particularly by 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. High and low temperatures of 10-25F above average will be widespread through at least 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 moderate nearer to normal in the Northwest by Thursday, but may remain much above normal for California stretching into the Northern Plains, gradually moderating on Friday and Saturday. 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 highs that are below normal by a few degrees on Tuesday, before gradually warming to above normal through the week. A meandering frontal boundary is likely to stay draped along the Gulf Coast into southern Texas through the week, which combined with Gulf moisture will produce continued showers and thunderstorms, along with some potential for flooding issues given the recent wet pattern. The recent forecast/models show a trend toward the rainfall focus spreading into the central/eastern Gulf Coast through the latter part of the week (with reduced amounts in Texas as the front pushes south), 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 good likelihood of forming per the National Hurricane Center. 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