Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Sun Sep 04 2022 Valid 12Z Wed Sep 07 2022 - 12Z Sun Sep 11 2022 ...Record-setting heat will continue across the West into the north-central U.S. through Thursday and may persist in California through Friday... ...Overview... Widespread record-breaking heat remains a weather hazard through the medium range period across California through the Intermountain West and into the northern half of the Plains through Thursday as the anomalous upper high/ridge persists, with some gradual moderation of the heat late in the week. Across the eastern U.S., a broad upper trough and weak closed low will be in place, and a couple of frontal systems and Gulf moisture should allow for rain and thunderstorms in the Southeast and possibly into the Ohio Valley/Great Lakes regions by late week. Then there is potential for tropical moisture to stream into the Southwest over the weekend as the recently formed Tropical Depression Twelve-E tracks northward through the eastern Pacific. ...Guidance Evaluation/Predictability Assessment... While there has always been good model agreement for the large upper high/ridge centered over the Intermountain West, fortunately model agreement has also increased considerably over the last day or so with the timing of troughing moving into the Northwest Wednesday-Thursday and tracking across the northern tier through next weekend, which temporarily reshapes and suppresses the northern part of the upper ridge. There are still certainly some differences in timing and the evolution (and potential widening) of the trough as additional energy gets integrated, but more along the lines of average/typical differences for the medium range period rather than the below average model agreement and thus forecast confidence from the last several cycles. The ECMWF has the most run-to-run consistency for the trough's timing while GFS runs have waffled, but the 06Z GFS was in good alignment with the 00Z ECMWF. The 00Z UKMET and CMC were a bit slower than those models and the 00Z ensemble means, but it does seem like the 12Z model suite may be a bit slower as well Friday-Sunday. Then while models agree on an upper trough/low with its axis across the east-central U.S. through late week, there becomes some uncertainty with the possible combination of this feature and the incoming northern stream trough. Model guidance and the forecast shows some phasing, with a trough axis in the Upper Great Lakes south-southwest into the Lower Mississippi Valley by day 7/Sunday, with the upper ridge persisting to the trough's west. The WPC fronts/pressures forecast utilized a multi-model deterministic blend early in the period, gradually including and increasing the proportion of the ECMWF mean and eventually some GEFS mean 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 increased QPF in the Great Lakes and Ohio Valley by next weekend along and ahead of the cold front. ...Weather/Hazards Highlights... Considerably hot temperatures particularly by September standards are forecast to continue into through Wednesday and Thursday from California through the Great Basin into central/northern portions of the Rockies and High Plains. Daytime high and overnight/morning low temperatures of 10-25F above average will be widespread, with actual highs of 110+ 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. Recent forecasts show better agreement for a significant cooling trend behind a cold front in the north-central U.S. on Friday, though California is likely to remain hot with the reorientation of the upper ridge atop the region. By next weekend, temperatures could be warmer than normal across the Northwest Saturday into the Northern Rockies/High Plains next Sunday, but likely not to the same anomalous levels as earlier in the week. Meanwhile, slightly below normal highs and around normal lows are expected for the Lower Mississippi Valley and Southeast given the cloudy and showery forecast. Parts of the Northeast/Mid-Atlantic could see highs that are below normal by a few degrees on Wednesday, before gradually warming to above normal through the latter part of the week. The main focus for rain during the period will be across the southeastern quadrant of the country, as ample Gulf moisture flows over a meandering frontal boundary near the Gulf Coast with the broad upper low in the vicinity, promoting showers and thunderstorms. There may be some flooding concerns with slow-moving storms across the Gulf Coast, Southeast, and into the Southern Appalachians. By next weekend, moisture could spread farther north ahead of a cold front trekking across the Great Lakes and Ohio Valley, and the recent forecast has risen rainfall totals for those areas. Additionally, a surge of increased moisture may arrive in the Southwest in conjunction with what is currently Tropical Depression Twelve-E. 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