Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EDT Thu Aug 22 2024 Valid 12Z Sun Aug 25 2024 - 12Z Thu Aug 29 2024 ...Overview... The persistent blocky pattern is expected to transition into early next weeks as the eastern Pacific upper trough lifts inland across southern Canada the Pacific Northwest. As this occurs, the expansive upper ridge over the south-central U.S. will sprawl into the east-central portion of the country. This will lead to a relative lull in monsoonal flow into the Southwest before it resumes toward the end of the extended period. The increasingly more progressive and energetic northern stream upper trough energy will direct cooler airmasses into the West and downstream over the U.S. northern tier states mid-later next week. ...Guidance/Predictability Assessment... The timing of the pattern transition seems to not have resolved much with the latest runs of guidance. The ECMWF has consistently been the slower less progressive solution for several days and now the UKMET is closely clustered. The CMC has become increasingly faster like the GFS the last couple of cycles, both seem approximately 12 to 18 hours faster with the evolution that the ECMWF/UKMET. The ensemble means remain fairly clustered with the ECMWF/UKMET or somewhere between the two clusters through the extended period. The WPC forecast was derived from a model and ensemble blend with enhanced weighting applied to solutions on the slightly less progressive side of the full envelope of guidance given recent flow nature, but the result is not as slowed as continuity. ...Weather/Hazards Highlights... Unsettled and cooler conditions will filter into the Northwest/West over the weekend and into next week as an amplified trough moves onshore the West Coast. Periods of rain and spotty high elevation mountain snows will be possible across the northern Intermountain West/Rockies. For areas downstream, showers and thunderstorms may develop across the north-central states by early next week given focus from possible moderate cyclogenesis/frontogenesis, with modest focus shifting downstream as the week progresses with system/frontal translation. Renewed northern stream upper trough approach and digging back into the Northwest into mid-later next week offers potential for development and digging/intrusion of unsettled and cooled flow. Hot temperatures under the upper ridge will persist across the south- central U.S. while expanding to the east-central states during this period. Daily temperatures will routinely reach upwards of 10 degrees above normal. Heat risk guidance identifies an associated region with upwards to major level impacts with main focus set to shift slowly from the south-central Plains and Mid- Mississippi Valley Sunday southeastward through the South- Southeast into next midweek. A lingering trailing front stalling across Florida will maintain periodic rain/thunderstorm chances there and into Southeast U.S. coastal areas into next week. Moist flow and pattern longevity may lead to possible local runoff issues, especially for urban areas. Campbell 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw