Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Aug 31 2024 Valid 12Z Tue Sep 03 2024 - 12Z Sat Sep 07 2024 ...Overview... A fairly progressive northern stream upper level pattern will be in place next week during the medium range period. On the large scale, upper ridging across the Southern U.S. will expand and build into the West as systems ride the top of the ridge from west to east, eventually building a deepening trough over the Great Lakes to Northeast mid to late next week. This pattern should push a couple of frontal boundaries into the central and southeastern U.S., with periods of stormy weather expected to accompany them. Meanwhile, embedded energy within the larger scale upper ridge and a persistent surface trough and embedded waves near the Gulf Coast should provide focus for heavy rainfall from Texas to the Lower Mississippi Valley and Gulf Coast. ...Guidance/Predictability Assessment... Models are in reasonable agreement with respect to the overall synoptic evolution across the CONUS during the medium range period. The 00z Euro clusters reasonably well with the ML models and 00z CMC/UKMET/ECE/CMCE and 06z GEFS. The deterministic GFS trended drier and more suppressed with QPF along the Gulf coast, Lower Mississippi Valley and Tennessee Valley throughout the period. The 00z European suite was favored in the medium range blend for those reasons. The day 3 blend consists of the 00z ECMWF/CMC/UKMET and 06 GFS with weighting tilted toward the operational Euro and GFS. The Euro emerges a favorite in the blend on day 4 at the expense of the GFS which begins to split flow over the eastern U.S./Canada. The ensembles filter into the blend on day 5 and are favored through day 7. ...Weather/Hazards Highlights... The main heavy rainfall focus during the period will be across the South as shortwave energy moves inland from the western Gulf and a surface front lingers along/near the coast. Precipitation coverage and totals remain highly uncertain, but the overall dynamics and anomalous moisture support a heavy to excessive rainfall threat with slow- moving storms. This is covered by fairly broad Marginal Risks on the Days 4 and 5 EROs (covering Tuesday to Wednesday night), with a possible focus across the Gulf Coast and farther inland across much of Texas Tuesday, spreading eastward with time into the Lower Mississippi Valley. For midweek and beyond, the lingering frontal boundary across the South will promote additional rain and locally heavy downpours. Amounts and locations remain very uncertain, but the trend is for increased rain totals along the Gulf Coast and into parts of the Southeast. Eventually by late week, some moisture may spread into the Ohio Valley and/or Eastern Seaboard. Meanwhile, some light to modest precipitation should accompany a shortwave moving through the Interior West, and rain chances could spread into the Northern Plains and Midwest/Great Lakes later next week along and ahead of the northern front. Modestly above normal temperatures across the Northern Plains on Tuesday should shift east and moderate. Meanwhile, increasing heat is expected across much of the West, with moderate to major HeatRisk threats across the Southwest and central valleys of California. From the Southern Plains to the Midwest and the East, temperatures should be near or below normal for this time of the year within a rather pleasant air mass through much of next week. Kebede/Santorelli 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