Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM 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 show fairly good agreement on the overall large scale pattern through much of next week, but the typical differences in the details of individual systems cause some notable differences in sensible weather. One feature of note is shortwave energy in the Southern Plains or so that, along with surface troughing and fronts in the vicinity, will help force convection in Texas and spreading eastward with time. Models may be coming more in line with each other, but there is still a considerable amount of spread. To the north, a shortwave over the Northwest will phase with a shortwave through central Canada as it shifts east, eventually resulting in a deepening trough (or closed low) over/near the Great Lakes by Friday. There are some timing differences between the GFS, ECMWF, and Canadian, but not enough confidence to throw out any particular solution at this time. This does have implications for timing of the main frontal boundary that pushes from the north-central U.S. into the East late period. The WPC forecast for tonight used a blend of the deterministic models the first half of the period, transitioning to just under half of the ensemble means into the blend amidst increasing late period spread. ...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. 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