Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 PM EDT Sun Sep 1 2024 Valid 12Z Wed Sep 04 2024 - 12Z Sun Sep 08 2024 ...Overview... An upper ridge over the southern tier to start the period will strengthen over the West mid to late week as a shortwave shifts into the northern Plains. This looks to interact with energy dropping out of Canada to produce a fairly amplified trough over the Great Lakes, moving into the Northeast next weekend. This should push a mean boundary through the northern tier during the period. Meanwhile, embedded energy within the larger scale upper ridge and a persistent surface boundary draped near the Gulf Coast should provide focus for heavy rainfall that shifts slowly from the Lower Mississippi Valley/Gulf Coast into the Southeast and possibly points north. ...Guidance/Predictability Assessment... Models are slowly consolidating around a split flow pattern over eastern North America toward the end of the week. The operational GFS has trended slower and closer in line with the rest of the guidance with respect to the mid-level trough propagating through the Great Lakes and Midwest late week. The operational Canadian continues to be faster and more amplified than the rest of the guidance. In terms of QPF, the GFS suite has increased signals for heavy rainfall over the Lower Mississippi/Tennessee Valley on Wednesday. The ensemble means appear to signal similarly. A general model blend consisting of the 00z EC/CMC/UKMET and 06z GFS were utilized through day 4 with the EC and GFS favored. By day 5 the 00z ECE and 06z GEFS are introduced at the expense of the 00z UKMET, which is removed from the blend. The deterministic GFS is replaced with the 00z Canadian ensemble mean on day 6, while the GEFS and ECE are favored in the blend 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 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 Wednesday to Thursday night), with a focus for a possible eventual slight upgrade across the Gulf Coast and/or farther inland across portions of the Lower Mississippi/Tennesse Valley on Wednesday and Thursday. Later, the lingering frontal boundary across the South will promote additional rain and locally heavy downpours with amounts and locations still very uncertain, but the trend is for increased rain totals along the Gulf Coast and into parts of the Southeast. By the weekend, 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. Increasing heat is expected across much of the West, with moderate to major HeatRisk threats across the Southwest and central valleys of California. Elsewhere, from the central U.S. into the East, temperatures will be below or near normal as systems move through. 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