Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 continue to show reasonable agreement the first half of the period as a shortwave shifts from the Northwest into the northern Plains. By next weekend though, there is significant uncertainty in the evolution of the pattern as an amplified trough and additional energy drop southward. Through yesterdays 12z/18z runs, there was uncertainty in how amplified this trough might get near the Great Lakes, but now most of the new 00z guidance (available after the forecast was generated) have trended towards some sort of deep closed low development but huge distance differences in location. The GFS drops this low south into the Midwest, while the ECMWF pulls it East towards eastern Canada or the Northeast, with other models somewhere in between. This evolution is obviously very fluid and will change several times between now and then. The WPC forecast for tonight used a general blend of the deterministic models (12z/18z runs) for the first half of the period. Trended towards 60 percent ensemble means for next weekend given the uncertainty in the northern stream pattern. ...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 Wednesday to Thursday night), with a focus for an possible eventual slight upgrade across the Gulf Coast and farther inland across the Lower Mississippi Valley on Wednesday and towards the Tennessee Valley/Alabama on Thursday. Beyond, 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. 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