Extended Forecast Discussion NWS Weather Prediction Center College Park MD 229 AM EDT Mon Sep 2 2024 Valid 12Z Thu Sep 05 2024 - 12Z Mon Sep 09 2024 ...Hazardous heat likely late week across parts of the Southwest and California with heavy rainfall threats across the Lower Mississippi Valley to the Southeast... ...Overview... Upper ridging will build over the West during the period, with heat threats for parts of the Southwest into California. Downstream, an amplified ridge and eventual closed low will dig into the Eastern third of the nation within an increasingly blocky late period regime. Meanwhile, weak energy near the Gulf Coast interacting with a persistent surface boundary should provide focus for heavy rainfall that shifts from the Lower Mississippi Valley/Gulf Coast into the Southeast and East Coast late this week. Weather across the nation should be very quiet by Sunday and beyond. ...Guidance/Predictability Assessment... Models and ensembles show fairly good agreement through much of the period on an increasingly amplified and blocky upper flow pattern across the lower 48. The greatest area of uncertainty remains surrounding the deep trough/closed low near the Great Lakes that should slowly lift north and timing of the associated front into the Northeast. The CMC remains displaced slightly south of the GFS/ECMWF and the ensemble means with the closed low. Timing uncertainties also surround an additional shortwave entering the Northwest early next week. The WPC forecast used a blend of the deterministic guidance for the first half of the period, increasing to an equal mix of deterministic (GFS and ECMWF) and ensemble means by the late period to smooth out some of the differences. ...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 which slowly shifts across the Deep South. The Day 4 Excessive Rainfall Outlook (covering Thursday-Thursday night) shows a broad marginal risk across the Lower Mississippi Valley into the lower Tennessee Valley and parts of the Southeast. An embedded slight risk was added near the coast from southeast Louisiana to the far western Florida panhandle where models show overlap in heavy QPF from the short range into the medium range period. The overall threat shifts east on Friday into the Southeast and northward towards the Mid-Atlantic as a weak area of low pressure lifts up the coast. This is covered by a large marginal risk on the Day 5 ERO, where and embedded slight may be needed in future issuances once models show better consistency. Wrap back moisture and rainfall should continue across the Northeast on Saturday on the backside of the cyclone, but otherwise rainfall should clear the East Coast. By Sunday and beyond, much of the country looks quiet and dry. Increasing heat and above normal temperatures are expected across much of the West, with moderate to major HeatRisk threats across the Southwest and central valleys of California through at least Friday. A much cooler airmass will filter eastward from the northern Plains and Great Lakes into the East this weekend and early next week, with daytime highs 10 to 20 degrees below normal. 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