Extended Forecast Discussion NWS Weather Prediction Center College Park MD 220 AM EDT Wed Sep 4 2024 Valid 12Z Sat Sep 07 2024 - 12Z Wed Sep 11 2024 ...Heavy rainfall threats to impact Northern Florida and the Northeast on Saturday... ...Overview... An amplified upper-level pattern will be in place by the weekend with an upper ridge over the West that shifts east with time, and an upper low slowly moving through the Northeast. This will support a period of enhanced rainfall over portions of New England this weekend, with a fall- like dry and cool air mass filtering in behind it across much of the eastern half of the country. A lingering front will keep rain chances from the central Gulf Coast eastward to Florida, with the best chance for heavy rain on Saturday across Northern Florida. Tropical moisture moving into the southwestern Gulf could bring heavy rain into southern Texas by early next week. ...Guidance/Predictability Assessment... The main area of uncertainty during the period continues to surround the deep upper low/trough as it shifts out of the Great Lakes. The ECMWF continues to be the southern outlier pulling the upper low through the Northeast, with the GFS and CMC having a more northward track with the system through far eastern Canada. The ensemble means and recent AI models also supported this farther north track, which pulls the rainfall out of the Northeast quicker. There are also timing inconsistencies with the next amplified trough in the Eastern Pacific next week, which guidance shows potential for a cutoff low to form off the West Coast next week. The timing of this, which is very uncertain at these later ranges, will affect the overall timing of the arrival of the larger scale trough into the West. The forecast for tonight used a blend of the GFS, CMC, and ECMWF for Days 3 and 4, beginning to incorporate minor amounts of the ensemble means into the blend by Day 5 as uncertainty with the Northeast low increased. By Day 7, the blend consisted of 70 percent of the ensemble means which showed good consistency with each other in an attempt to mitigate differences amongst the deterministic solutions. ...Weather/Hazards Highlights... The main heavy rainfall focus during the period will be across the Northeast on Saturday as the upper low/associated trough pivots through the Northeast, aided by wrap back moisture from a surface low along the coast. The Day 4 Excessive Rainfall Outlook for now continues to show just a marginal, but the potential exists for an eventual upgrade to a slight risk if models come into better agreement on exactly where pockets of heavier rain will set up. Farther south, a lingering front across northern Florida and along the Gulf Coast will continue to focus showers and storms, locally heavy, through the weekend. The better chance for heavier QPF is on Saturday, covered by a marginal risk on the Day 4 ERO. Depending on what falls where during the short range period, a marginal risk may be needed on Sundays ERO as well, but for now the QPF does not support any risk. From Sunday into early next week, much of the country should dry out, though shortwave energy within and around western U.S. upper ridging may produce isolated to scattered rainfall with mostly light amounts. Meanwhile, tropical moisture moving into the southwestern Gulf could bring an increasing threat of heavy rain into southern Texas by early next week, and rainfall may move back into the Northwest early to mid next week ahead of the upper trough. Hazardous heat across the Southwest and into California during the short range period should be waning by this weekend as the upper ridge begins to shift out of the West. Above normal temperatures will also slide east with time from the West into the Central U.S., however it does not look to be particularly hazardous at this time. In contrast, 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 10-20 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