Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Thu Sep 5 2024 Valid 12Z Sun Sep 08 2024 - 12Z Thu Sep 12 2024 ...Overview... An amplified upper-level pattern will be in place this weekend with an upper ridge over the West that shifts east with time, as a mean upper low/trough slowly works through the Northeast. This will usher in a fall-like dry and cool air mass across much of the eastern half of the country. Elsewhere, a lingering and wavy front will keep rain chances across the Gulf Coast to Florida, while tropical moisture moving into the southwestern Gulf could bring heavy rain into southern Texas by early next week. ...Guidance/Predictability Assessment... Models show better consistency with the upper low over the Northeast as it lifts northward with time, and a general model blend seemed to suffice for this feature. The greatest area of uncertainty during the period is out West as the next amplified trough (and possible cut off low) shifts east across the Eastern Pacific towards the West Coast. There are some notable timing differences with this and plenty of run to run variance. Through the 12/18z runs from yesterday, the GFS was by far the quickest to bring this system inland, with the ECMWF the slowest (which had support from the AI models). However, most of the new 00z runs trended faster, including the ECMWF. For the later periods, the WPC forecast blend incorporated increasing amounts of the ensemble means late period. Did lean more towards the slower solutions with the western trough, given support from the 12z AI models, but based on the new 00z model trends (which were available after forecast generation time), the WPC forecast may well be too slow now. ...Weather/Hazards Highlights... A lingering front across northern Florida and along the Gulf Coast will continue to focus showers and storms, locally heavy, into next week. At this time, no excessive rainfall risk areas are depicted on either the Day 4/Sunday or Day 5/Monday graphics, but depending on what falls where during the short range period, a marginal risk may eventually be needed across northern Florida. Tropical moisture moving into the southwestern Gulf may bring an increasing threat for heavy rains back into southern Texas, which is covered by a marginal risk on the Day 5 ERO. Some guidance shows potential of locally significant amounts to monitor. Otherwise much of the country should be quiet and dry, though shortwave energy within and around western U.S. upper ridging may produce isolated to scattered rainfall with mostly light amounts. Rainfall ahead of the next Western U.S. trough should begin to move into the region around next Tuesday. Above normal temperatures across the Northwest on Sunday should shift into the Northern Plains with time. Daytime highs look to be 10-15 degrees above normal, but this may only equate to a moderate HeatRisk threat. Above normal temperatures may reach into the Midwest/Upper Great Lakes as well late period. In contrast, a much cooler airmass will be in place across the East this weekend, but may trend back towards normal next week. An arriving upper trough should signal below normal temperatures back into the West Coast states mid 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