Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 PM 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 sluggishly shifts east with time, and as a mean upper low slowly works through the Northeast. This will support a lingering 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 and wavy front will keep rain chances across the Gulf Coast 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... One main areas of system uncertainty during the period continues to surround the deep upper low/trough as it shifts out of the Great Lakes/Northeast. There are also timing inconsistencies with the next amplified trough in the Eastern Pacific next week, which guidance shows potential for a cutoff low off the West Coast with a possible slow shift of the larger scale trough to/into the West. A composite blend of model and ensemble guidance offered a seemingly reasonable forecast basis. However, given amplified flow tends to linger on the slower side of the full envelope of guidance, opted to apply slightly greater weighting to those components. This stays in line with the National Blend of Models. ...Weather/Hazards Highlights... One 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 WPC Day 4 Excessive Rainfall Outlook shows a Marginal Risk. Farther south, a lingering front across northern Florida and along the Gulf Coast will continue to focus showers and storms, locally heavy, through at least the weekend. The better chance for heavier QPF is on Saturday, as covered by a WPC Marginal risk on the Day 4 ERO. Depending on what falls where during the short range period, a marginal risk may eventually be needed on the Day 5/Sunday ERO as well, but the QPF signal still does not show sufficient focus. 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 may bring an increasing threat of heavy rain into southern Texas by early next week. Some guidance show potential of locally significant amounts to monitor. 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/Schichtel 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