Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Thu Sep 12 2024 Valid 12Z Sun Sep 15 2024 - 12Z Thu Sep 19 2024 ...General Overview... The beginning of the forecast period Sunday will feature an upper level trough amplifying across the West Coast, with another wave acting to reinforce and maintain troughing across the West into next week. This combined with a cold front will result in more widespread showers, mountain snow, and cooler temperatures. Across the Eastern U.S., a rex block type pattern is likely for Sunday into early next week with an upper high situated over the Northeast U.S., and a weak upper low over the Carolinas (partially from the remnants of Francine and another disturbance over the Atlantic). This feature may linger for a few days, which would limit the eastward progression of the large scale trough out West. ...Guidance/Predictability Assessment... Latest models guidance continues to show good overall synoptic scale agreement on the pattern described above, but does offer plenty of uncertainty in the details. The remnant low from Francine should not be much more than a surface trough or weak reflection by Sunday, but there is a lot of uncertainty regarding how this lingering energy may interact with additional energy off the Southeast coast. The GFS is the strongest in suggesting possible tropical or sub-tropical development of this system near the Southeast Coast, but the remainder of the guidance is much weaker with the system. Out West, the initial low Sunday-Monday has good agreement as it lifts through the Great Basin and northern High Plains. Some timing uncertainties arise as the next low arrives by mid next week. The WPC forecast today used a blend of the latest deterministic guidance through day 5 amid good model agreement. For 6 and 7, incorporated 40 percent of the ensemble means with the GFS and ECMWF to account for greater detail and timing differences. Overall, this maintained good agreement with the previous WPC forecast as well. ...Weather/Hazards Highlights... The remnant low from Francine should be nearly dissipated by Sunday, though with some lingering rain across portions of the Tennessee River Valley and into the southern Ohio Valley. Since the ground across central Tennessee and northern Alabama will be quite saturated by this time, a Day 4 Marginal Risk is still planned for Sunday across this region. Farther to the east across the coastal Carolinas, a developing wave of low pressure along an old stationary front will likely result in a plume of enhanced moisture and onshore flow with the potential for 1 to 3 inches of rainfall, and this may move farther inland on Day 5/Monday across North Carolina. Therefore, a Marginal Risk area is planned for both days for portions of this region. Rainfall amounts and location is highly uncertain given differences in how the models handle this feature off the Southeast Coast. Rainfall chances could continue into next week with this into parts of the Mid-Atlantic. Showers and thunderstorms are likely Sunday for parts of the Southwest with a increased moisture connection ahead of Tropical Depression Nine-E lifting north over the southern Baja Peninsula. Marginal Risk ERO areas are in place for portions of Arizona and into western New Mexico on Sunday, and extending farther north into southern Utah and southwest Colorado on Monday. Elsewhere across the West, increasing showers and storms are likely by early next week as the second trough builds in across the West Coast and then the Intermountain West, and scattered showers/storms also for the southern Plains as tropical moisture gets wrapped in ahead of the Western trough. Some snow is likely for the highest mountain ridges of the Northern Rockies as snow levels drop. Another cool-down is expected going into early next week for the West Coast with readings 10 to perhaps 20 degrees below normal for portions of the interior West Coast states by Tuesday. Frost may be a concern for some across the northern-central Great Basin. Meanwhile, warm conditions are forecast to continue from the central/northern Plains to the Great Lakes and parts of the Northeast where upper level ridging will tend to govern the overall weather pattern. Santorelli/Hamrick 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