Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM EDT Fri Sep 13 2024 Valid 12Z Mon Sep 16 2024 - 12Z Fri Sep 20 2024 ...Overview... Upper low swings from CA to MT Monday through Tuesday night with a reinforcing trough shifting down the PacNW coast Tuesday to at least the central CA Coast Thursday, which will promote cooler weather with rain and higher elevation snow. Meanwhile in the East, ridging in the Northeast looks to direct any subtropical lows off the Southeast Coast into the Carolinas Monday/Tuesday. The evolution of the pattern in the East retains uncertainty with the positioning of the upper low and progress toward/into the eastern states. ...Guidance/Predictability Assessment... Great agreement of both initial and second low over the West through next week. The timing of the first low is even in better agreement with the 12Z cycle as the CMC is a bit slower and the ECMWF is a bit quicker. And the 12Z UKMET slowed a bit with the second low progressing down the West Coast Tuesday night/Wednesday.--- There was considerable uncertainty in the 00Z/06Z guidance with low pressure into the Carolinas Monday/Tuesday as the ECMWF had suppressed troughing while the GFS maintained a low into NC. The 12Z GFS held serve, the CMC came in stronger with troughing, and the 12Z ECMWF now has a low similar to the GFS. For now this increases confidence in the forecast. The WPC forecast was based on a general model blend for the higher confidence west and a non-GFS solution for the east given the 06Z GFS was an outlier. Encouragement would be to now go more toward the 12Z GFS/ECMWF. ...Weather/Hazards Highlights... Drastic shift in the 12Z ECMWF toward recent GFS solutions warrants a Slight Risk ERO upgrade on Day 4 given the presence of low pressure (likely subtropical in nature - please stay tuned to NHC forecasts for further updates) moving into NC. For now the Slight Risk is for southern NC. This low tracks inland per the 12Z GFS/ECMWF with a Marginal Risk introduced for much of VA and northern NC. The presence of terrain in the eastern Appalachians may warrant further expansion of excessive rainfall risks as this forecast continues to evolve. Also on Day 5 is a lingering frontal zone over the central Gulf Coast/FL Panhandle where heavy rain should develop and a Marginal Risk was introduced. Lingering moisture from the Pacific Tropical Storm Ileana will be drawn north ahead of the low over NV, so the Day 4 Marginal Risk is maintained/expanded a bit for the Four Corners states. The Day 5/Tuesday Marginal Risk is maintained for the western half of MT as heavy rain rates ahead of the initial low track look to persist over the same areas for multiple hours with some instability in place. The higher elevations can expect snow with this system given the depth of the upper low. Another round of energy will cause the Pacific Northwest to see precipitation on Tuesday, shifting into much of the Great Basin midweek and then into the northern Rockies. Farther east, scattered rain and thunderstorms are likely in the Plains for multiple days next week near and ahead of frontal systems. The West can expect much cooler than normal temperatures next week under the upper trough. High temperatures will be 15-25F below normal across much of the West, gradually moderating closer to normal across the West Coast states but persisting in the Interior West. Some areas may reach the freezing mark in the higher elevations of the Great Basin/Rockies. Meanwhile, the central U.S. should see warmer than normal highs by 10-15F and lows by 15-20F, including highs in the 90s in the southern half of the Plains. Warmer than average temperatures will stretch into the Great Lakes and Northeast, with more typical temperatures south of there. Jackson 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