Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM 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 be featured with an upper level trough amplifying across the West Coast, and then the trough should encompass most of the interior West by Tuesday. This combined with a cold front will result in more widespread showers 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), and this could have some staying power for several days. This will tend to limit the eastward progression of the large scale trough out West. ...Guidance/Predictability Assessment... The 00Z model guidance suite features good overall synoptic scale agreement across the nation for the upcoming weekend. The remnant low from Francine continues to weaken further and become just a surface trough or weak reflection based on the guidance by Sunday. Looking ahead to the first half of next week, the model guidance and the machine learning guidance has consolidated with the arrival of the next trough across the West Coast. The latest GFS has slowed compared to earlier runs and better matches the CMC and ECMWF models. By midweek, the CMC is stronger with a reinforcing trough across the Pacific Northwest and begins to stray more from the model consensus. For the WPC forecast, a blend of the deterministic guidance was used through early Tuesday as a starting point in the forecast process. Ensemble means were added gradually to make up about 40% of the blend by midweek, which helped to smooth out increasing mesoscale differences. Overall, good continuity was maintained with the previous WPC forecast. ...Weather/Hazards Highlights... The remnant low from Francine should be nearly dissipated by Sunday and evolve into a surface trough, 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 moves farther inland on Day 5/Monday across North Carolina. Therefore, a Marginal Risk area is planned for both days for portions of this region. Showers and thunderstorms are likely Sunday for parts of the Southwest with a increased moisture connection ahead of a possible tropical disturbance lifting north over the southern Baja Peninsula. To account for this possibility, Marginal Risk areas are planned 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. 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. 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