Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Wed Sep 11 2024 Valid 12Z Sat Sep 14 2024 - 12Z Wed Sep 18 2024 ...General Overview... The remnant low associated with what is now Hurricane Francine will continue to dissipate across the Ohio Valley over the weekend. Meanwhile, a closed upper high is expected to reside over the Great Lakes and into the Northeast with above average temperatures going into early next week. Across the Western U.S., another trough builds in near the West Coast after the first one lifts out across the Northern Plains late this week, with more of an amplified upper level pattern evolving. There may be a coastal low that tries to develop near the coastal Carolinas by mid-week as the frontal boundary over the western Atlantic tries to lift back north. ...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 past few runs of the GFS remain quicker with the arrival of the next trough across the West Coast. The recent machine learning guidance suggests the timing of the trough and closed low would be slower, more in line with the ECMWF/CMC solutions. For the WPC forecast, a blend of the deterministic guidance was used through the weekend as a starting point in the forecast process. Ensemble means were added gradually to make up about 40% of the blend by Wednesday, which helped to smooth out increasing mesoscale differences. The ECMWF was weighted more than the GFS late in the period given better consistency with the ensemble means and ML guidance. Overall, good continuity was maintained with the previous WPC forecast. ...Weather/Hazards Highlights... The remnant low from Francine should be nearly dissipated by Saturday and evolve into a surface trough, and the majority of the remaining rainfall should be located to the east and southeast of the low. There will likely be a remaining inflow band across central Tennessee and extending into northern Alabama, where locally higher higher rainfall totals could be observed. Orthogonal low level flow across the southern Appalachians could also result in some higher rainfall totals here as well, and both of these regions are encompassed in a broad Marginal Risk area for Day 4/Saturday. Showers and thunderstorms are likely this weekend 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 southern portions of Arizona on Saturday, and extending farther north into Arizona and western New Mexico on Sunday. 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. Temperatures should moderate some across the northwestern U.S. by the weekend as the first trough lifts out, with highs returning to near seasonal averages. Another cool-down appears likely 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