Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Tue Sep 10 2024 Valid 12Z Fri Sep 13 2024 - 12Z Tue Sep 17 2024 ...General Overview... The main thing that will make weather headlines for the end of this week will be the eventual remnants of Francine progressing inland across the Tennessee and Ohio River valleys. The result will be moderate to heavy rainfall that could produce some instances of flooding. Elsewhere, a trough exiting the Northern Rockies Friday will likely be replaced by another large scale trough across the West Coast by Monday, bringing another round of cooler weather and increased rainfall chances. Meanwhile, a general upper level ridge to the north of the remnant low from Francine will be in place from the Midwest/Northern Plains to the Great Lakes, keeping summer-like temperatures in place. ...Guidance/Predictability Assessment... The 00Z model guidance suite features good overall synoptic scale agreement across the nation on Friday and continuing into the weekend. For the remnant tropical low moving inland across the Ohio Valley, the 00Z is slightly faster than the ECMWF/GFS solutions and also ahead of the NHC track forecast. The low continues to weaker further and become just a surface trough based on the guidance by Sunday. Looking ahead to Monday, the GFS is quicker with the arrival of the next trough across the West Coast, and is a few hundred miles to the east of the CMC/ECMWF solutions by Tuesday. There is growing model spread with timing of northern stream shortwaves across Canada, but these should not be all that impactful for the weather forecast across the U.S. For the WPC overnight forecast, a blend of the deterministic guidance was used as a baseline on Friday, and then tapered down the CMC and UKMET and placed more weighting to the GFS/ECMWF for the weekend. Ensemble means were added gradually to make up about 40-50% of the blend by early next week, which helped to smooth out increasing mesoscale differences. ...Weather/Hazards Highlights... The center of steadily weakening T.D. Francine should be reaching the western Ohio Valley by Friday night, and the majority of the rainfall should be located to the east and southeast of the low pressure center. There will likely be a remaining inflow band across central Tennessee and extending into Kentucky, where 1-2 inches of rain can be expected with locally higher amounts. 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/Friday. Not much in the overall atmospheric set-up changes for Day 5/Saturday, except for a slightly weaker signal in the model guidance and further weakening of the remnant low. However, the Marginal Risk is still planned for many of those same areas in Day 4 given the grounds will likely be more saturated. Elsewhere across the country in terms of rainfall, showers should be decreasing in coverage across Montana and the northern High Plains as the first western U.S. trough lifts out of the region in time for the weekend. Showers and some thunderstorms are likely to make a return going into early next week from Arizona to the Pacific Northwest as the second trough builds in across the West Coast and then the Intermountain West, and scattered showers/storms also for the southern Plains. 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 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