Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM EDT Mon Sep 9 2024 Valid 12Z Thu Sep 12 2024 - 12Z Mon Sep 16 2024 ...Heavy rainfall and potential flooding for the lower Mississippi River Valley on Thursday as TS Francine moves inland... ...General Overview... The main thing that will make weather headlines for the end of this week will be Tropical Storm Francine progressing inland across Louisiana, Arkansas, and Mississippi. The result will be heavy rainfall that could produce flooding and gusty winds, and the rain from this system will likely lift northward towards the Ohio Valley as it weakens further by the weekend. Elsewhere, a pattern change will be arriving to the West Coast and Intermountain West as an upper trough amplifies over the region and heralds the arrival of much cooler temperatures and increasing rainfall chances, compared to recent conditions. Meanwhile, warm temperatures are expected across much of the north-central U.S. to the Great Lakes region with an upper level ridge building in. ...Guidance/Predictability Assessment... Latest guidance continues to show good overall agreement on Tropical Storm Francine as it makes landfall and moves northward through the Lower Mississippi into the Middle Mississippi Valley later this week. The CMC is a bit on the faster side of the guidance envelope, so a blend of the ECMWF and GFS seemed closest to the official track from the National Hurricane Center for both the pressure/fronts progs and the QPF. Otherwise, later in the period, there continues to be timing differences with the next trough into the West Coast. The 06z GFS was significantly faster than the rest of the guidance, but the new 12z run today did trend slower. The WPC progs were based on a blend of the deterministic models, weighted towards the ECMWF and GFS, for the first half of the period. Incorporated up to 50 percent of the ensemble means for the latter half to mitigate the differences out West. ...Weather/Hazards Highlights... The core of the heaviest rainfall associated with a weakening TS Francine is forecast to be across eastern Arkansas and into western Tennessee and northern Mississippi by Thursday, and the QPF from the ECMWF/GFS best matched the NHC track and is overall more progressive compared to earlier forecasts. Therefore, the Slight Risk area on Day 4/Thursday has been nudged northward some from overnight continuity, and also extends eastward over western Alabama where the inflow band may reside and result in locally higher rainfall totals as well. Given the recent dry conditions and remaining uncertainty in the swath of heaviest QPF, there is no need for any Moderate Risk areas for the Day 4 time period at this time, but may be needed for future updates. For the Day 5/Friday time period, the overall QPF decreases as the low lifts farther north and steadily weakens across the Ohio Valley region, where a Marginal Risk is planned. This Marginal Risk also includes the southern Appalachians where moist upslope flow could produce even higher rainfall totals, and southward towards the Florida panhandle along the front/inflow. Otherwise, the trough arriving over the West will result in numerous showers across the Northern Rockies. The heaviest QPF from this will likely be over northwestern Montana, with snow for the highest mountain ridges. Closed upper low passage and increased ensemble enhanced QPF probabilities seems to also support a Marginal Risk ERO area for Day 4/Thursday. Showers and some storms then reach the central/northern Plains by Friday, while the Great Lakes and the Northeast U.S. remains mostly dry. Temperatures will still feel more like August across much of the north-central U.S. through the end of the week and into the weekend for both highs and lows, with potential +20 degree anomalies for lows across North Dakota and northern Minnesota by Friday morning. Meanwhile across the West Coast region and the Intermountain West, a return to refreshingly cooler temperatures is expected by midweek in response to the building upper trough, and feeling more like October. Widespread highs in the 60s can be expected for the lower elevations behind the cold front. It will likely be cooler than average across portions of the Mid-South for daytime highs to close out the week owing to increased clouds and rainfall. 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