Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Sun Sep 8 2024 Valid 12Z Wed Sep 11 2024 - 12Z Sun Sep 15 2024 ...Central Gulf Coast to Lower Mississippi/Tennessee Valley heavy rainfall/flooding threat into mid-later week, with possible tropical system development and landfall as monitored by NHC... ...Overview... Widespread and intense rain/thunderstorms with flooding issues will make continued major weather headlines with spread across for the central Gulf Coast states and Lower Mississippi/Tennessee Valley and vicinity with inland return resurgence of deep Gulf moisture with a leading lifting warm front mid-later this week. The threat could be significantly enhanced with possible tropical system development and approach/landfall as currently monitored by NHC. Rain from this system will likely lift northward towards the Ohio Valley as it weakens through next weekend in slowed flow. Out West, the persistent upper ridge and widespread anomalous warmth is forecast to abate by the second half of the week as an upper trough works across the West Coast, the Intermountain West and eventually through the northern Rockies/Plains in an organized rain pattern with some potential for local/terrain enhanced rains. ...Guidance/Predictability Assessment... Latest model guidance features good overall synoptic scale agreement across the nation, with differences only slowly increasing through longer time frames. Importantly, the forecast signal continues to increase in favor of possible tropical system development over the Gulf of Mexico with likely impacts for the western to central Gulf coast and inland over the east-central U.S.. The WPC medium range product suite was primarily derived from a broad model and ensemble blend along with the 13 UTC National Blend of Models and WPC overnight continuity. Blend weight focus gradually transitions from the models to the ensemble means to address slowly growing forecast spread. This solution works well with latest NHC guidance, but remains dependent on updated info. ...Weather/Hazards Highlights... Heavy rainfall, wind/coastal affects and the potential for flooding will indeed be the most important facet of the medium range forecast across the Gulf Coast region, and then across interior portions of the South and northward. This would occur as a potential western Gulf tropical low lifts; leading to possible landfall and northward inland track with extratropcial transition and weakening over time. A WPC Excessive Rainfall Outlook Slight Risk area has been depicted from the greater Houston metro area to the central Gulf Coast, where the overall model signal is greatest for 2-4+ inch totals, and potentially double that for areas where convection repeatedly trains. The same general thinking holds true for the new Day 5 period Thursday as the main QPF axis lifts north across Mississippi and western Tennessee, but there remains noteworthy longitudinal differences in the track of the inland low. Although a Moderate Risk may eventually be needed for parts of these regions, there is still not enough confidence to place one at this time, especially since the low has not fully developed yet. Otherwise, the trough arriving over the West will likely produce mainly light to moderate showers across portions of the Pacific Northwest and into the Rockies. The heaviest QPF from this will likely be over northern Wyoming and into Montana, with snow for the highest mountain ridges. Closed low passage and increased ensemble enhanced QPF probs seems to also support a Marginal Risk ERO area. Showers and some storms then reach the central/northern Plains by Friday, while the Great Lakes and the northeastern quadrant of the nation remains mostly dry. Temperatures will still feel more like August across much of the north-central U.S. through the end of the week 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 providing badly needed relief from the recent hot and dry weather this region has endured over recent days. Widespread highs in the 60s can be expected for the lower elevations behind the cold front. Hamrick/Schichtel 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