Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sun Sep 8 2024 Valid 12Z Wed Sep 11 2024 - 12Z Sun Sep 15 2024 ...Deteriorating conditions expected for southeast Texas and into Louisiana as a tropical system likely affects the region midweek... ...Overview... The main thing that will make weather headlines for the middle of next week will be the increasing threat of an organized tropical system, which is being closely monitored by the National Hurricane Center with a high chance of development. The result will likely be heavy rainfall that could produce flooding and gusty winds for portions of the central and western Gulf Coast, and the rain from this system will likely lift northward towards the Ohio Valley as it weakens late in the forecast period. Elsewhere, a pattern change will be arriving to the West Coast and Intermountain West as an upper trough amplified over the region and heralds the arrival of cooler temperatures. Meanwhile, a generally warmer trend is expected for the East Coast region as upper level ridging develops in the wake of the recent trough across the Northeast. ...Guidance/Predictability Assessment... The 00Z model guidance suite features good overall synoptic scale agreement across the nation on Wednesday and continuing into Thursday, especially with the development and evolution of the trough across the West. For the tropical low approaching the western Gulf Coast, the UKMET is a northeastern outlier compared to the other guidance and the ensemble means, and the GFS slightly east of the CMC/ECMWF tracks. Looking at individual ensemble members, there is still a big southwest to northeast spread with the position of the low, and intensity varies from about 980-1000 mb as it approaches the coast. For the WPC overnight forecast, a blend of the deterministic GFS/CMC/ECMWF was used as a baseline through the end of the week, and less of the UKMET owing to its eastern solution with the tropical system. Ensemble means were added gradually to make up about 40% of the blend by Sunday, which helped to smooth out increasing mesoscale differences. ...Weather/Hazards Highlights... Heavy rainfall, wind, 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 Deep South as the western Gulf tropical low lifts slowly towards the northeast. The core of the heaviest rainfall from the low will likely be just approaching the southwest Louisiana/southeast Texas coast by late Wednesday, with outer feeder bands likely reaching north of the Interstate 20 corridor by early Thursday. A Slight Risk remain valid from the greater Houston metro area to central Louisiana, where the overall model signal is greatest for 2-4+ inch totals, and potentially double that for areas where convection repeatedly trains over the same areas. 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, with the UKMET on the eastern side of the guidance and the GFS trending a little slower with arrival of heaviest rainfall. 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 through Thursday night. The heaviest QPF from this will likely be over northern Wyoming and into Montana, with snow for the highest mountain ridges. 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 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