Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Sat Sep 7 2024 Valid 12Z Tue Sep 10 2024 - 12Z Sat Sep 14 2024 ...Multi-day Western Gulf Coast States Heavy Rainfall/Flooding Threat through early-mid next week also being monitored by NHC... ...Overview... The beginning of next week will be featured with an upper level trough exiting the Northeast U.S. and a large surface high. The initially fall-like airmass will undergo a moderation trend going into the middle to end of the week with widespread 80s for highs returning. Out West, the persistent upper ridge and widespread anomalous warmth is forecast to abate for the second half of the week as an upper trough builds in across the West Coast and then the Intermountain West. Heavy rain and thunderstorms will continue to make weather headlines for the western Gulf Coast as another resurgence of Gulf moisture (currently monitored by NHC) moves inland across Texas by Tuesday night into Wednesday, but not much in terms of heavy rainfall elsewhere across the nation with mainly lighter rainfall for the northwestern U.S. and the Rockies. ...Guidance/Predictability Assessment... The 00Z model guidance suite features good overall synoptic scale agreement across the nation on Tuesday, with the greatest model spread generally north across southern Canada. The guidance has remained consistent with the amplifying trough over the northwestern U.S., with the GFS a little quicker to lift the trough out across the Dakotas on Friday. With the disturbance approaching Texas, the ensemble members still vary quite a bit with low placements and intensity, so this is something to closely monitor in the days ahead. For the WPC overnight forecast, a blend of the deterministic GFS/CMC/UKMET/ECMWF was used as a baseline through Wednesday, with a little more weighting to the ECMWF guidance. Ensemble means were added gradually to make up about 50% of the blend by Saturday, which helped to smooth out increasing mesoscale differences. ...Weather/Hazards Highlights... Increasing tropical moisture over the southwestern Gulf of Mexico/Bay of Campeche over the next few days is expected to bring an increasing threat for heavy rain and strong storms to the coastal areas of the Texas Gulf Coast by early to middle next week. This disturbance is also being monitored by the National Hurricane Center for possible development of a tropical depression or storm as the system gets closer to the Texas Coast. For the Day 4 period Tuesday, the guidance generally keeps the heaviest rain close to the coast or just offshore the Texas Coast. Owing to wet antecedent conditions from heavy rain in the days leading up to this, a Slight Risk remains valid from the greater Corpus Christi area to near Lake Charles, where a few inches of rainfall will be possible for some of these areas. Going into the Day 5 period Wednesday, there is a decent overall model signal for a swath of enhanced rainfall moving inland across southeast Texas and into central Louisiana, with potentially a sharp western gradient of the QPF across the central Texas Gulf Coast. A Slight Risk is planned from the greater Houston metro area to near Baton Rouge, and a Marginal Risk extending eastward to the Georgia Coast. 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 northeastern quadrant of the nation remains dry. Temperatures are expected to be unseasonably warm by September standards across much of the north-central U.S., with highs running about 5 to 15 degrees above average from the Front Range of the Rockies to the western Great Lakes for much of the week. The hot weather expected across much of the West Coast and Intermountain West during the short range period is expected to improve going into the middle of next week as the upper trough builds southward and heralds the arrival of a cold front, bringing temperatures of 5 to 15 degrees below normal for portions of the West Coast states and into Nevada. Slightly below average readings are also likely for the Gulf Coast region (but still humid) owing to increased cloud cover and rain. 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