Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Sat Sep 7 2024 Valid 12Z Tue Sep 10 2024 - 12Z Sat Sep 14 2024 ...Multi-day western to central Gulf Coast States heavy rainfall/flooding threat through early-mid next week, with possible tropical system development being monitored by NHC... ...Overview... Widespread and protracted heavy rain/thunderstorms with flooding issues will make weather headlines for the western to central Gulf Coast states with inland return resurgence of deep Gulf moisture with a slowly lifting warm front early to mid-later next week. This threat could be significantly enhanced with possible tropical system development and approach as currently monitored by NHC. Elsewhere, it generally remains the case that 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 next week as an upper trough and light to moderate precipitation focus works across the West Coast, the Intermountain West and then eventually out through the northern Rockies/Plains. ...Guidance/Predictability Assessment... Latest model guidance still features good overall synoptic scale agreement across the nation into Tuesday/Wednesday, with differences slowly increasing through longer time frames. Guidance has shown measured cycle to cycle uncertainty with the amplifying trough over the northwestern U.S., but a favored composite has trended toward the more amplified side of the full envelope of solutions. 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. The WPC medium range product suite was primarily derived from a 00 UTC ECMWF/NAEFS ensemble mean blend along with the 13 UTC National Blend of Models and WPC overnight continuity. This solution works well with latest NHC guidance and also has good composite model support over much of the rest of the country for next week. ...Weather/Hazards Highlights... Increasing tropical moisture over the southwestern Gulf of Mexico/Bay of Campeche in the short term is expected to bring an increasing threat for heavy rain and strong storms to the coastal areas of the Gulf Coast states through 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 South Texas to near Lake Charles, where a multiple 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 inland from the Texas Gulf Coast. A Slight Risk remains from the cental Texas coast to near Baton Rouge. A Moderate Risk upgrade was considered and may be needed pending future NHC updates. 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/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