Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Aug 19 2024 Valid 12Z Thu Aug 22 2024 - 12Z Mon Aug 26 2024 ...Hazardous heat to persist for the southern Plains into this weekend... ...Overview... The amplified upper pattern of a central CONUS ridge centered over the southern Plains with troughing over the Northeast into this weekend and a low moving over the Northwest into next week. This pattern maintains hazardous heat over Texas, moist monsoonal flow over the Southwest/Four Corners, cooler surface high pressure along the Eastern Seaboard, and Tropical moisture lurking near the Gulf Coast and across Florida. ...Guidance/Predictability Assessment... The 12Z global suite is coming into decent agreement with the progress of the western low/trough over the weekend with a somewhat more progressive track than the 00Z consensus. The 06Z GFS was the most progressive with the arrival of the low to the northern Plains by Sunday, so it was disregarded from the morning model preference. However, now the 12Z GFS has slowed considerably and is in line with the 12Z consensus with the upper low/trough reaching the eastern MT border around 12Z Monday. For Days 4-7 QPF the 13Z NBM was blended with the 00Z ECMWF/CMC to reduce the input from the 06Z GFS. ...Weather/Hazards Highlights... A monsoonal convective pattern will linger over the Southwest/Four Corners states through Friday with moist southerly flow west of a slow to dislodge upper ridge axis centered over the southern Plains. The WPC Day 4/5 Excessive Rainfall Outlooks for Thursday/Friday have been updated to depict Marginal Risks for united portions of the Four Corners states. Local storms with embedded heavy rain could cause flash flooding, particularly in vulnerable areas. Upper trough amplification proximity along the West Coast portends a cooler and wetter pattern for the Northwest and vicinity through later this week. Rains and spotty high elevation mountain snows may work inland across the north-central Intermountain West/Rockies this weekend into next week dependent on the uncertain track/timing of supporting upper trough/low ejection. Rains/convection may also consolidate downstream over the north-central U.S. given possible moderate cyclogenesis/frontogenesis for focus in this scenario. Upper ridging focus over the south-central U.S. will support 5-15 degree above normal temperatures into this weekend. Highs well into the 100s in much of Texas in particular could set daily records and high dewpoints should produce heat indices likely above 110 degrees. Low temperatures in the upper 70s to low 80s will exacerbate heat stress and could set records for warm minimum temperatures. Experimental HeatRisk continues to indicate Major to Extreme impacts from this heat through Sunday. This will be in contrast to cooling conditions into the West/Northwest and over the eastern U.S. where some record cool overnight temperature values are possible. Jackson 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