Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 AM EDT Mon Aug 19 2024 Valid 12Z Thu Aug 22 2024 - 12Z Mon Aug 26 2024 ...Hazardous heat to persist for the southern Plains this week... ...Overview... The prevailing amplified upper pattern across much of our country is forecast to persist through the end of this week. This pattern will be highlighted by an upper high centered over the southern Plains to sustain hazardous heat there and moist monsoonal flow on the western periphery over the Southwest/Four Corners. The northward building ridge/heat over time will be sandwiched by an amplified upper trough upstream into the West Coast and an amplified upper trough downstream over the East in a pattern with a cool high pressure system at the surface. Tropical moisture will lurk near the Gulf Coast and across Florida. ...Guidance/Predictability Assessment... Forecast spread remains the main concern in the western U.S. through the medium-range forecast period, primarily as attributed to differences with the handling of deep eastern Pacific upper trough/low energies inland into the West/Northwest starting late this week. The 18 UTC GFS has been the latest model to favor a solution on the more progressive side of guidance while most other guidance has been trending slower with recent runs through 12 UTC. Prefer to favor a slower solution and that trend seems at least modestly supported by latest 00 UTC guidance. A composite blend of more compatible guidance of the 12 UTC GFS/UKMET/ECMWF/Canadian models was used to derive the WPC medium range product suite for Thursday into Saturday in conjucture with the 01 UTC National Blend of Models. Forecast uncertainty grows quickly later weekend into next week, but the 12 UTC GFS/ECMWF at least offer a reasonably slow pattern progression while maintaining better system continuity/amplitude/identity and focus out through the West. The even slower translation of recent Canadian runs is plausible given the pattern, but was hesitant to fully embrace given lackluster ensemble support, even from Canadian ensembles. ...Weather/Hazards Highlights... An eastern U.S. upper trough offers some lingering unsettled conditions given slow translation, but rainfall will taper off midweek as surface high pressure builds in behind a quite refreshing cold frontal push in the wake of Ernesto lifting well off into the North Atlantic. However, trailing fronts slated to stall farther south across Florida and vicinity will maintain periodic thunderstorm chances there over a multi-day period later week through the weekend. Flooding issues are a possibility if heavy rain falls in urban areas. This front moves slowly as it bends back to the west atop the west-central Gulf Coast and back north into the Plains. Scattered storms are possible near the front as impulses ride and dig to the lee of the upstream ridge aloft, but with generally low confidence in the predicted placement. A monsoonal convective pattern will linger over the Southwest/Four Corners states this week with moist southerly flow west of a slow to dislodge upper ridge axis. The WPC Day 4/5 Excessive Rainfall Outlooks for Thursday/Friday depict a Marginal Risk area for portions of Arizona, Utah, southern Nevada and western Colorado. Local storms with embedded heavy rain could cause isolated 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. 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. 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. 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