Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun Aug 18 2024 Valid 12Z Wed Aug 21 2024 - 12Z Sun Aug 25 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. The northward building ridge over time will be sandwiched by an amplified upper trough upstream near the West Coast and an amplified upper trough downstream over the East with a cool high pressure system at the surface while tropical moisture will be lurking 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 ECMWF has switched to the fastest side of the guidance envelop since yesterday while the GFS and UKMET are slower, with the CMC the slowest solution. Their ensemble means also depict the same kind of timing differences although not as pronounced as their deterministic counterpart. The latest EC-AIFS also favored a more progressive and less amplified solution. The faster solution affects how much downstream cyclogenesis will occur on the lee of the northern Rockies and Canadian Rockies. Model spread remains modest regarding the timings and amplitudes of the downstream ridge over the mid-section of the country and near the East Coast. The WPC morning forecast package was based on a composite blend of the 06Z GFS/GEFS, 00Z ECMWF/EC mean, as well as the 00Z CMC/CMC mean. Again, more noticeable descrepancy from yesterday's forecast package was across the northwestern U.S. with the formation of an occluded low pressure system during the weekend on the lee of the southern Canadian Rockies. ...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 into next 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 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. Meanwhile, a monsoonal convective pattern will continue to affect the Southwest/Four Corners states this week with moist southerly flow west of the upper ridge axis. The WPC Day 4/5 Excessive Rainfall Outlooks for Wednesday/Thursday maintain a Marginal Risk for Arizona and portions of Utah, Southeast California, southern Nevada and western Colorado. Local storms with embedded heavy rain could cause isolated flooding, particularly in vulnerable areas. A Slight Risk may be considered within the Marginal Risk but has been held off due to model differences. Meanwhile, upper trough amplification proximity along/off the West Coast portends a cooler and wetter pattern for the Northwest. Some rains are also forecast to broadly focus downstream across the U.S. northern tier as the week progresses. Some convection may break out along/ahead of relatively slow moving frontal systems across the north-central U.S. as the week progresses, but the aforementioned forecast plan downplays faster/organized ECMWF/UKMET potential. Upper ridging over the south-central U.S. will support 5-15 degree above normal temperatures. Highs well into the 100s in much of Texas 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 to creep into the West/Northwest and over the East to include some record values. Kong/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