Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Wed Aug 21 2024 Valid 12Z Sat Aug 24 2024 - 12Z Wed Aug 28 2024 ...Hazardous heat focus over the Southern Plains... ...Overview... An expanding ridge will remain in place over the Southern Plains, cooling trough over the Northeast/East while the Pacific Northwest to northern-central U.S. transitions to a wet pattern as a Pacific trough/low ejects inland. This pattern offers a hazardous heat focus over Texas, moist monsoonal flow lingering into the weekend over the Four Corners states, cooled surface high pressure along the Eastern Seaboard and Tropical moisture lurking near the Gulf Coast and especially across South Florida. ...Guidance/Predictability Assessment... The latest runs of guidance continue to show differences in the timing and depth of an eastern Pacific trough that makes its way inland across the Northwest/West this weekend. As noted in the previous discussion, multiple runs of the GFS had locked on to a more progressive solution and because of that the preferred model blend had been the ECWMF/EC ensemble mean/CMC. The latest runs of CMC remains synced with the ECWMF/EC ensemble through about hour 120 before speeding up more towards the GFS placement. Although the NAEFS mean consists of the CMC, it actually locked in well with the ECWMF and the EC ensemble by favoring a slow progression due to strong ridging. WPC preferred blend for today comprised of ECMWF/Canadian and ECMWF ensemble mean early on before reducing weighting of CMC and ultimately replacing it with the NAEFS. ...Weather/Hazards Highlights... With an amplifying upper trough approaching the West Coast and moving inland much of the Pacific Northwest will trend cooler and wetter. Rains and spotty high elevation mountain snows may work inland across the north-central Intermountain West/Rockies this weekend into early next week dependent on the track/timing of the supporting upper trough/low ejection. There's potential for enhanced rainfall amounts associated with convection. Rains/convection may also consolidate downstream over the north- central U.S. early next week given focus from possible moderate cyclogenesis/frontogenesis. For the Southwest, the monsoonal convective pattern will linger but gradually ease over the Four Corners region into Saturday with moist southerly flow west of a slow to dislodge upper ridge axis centered over the southern Plains. Some moisture may be pulled northward into the northern Intermountain West/Rockies this weekend ahead of Pacific upper trough/low translation over that region. WPC continues to carry a Day 4 Marginal Risk for excessive rainfall. Local storms with embedded heavy rain may cause flash flooding, particularly in vulnerable areas. With the upper ridging in place much of the south-central U.S. will quite warm. Daily temperatures are progged to be 5 to 15 degree above normal for late August. 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 also over the central to eastern U.S. under the influence of dammed surface high pressure. Some record cool overnight temperature values are possible over the East in this pattern into later week, but expect summer heat to then steadily return with spread across the central-eastern U.S.. A trailing front stalling across Florida will maintain periodic rain/thunderstorm chances there and into Southeast U.S. coastal areas later week into next week. Moist flow and pattern longevity may lead to possible flooding issues, especially if heavy rain falls in urban areas. WPC Day4/ ERO highlights a Marginal Risk area for South Florida given moisture. Campbell/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