Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Aug 21 2024 Valid 12Z Sat Aug 24 2024 - 12Z Wed Aug 28 2024 ...Hazardous heat focus over the southern Plains... ...Overview... A persistent and amplified upper pattern over our nation remains highlighted by a ridge expanding from the southern Plains, cooling troughing over the Northeast/East, and a trough/low ejecting inland in a wet pattern over the Northwest then north-central U.S. this weekend into next week. This pattern offers a hazardous heat focus over Texas, moist monsoonal flow lingeirng 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 espcially across South Florida. ...Guidance/Predictability Assessment... In a near repeat of trends inherent to the last few days worth of model and ensemble forecasts, differences linger with the handling of deep eastern Pacific upper trough/low energy inland across the West/Northwest this weekend. Recent 06/18 UTC GFS runs and to some extent 06/18 UTC GEFS runs have been prone to favor a solution on the more progressive side of guidance, while most other guidance has been on the slower side of the full envelope of solutions with recent runs, including many 00/12 UTC GFS/GEFS runs. The manually overseen forecast from here at WPC has tried to be consistent in favoring the less progressive pattern camp. Today is no exception. In this vein, a composite of best clustered guidance from the 12 UTC ECMWF/Canadian and ECMWF ensemble mean was primarily used to derive the WPC medium range product suite from this weekend into next midweek in conjucture with the 01 UTC National Blend of Models and WPC product continuity. ...Weather/Hazards Highlights... Upper trough amplification proximity along/off the West Coast portends a cooler and wetter pattern for the Northwest. 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. Meanwhile, a monsoonal convective pattern will linger but gradually ease over the Southwest/Four Corners 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. The WPC Day 4/5 Excessive Rainfall Outlooks for Saturday/Sunday show Marginal Risk areas. Local storms with embedded heavy rain may cause flash flooding, particularly in vulnerable areas. 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 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.. Elsewhere, 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 Day4/5 weekend ERO Mariginal Risk area was introduced for South Florida given moisture. 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