Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Aug 17 2024 Valid 12Z Tue Aug 20 2024 - 12Z Sat Aug 24 2024 ...Hazardous heat to persist from the southern Plains to the west-central Gulf Coast next week... ...Overview... An amplified and blocky upper pattern is forecast to prevail across North America through next week. This will be highlighted by a building upper high centered over the southern High Plains that will promote hazardous heat from the southern Plains to the central Gulf Coast. Upstream, amplified upper troughing is forecast to meander near/off the West Coast. Downstream, an amplified mean upper trough will slowly lift and exit the eastern U.S. ...Guidance/Predictability Assessment... Forecast spread and uncertainty in guidance continues to hold at below normal thresholds through medium range time scales for next week over much of our fine nation. Models have generally trended toward a less-amplified and more progressive solution regarding the deep upper low near/off the West Coast/Pacific Northwest, especially with the ECMWF. The ensemble means show rather good agreement that the mean trough axis will slowly come onshore into the West Coast by next weekend. The more progressive solution has resulted in a faster southward push of a cool air mass down western Canada later next week, while a southern-stream occlusion is forecast to develop in Alberta, Canada by next Saturday. The more progressive pattern also introduced some timing and location differences for a warm frontal wave and QPF to develop over the northern Plains late next week. Agreement among models remains very good regarding the timing of the low pressure system exiting the Northeast early next week, followed by a large dome of cool high pressure system building into the region and then farther south into the Southeast and the Deep South. Models generally agree that a large pool of tropical moisture will be confined in the Gulf of Mexico and Florida to the south of a stationary front, although there appears some tendency for the models to place the heavy rain closer to or near the Gulf Coast in the morning model cycle.for late next week into the weekend. Therefore, a general model and ensemble mean composite blend appears to provide a solid deterministic forecast base and ranges of solutions with the potential weather hazards, albeit with typically less predictable smaller scale warm season features and local weather focus to better address at closer time scales. ...Weather/Hazards Highlights... An east-central to eastern U.S. upper trough offers some lingering unsettled conditions, but rainfall during much of the earlier period is expected to taper off by midweek as surface high pressure builds in behind the cold front(s) 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. 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 the ridge" aloft, but with generally low confidence in the predicted placement. Meanwhile, monsoonal convection will affect the Southwest/Four Corners states next week with moist southerly flow west of the upper ridge axis. The WPC Excessive Rainfall Outlooks have Marginal Risks maintained across the interior Southwest into the Four Corners for next Wednesday, but the Marginal Risk for Tuesday had been trimmed to include only portions of Arizona, Utah, Southeast California into southern Nevada. Storms with embedded heavy rain could cause isolated flooding, particularly in vulnerable areas. 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 next week progresses, but focus is uncertain. Upper ridging over the south-central U.S. will maintain above normal temperatures by 5-15 degrees from the High Plains east into the west-central Gulf Coast. 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 wave. 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