Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Fri Aug 9 2024 Valid 12Z Mon Aug 12 2024 - 12Z Fri Aug 16 2024 ...Excessive heat lingers over portions of Texas and the central Gulf Coast states into much of next week... ...Overview... It is generally still the case that a rather stagnant summertime upper pattern will be in place across the lower 48 next week, composed of a central U.S. mean ridge and mean troughing in the western and eastern U.S. on either side. The south-central U.S. can expect high heat indices to continue underneath this upper ridge, but temperatures for many other areas of the country elsewhere should generally be near to below normal. Monsoonal moisture will maintain rain chances across the Four Corners states, while some moisture makes it farther north into the northern Plains to Midwest, and some advects east around the ridge for rounds of mesoscale convective systems that could focus particularly in the Mid-Mississippi Valley next week. The NHC is also indicating possible new tropical system development out from the tropical Atlantic and Antilles that subsequently offers an increasing signal off the Southeast U.S. and Bahamas in about a week to monitor. ...Guidance/Predictability Assessment... Latest model guidance still shows reasonably good agreement with the overall pattern, with some typical differences in shortwaves that track through the mean flow. Predictability seems slightly above normal in this period. One significant model difference was evident with the 06 UTC GFS by the latter half of next week. It moves western U.S. origin troughing farther inland than most 00 UTC guidance, even into the Plains by next Friday, unlike other guidance that maintains ridging there as troughing stays west. The 12 UTC GFS/ECMWF are now more in line with the 06 UTC GFS, but the 12 UTC Canadian has not trended in that direction. Quite uncertain. Accordingly, the WPC forecast used a deterministic model blend Monday into Wednesday with the GFS/ECMWF/Canadian/UKMET. Opted to switch to a composite of best compatible GEFS and ECMWF ensemble means Days 6-7 amid noted growing forecast spread and uncertainty. ...Weather/Hazards Highlights... Above normal moisture will be directed into the Four Corners states and north and east next week. Moderate to locally heavy rain looks to be most likely on Monday in areas like the central Rockies and central High Plains. Broad Marginal Risks are in place across much of the Four Corners states and into southern Wyoming for Monday and Tuesday. Scattered convection in the southwestern U.S. into the Rockies should continue into later week. Farther east, moisture rounding the upper ridge will spread into parts of the central U.S., and convection likely in the form of multiple mesoscale convective systems are forecast to move generally west to east. Marginal Risks stretch across central parts of the Plains into the Middle Mississippi Valley to cover the flash flooding threat with these storms. Convective storms like these "ridge riders" are notoriously difficult to predict in terms of exact placement, but given the potential for high rain rates in an unstable environment, Slight Risks in future cycles are certainly a possibility. The Middle Mississippi Valley may be of particular concern into Tuesday-Wednesday if storms repeat over similar areas as the forecast currently suggests. By Thursday there is a general trend toward convection moving east into the Ohio and Tennessee Valleys to southern/central Appalachians. To the north, a couple of shortwaves and a frontal system or two should help to focus rounds of rain across the northern Plains and Upper Midwest next week. Meanwhile, a wavy front stalling from the central Gulf Coast into Georgia and the Carolinas could help spark/focus scattered to widespread rain into Monday-Tuesday. In these areas, recent excessive rainfall from Debby has resulted in fully saturated soils that may only partially recover by then. Marginal Risks are in effect in this part of the Southeast for Days 4-5. The upper ridge axis over the south-central U.S. will maintain temperatures up to 10F above (already hot) normal values across the southern Plains into the central Gulf Coast. Combined with the high dewpoints in most areas, heat indices above 110F are possible, and experimental HeatRisk shows Major to Extreme impacts. North of this region though, most areas should see near to below normal highs from the western to north-central to eastern U.S., but lows could be a bit above average especially for the Intermountain West. Tate/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