Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 PM EDT Sat Aug 10 2024 Valid 12Z Tue Aug 13 2024 - 12Z Sat Aug 17 2024 ...Excessive heat lingers over portions of Texas and the central Gulf Coast/Southeast into much of next week... ...Overview... Upper ridging will be in place across the south-central U.S. through much of next week, leading to high heat indices that could be hazardous across the southern Plains to Gulf Coast/Southeast. Monsoonal moisture will maintain rain chances across the Four Corners states and adjacent Plains, while some moisture makes it farther north into the northern Plains to Midwest, and some advects east around the ridge in an unstable airmass for rounds of mesoscale convective systems that could focus particularly in the Mid- Mississippi Valley into the Ohio Valley next week. Mean troughing is likely to set up across the West Coast and the East, while an upper ridge starts to build farther west into the Four Corners states/Rockies late week. Meanwhile, a tropical system that is likely to develop per NHC could work from the Antilles to off the Bahamas in about a week, which will continue to be monitored, though fortunately most models and latest NHC guidance takes the low system away from the lower 48 at this point. ...Guidance/Predictability Assessment... Latest models and ensembles offer a reasonablty similar larger scale summer pattern evolution through medium range time scales. However, guidance continues to offer less than stellar forecast spread and cycle to cycle continuity variances with flow embedded small-mid scale systems and associated local weather/convection focus. A composite model and ensemble solution along with the National Blend of Models seems to offer a reasonable forecast basis with detail consistent with individual system predictability. The tropical cyclone that is forecast to develop in the tropical Atlantic could reach WPC's forecast domain by next weekend. The bulk of model and ensemble guidance indicates the system turning north near the Bahamas and then northeastward enough to take it out to sea rather than into the U.S., but will continue to monitor as the track will depend in part on a possible trough axis across the eastern U.S./western Atlantic. ...Weather/Hazards Highlights... It generally remains the case that above normal moisture will be directed into the Four Corners states and north and east especially into Tuesday. A broad Marginal Risk remains in place for Day 4/Tuesday for much of the Four Corners states and into the central High Plains. Coverage and amounts of rain may tick down a bit into Wednesday, but likely still sufficient for a Marginal Risk from eastern Arizona into New Mexico and Colorado for the Day 5 ERO. Some scattered convection in the southwestern U.S. into the Rockies should continue into later week, eventually focusing westward a bit (more into the Desert Southwest) as the upper ridge retrogrades. Farther east, moisture rounding the upper ridge will spread into parts of the central U.S., and thunderstorms likely in the form of multiple mesoscale convective systems are forecast to move generally west to east or northwest to southeast, with shortwave energies north of the ridge providing forcing. Marginal Risks stretch across central parts of the Plains into the Middle Mississippi Valley (and Tennessee Valley by Wednesday) 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 and latest guidance has trended slightly more in favor of this. By Thursday- Friday there is a general trend toward convection moving east into the Ohio and Tennessee Valleys to the 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. Model differences abound with timing and amounts of rain there in the north-central U.S., but did delineate a Marginal Risk for Day 5/Wednesday across the eastern Dakotas into Minnesota where wet antecedent conditions remain from heavy rains earlier this summer. Meanwhile, a wavy front stalling from the central Gulf Coast into Georgia and the Carolinas could help spark/focus scattered to widespread rain lasting into Tuesday. In these areas, recent excessive rainfall from Debby has resulted in fully saturated soils that may only partially recover by then. A Marginal Risk is in effect in this part of the Southeast for Day 4/Tuesday. The current forecast shows less potential for rain into Wednesday as the front pushes south, but rain and thunderstorm chances may rise once again by later week for this area. 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 slightly 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 to Midwest. 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