Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM 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 states into much of next week... ...Overview... An upper ridge 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. 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. 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 late week. Meanwhile, a tropical system that is likely to develop per NHC could approach the Bahamas in about a week, which will continue to be monitored, though fortunately most models take the low system away from the lower 48 at this point. ...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. Unfortunately these differences that are on the smaller side do have impacts on sensible weather like rainfall and frontal placement. Through the 12Z/18Z model cycle that was available for tonight's forecast, the ECMWF, GFS, and AIFS formed a cluster of solutions that were relatively agreeable with a shortwave tracking across the north-central U.S. Thursday into the Midwest Friday, along with a weak surface low/frontal system. This trough may dig a bit in the east-central U.S. into Saturday. While uncertain, did opt to lean toward this favorable cluster of guidance, and fortunately incoming 00Z guidance seems to show this same general idea. By late week, deepening troughing in the eastern Pacific may edge toward the West Coast, and ridging looks to build farther west in between these trough features. A model/mean blend seems reasonable with that. 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. The WPC forecast used a multi-model deterministic blend heavily favoring the 12Z ECMWF and 12/18Z GFS runs for Days 3-4, incorporating the EC and GEFS ensemble means by Day 5 and increasing the means to half the blend by Day 7 amid growing forecast spread and uncertainty. ...Weather/Hazards Highlights... 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 southern Wyoming. 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. By Thursday- Friday 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. 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 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