Extended Forecast Discussion NWS Weather Prediction Center College Park MD 115 AM EDT Sat Aug 22 2020 Valid 12Z Tue Aug 25 2020 - 12Z Sat Aug 29 2020 ...The heat wave continues in the Southwest and portions of the northern U.S. through the middle of next week... ...Two tropical systems forecast to affect the southern/southeastern U.S. next week... ...Overview and Weather/Hazard Highlights... A broad upper-level ridge axis is expected to persist across the CONUS southern tier during the extended forecast period. Two relatively persistent positive 500 hPa height anomaly centers are evident in ensemble means next week, one across the southwestern states, and another across the western Atlantic Ocean, between Bermuda and the Bahamas. The National Hurricane Center is currently issuing advisories on two tropical systems forecast to potentially affect the U.S. next week. Tropical Storm Laura is east-southeast of Puerto Rico, and is forecast to enter the central Gulf of Mexico early next week, potentially strengthening to hurricane intensity. Tropical Storm Marco, currently over the northwestern Caribbean, is forecast to skirt the Yucatan Peninsula this weekend before moving northwest across the central/western Gulf of Mexico, eventually reaching the western Gulf Coast. Refer to advisories issued by the National Hurricane Center for the latest information on these systems. The combination of these two systems, and their associated tropical moisture will bring the potential for heavy rains next week from Florida to Texas, with the potential for heavy rain spreading inland over time by the middle/end of the week across the Southeast and lower Mississippi Valley. Considerable uncertainty remains with these systems, however. Any other potential impacts from these systems will become more clear over the next few days as confidence in the forecast tracks increases. The aforementioned positive height anomalies will serve to help steer the tropical systems, with T.S. Marco heading into a weakness across the south-central U.S., and Laura initially taking a more westerly course south of the West Atlantic ridge center before potentially turning north over the Gulf of Mexico. The western ridge will continue to support above average temperatures across much of the western U.S., also extending into the north central U.S. early next week. Max temperatures across the Southwest are forecast to continue soaring well past the 100 degree mark. While temperatures will not be quite as hot as seen in recent days, isolated record max temps will still be possible next week. Additionally, max temps are forecast to be 10 to nearly 20 deg F above average across much of the north-central U.S. Tue-Wed, with a few locations in the north-central Plains forecast to surpass 100 degrees, and a couple record max temperatures possible. Scattered showers and storms will be possible across the central/northern Rockies in association with a lingering frontal boundary, with convective activity gradually migrating eastward toward the central/northern High Plains later in the week. Farther east, the Great Lakes and Northeast may become convectively active by Thu-Fri as a shortwave and frontal system cross those regions. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF/18Z GFS served as a basis for the WPC forecast during days 3-4 (Tue-Wed). After that, emphasis was increased on the ECENS/GEFS ensemble means. Forecast confidence (outside of the tropical systems) was average, with guidance struggling on the timing/amplitude of relatively small-scale features traversing the jet across Canada and the CONUS northern tier. Ensemble consensus at the larger scales remained relatively good through the extended forecast period, however. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml