Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sat Aug 22 2020 Valid 12Z Tue Aug 25 2020 - 12Z Sat Aug 29 2020 ...Two tropical systems (Marco and Laura) are forecast to affect the southern/southeastern U.S. next week... ...Heat continues for the Desert Southwest while much above normal temperatures spread across much of the northern U.S. through Thursday... ...Overview and Guidance/Predictability Assessment... Mid/upper ridging is forecast to generally persist across the southern tier of the U.S. toward the western Atlantic, but get disrupted by tropical systems Marco and Laura during the medium range period. Meanwhile, the jet along the northern tier has some smaller-scale, less predictable shortwaves embedded, but the general longwave pattern does not differ much between model guidance. It appears energy from Laura could get pulled into the northern stream by Fri/Sat as it gets suppressed south by a shortwave anyway. For mass fields, a blend of the 00Z and 06Z GFS operational runs, the 06Z GEFS mean, and the 00Z ECMWF and its mean were used, with increasing weight on the means as the days progressed, with National Hurricane Center forecasts used for the tropical systems. ...Weather/Hazard Highlights... Currently, Tropical Storm Marco is expected to move through the Gulf of Mexico during the short range period and move into the western or central Gulf Coast around Tuesday. Rainfall totals may not be huge with Marco due to weakening of the system. Then Tropical Storm Laura, currently moving near Puerto Rico, is forecast to enter the Gulf of Mexico and potentially reach hurricane intensity before approaching the Gulf Coast by Wed/Thu. With a trough and front approaching from the north interacting with the moisture from Laura, rainfall totals could be heavy and cause flooding concerns from the Lower Mississippi Valley and farther north and east. However, uncertainty remains high with the track and intensity of these tropical systems, affecting where rainfall and wind impacts will be. Please see the National Hurricane Center for additional and updated information. The upper high persisting in the Four Corners region will lead to continued heat across the Desert Southwest in particular, with rounds of above normal temperatures of 5-10 degrees for the remainder of the West as well. The influence of the ridge should also spread above normal temperatures of 10 to 20 degrees across northern and central parts of the Plains into the Midwest on Tue-Thu before moderating back near normal. 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, and rain could then spread into the Great Lakes and Northeast. Tate 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