Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 AM EDT Sat Aug 07 2021 Valid 12Z Tue Aug 10 2021 - 12Z Sat Aug 14 2021 ...Overview... A typical August-like pattern is expected to evolve during the medium range period with building heights aloft across much of the western and southern CONUS. This will bring periods of excessive heat to some as temperatures rise above normal. Frontal systems should be confined to northern parts of the country as an upper trough shifts east from the northern Plains Tuesday and settles across the Great Lakes/Northeast by next weekend. ...Guidance/Predictability Evaluation... Compared to recent days, the guidance is showing better agreement regarding the details of individual shortwaves moving through within the broad northern stream trough. Run to run continuity has improved the last couple of model cycles with some wobble in the exact trough axis, though fairly typical at these time ranges. A general blend of the deterministic solutions provides a good starting point for the forecast. The only other thing to note is some timing differences in troughing as it moves into the Gulf of Alaska or far western Canada. The GFS and CMC are faster while the ECMWF holds the energy farther west. A blend of the GFS and ECMWF along with modest amounts of the more agreeable ensemble means seems to sort this out. This overall approach also provides good continuity with the previous WPC forecast. ...Weather Highlights/Hazards... Upper troughing and surface frontal systems should bring showers and thunderstorms to portions of the Midwest/Upper Great Lakes next week with rain chances moving into the Northeast as well by late week and next weekend. Rain could be heavy at times in some places, but detail issues results in plenty of uncertainty exactly where and how impactful it may be. Across the Eastern U.S. into the Deep South, daily scattered storm chances are likely within a warm and moist environment. Monsoonal moisture is also expected to result in diurnally driven rain and thunderstorms across parts of the Southwest as well through much of the week. Much of the country will be either near or above normal in terms of temperatures through next week. Values 5-10 degrees, locally higher, will shift eastward from the Central Plains/Midwest into the Ohio Valley and the Northeast ahead of the main cold front. Even though the temperature anomalies aren't all that impressive, the combined heat and humidity could bring heat indices into the low 100s for many. With little relief expected overnight, excessive heat is possible most notably in and around Arkansas and also parts of the Mid-Atlantic I-95 urban corridor. Out West, temperatures will rise once again as strong upper ridging moves in. By mid-week, expect daytime highs 10 to 15+ above normal, especially for parts of the interior Northwest where daily high temperature records could be challenged for a handful of locations. Santorelli 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, 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