Extended Forecast Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Tue Jul 05 2022 Valid 12Z Fri Jul 08 2022 - 12Z Tue Jul 12 2022 ...Prolonged Heatwave This Week and Upcoming Weekend Across Southern Plains to Mid South... ...Overview... A persistent and strong upper level high centered over the southern U.S. through this week and into early next week will keep very hot temperatures in place across much of the central CONUS. Combined with the humidity, heat indices are likely to exceed 105-110F in places and with very little nighttime relief in temperatures, a prolonged heatwave is expected across the southern half of the Plains and Mississippi Valley. On the periphery of the upper ridge, troughing initially across the eastern Pacific Friday will shift eastward across the northwestern to north-central U.S. over the weekend into early next week, while troughing over the Northeast through the weekend may shift away around Monday. Multiple impulses moving through the flow north of the ridge should produce multiple episodes of potentially strong storms and locally heavy rainfall across the Ohio/Tennessee Valleys and Appalachians to the East Coast, as well as across the northern Plains to Upper Midwest. ...Guidance Evaluation/Predictability Assessment... Recent deterministic and ensemble guidance remains in good agreement for the large scale setup and evolution for late week as the center of the upper high drifts westward while the ridge axis strengthens over the High Plains. Models for the most part show deepening troughing across the East Coast into the weekend which should help push the cold front southward. Out West, a couple of shortwaves should slide inland, the first on Friday and then another this weekend which is forecast to essentially ride the southern U.S. ridge across the far northern tier and possibly amplify some as it moves into the Great Lakes early next week. There are some timing questions with this second wave, which also affects how quickly the initial trough over the East is able to move out ahead of it. The CMC is a little faster with the wave as it comes into the Upper Great Lakes next Monday-Tuesday so it was excluded from the blend after day 5. WPC used a blend of the deterministic guidance for days 3-5, amidst good agreement, transitioning to a 50/50 model/ensemble mean blend by day 7 between the GFS/ECMWF and ECENS/GEFS means. This approach maintained good continuity with the previous WPC forecast as well. ...Sensible Weather and Hazards... Scattered showers and storms are likely across much of the U.S. from the Rockies eastward at times this week into early next week, with enhanced totals in the vicinity of a couple of frontal boundaries initially stretching generally west to east across the northern half of the country. Northwesterly flow aloft and abundant moisture and instability will create a favorable pattern for mesoscale convective systems and high rainfall rates from the Ohio Valley and Appalachians to the East Coast. Areas of excessive rainfall/flash flooding are possible through the weekend, with considerable uncertainty with the exact placement of heavy rain amounts given the fast-moving nature of storms. However, despite this, a slight risk was added to the experimental day 4 Excessive Rainfall Outlook for the central Appalachinas given heavy rainfall sensitivity and enough of a consistent signal in the models for heavier activity in that region. A cold front in the East will finally begin to sink southward over the weekend in response to slightly deepening troughing aloft. This could provide additional support and focus for thunderstorms across the Carolinas, Southeast, and eventually reaching the Gulf Coast, while clearing is expected over the Northeast behind the front. Given the potential for several rounds of heavy rainfall in the short range for parts of eastern North Carolina, a second slight risk was added to the day 4 ERO for that area. Meanwhile, frontal boundaries and upper-level energy could lead to convection across the northern Plains to Upper Midwest through the weekend with some potential for flash flooding. Thunderstorms may push southeastward from those areas by Monday-Tuesday along and ahead of a cold front. Elsewhere, monsoonal moisture could persist through much of the week with moist southwesterly flow/embedded shortwaves leading to mainly diurnally driven showers and storms over the Four Corners states. The southern Plains and California into the Great Basin should be the most consistently dry areas during the period. Very hot weather is expected to persist across the southern half of the Plains/Mississippi Valley underneath the strong upper ridge/high. High temperatures near or exceeding 100F are expected, which is between 5 and 15 degrees above normal. Combined with the humidity, daily maximum heat indices 105-110F+ are likely at times across a large portion of the Mid-Mississippi Valley southward into the southern Plains and Gulf Coast/Southeast. The repeating days of high heat/humidity and lack of any appreciable nighttime relief (lows in the upper 70s to low 80s for some areas) will increase the potential for impacts from this prolonged heatwave to the more sensitive and vulnerable populations. Some daily record high temperatures are possible, particularly for warm lows. Meanwhile, the West Coast could see cooler than normal conditions late week before moderating closer to average and eventually above average early next week, while the Desert Southwest should see near normal temperatures becoming a few degrees above average. Parts of the East may see slightly below normal temperatures spreading south this weekend into Monday behind a cold front. Santorelli/Tate Hazards: - Heavy rain across portions of the Middle Mississippi Valley, the Ohio Valley, the Tennessee Valley, the Central Appalachians, and the Mid-Atlantic, Fri-Sat, Jul 8-Jul 9. - Heavy rain across portions of the Carolinas, Fri-Sun, Jul 8-Jul 10. - Heavy rain across portions of the Northern Plains and Upper Midwest, Sat-Sun, Jul 9-Jul 10. - Heavy rain across portions of the Southeast, Sun, Jul 10. - Flooding occurring or imminent across portions of the Upper Mississippi Valley and the Northern Plains. - Excessive heat across portions of the Central Plains, the Middle Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Fri, Jul 8. - Excessive heat across portions of the Southern Plains, Fri-Mon, Jul 8-Jul 11. - Excessive heat across portions of the Lower Mississippi Valley and Texas, Fri-Tue, Jul 8-Jul 12. 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, experimental excessive rainfall outlook, 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/#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