Extended Forecast Discussion NWS Weather Prediction Center College Park MD 318 PM EDT Fri Jul 08 2022 Valid 12Z Mon Jul 11 2022 - 12Z Fri Jul 15 2022 ...A prolonged heat wave will last through early next week across the southern Plains and Mid-South... ...Overview... It remains the case that a persistent and strong upper level high drifting west from the southern High Plains and then meandering atop the Four Corners states next week will keep very hot temperatures in place across much of the central U.S. particularly through around Tuesday. Combined with the humidity, oppressive heat indices are likely in places and with very little nighttime relief in temperatures, a prolonged heat wave is expected to continue across the southern half of the Plains and Mississippi Valley through midweek before heat overspreads the northern Plains by late week. On the periphery of the upper ridge, troughing is forecast over the Northeast this weekend while another trough across the Northwest Sunday will track quickly across the north-central U.S. and redevelop troughing centered in the northeast quadrant of the lower 48 by Tuesday-Friday. Rain and thunderstorms are forecast to be heaviest near the Southeast U.S. coast as a cold front stalls. The stalled front is forecast to interact with an upper low from the sub-tropics and may result in heavy rain potential near the central Gulf Coast midweek. The northern Plains to Upper Midwest should also see chances of rain and potentially severe thunderstorms ahead of another cold front, and some showers and storms could shift southeastward with the front stretching from the Northeast back into the central/southern Plains Tuesday-later next week. Meanwhile, a persistent monsoonal moisture flow into the Southwest offers multi-day chances for periods of locally enhanced rainfall. ...Guidance Evaluation/Predictability Assessment... The most uncertain aspect in the medium-range period is near the central Gulf Coast where an upper low moving northwest across the Gulf of Mexico is forecast to interact with a stalled front and could result in heavy rainfall in the vicinity. The ECMWF and EC mean have been placing the heaviest rains farther east toward MS/AL coasts and western Florida Panhandle than the GFS and GEFS. The CMC also indicates such interaction but with less QPF. The ECMWF might introduce too much convective feedback on this system so that a tropical cyclone was forecast to race up the East Coast in yesterday's 12 UTC run, and with a similar tendency in subsequent runs. A blend of the GFS/GEFS with ECMWF/EC mean together with the ensemble bias-corrected QPF was used, which tends to keep a heavy rain threat near the central Gulf Coast region midweek. The WPC medium range product suite was derived from a composite blend of the reasonably well-clustered model guidance with 40% from the 06 UTC GFS/GEFS, 40% from the 00Z UTC ECMWF/EC mean, and 20% from the 00Z CMC/CMC mean, using more of the ensemble means with increasing forecast lead time. This forecast strategy maintains good WPC product continuity in a pattern with overall above normal predictability. ...Sensible Weather and Hazards... As the eastern part of a cold front continues to drop southward and then stall early next week in the Southeast/central Gulf Coast/Florida, it will provide a focus for anomalously high moisture to pool and lead to rain and thunderstorms chances continuing from the short range period. While rain totals are expected to gradually drop as the front slowly weakens and dissipates, localized areas of flash flooding cannot be ruled out especially if multiple rounds of heavy rain affect the same areas. Farther north, shortwave energy and a frontal system or two are forecast to lead to rain and thunderstorms across the northern Plains and Upper Midwest into Monday. Although moisture and instability levels will be sufficient, the possibly quicker storm motions and the uncertainty in the details of the shortwaves/frontal placement preclude Slight Risk issuance in the WPC experimental Excessive Rainfall Outlook at this point. Rainfall activity should shift toward the Great Lakes region and then the Northeast Monday-Wednesday, with scattered storms also possible along the trailing cold front in the Ohio Valley and even back into central and then southern parts of the Plains and Mississippi Valley. Elsewhere, monsoonal moisture could persist into next week with moist southwesterly flow/embedded shortwaves leading to mainly diurnally driven showers and storms over the Four Corners states. The West Coast should be the most consistently dry area through the period. An upper level weakness near the central Gulf Coast interacting with the stalled front and an upper low moving across the Gulf of Mexico could bring some locally heavy rainfall to coastal locations from early to middle of next week. This system could focus slow-moving/repeat cells with very heavy downpours leading to local runoff risk. A slight risk of excessive rainfall has been introduced near the Louisiana coast on Day 4. Very hot weather is expected to persist across the southern half of the Plains/Mississippi Valley underneath the strong upper ridge/high early next week, before finally some slight moderation around next midweek. 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 the southern Plains and Lower Mississippi Valley through early next week. 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. Several daily record high temperatures are possible, particularly for warm lows but a few highs may be close to records in Texas. As the upper ridge drifts westward, the West should see temperatures warming from slightly below normal to above normal (by 10+ degrees in the Northwest) as next week progresses. The East can expect slightly cooler than normal temperatures early in the week behind the cold front, before warming to near/just above normal. By late week, heat is forecast to develop and overspread the northern Plains as an upper ridge exits the northern Rockies ahead of an upper trough advancing into southwestern Canada. Kong/Schichtel, et al. 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 Hazards: - Heavy rain across portions of the central Gulf Coast, Mon-Thu, Jul 11-Jul 14. - Heavy rain across portions of the Southeast, Mon, Jul 11. - Heavy rain across portions of the Southeast and the southern Mid-Atlantic, Thu-Fri, Jul 14-Jul 15. - Flooding possible across portions of the Middle Mississippi Valley and the Upper Mississippi Valley. - Flooding occurring or imminent across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley. - Excessive heat across portions of the Central and Southern Plains, Mon, Jul 11. - Excessive heat across portions of the lower and middle Mississippi Valley and the Southern Plains, Mon-Wed, Jul 11-Jul 13. - Excessive heat across portions of the Southern Plains, Mon-Thu, Jul 11-Jul 14. - Excessive heat across portions of the Central and Northern Plains, Thu-Fri, Jul 14-Jul 15. 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