Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Wed Jul 06 2022 Valid 12Z Sat Jul 09 2022 - 12Z Wed Jul 13 2022 ...A prolonged heat wave will last through early next week across the southern Plains and Mid-South... ...Overview... A persistent and strong upper level high centered over the southern U.S. through this week drifting into the Four Corners region 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 heat wave is expected across the southern half of the Plains and Mississippi Valley. On the periphery of the upper ridge, troughing is forecast over the Northeast this weekend while another trough across the West Coast Saturday will quickly track across the north-central U.S. and redevelop troughing centered in the northeast quadrant of the lower 48 by Tuesday-Wednesday. Rounds of storms are expected to push southward ahead of a cold front across the southeastern U.S. through the weekend, with flash flooding possible. The northern Plains to Upper Midwest should also see rain and thunderstorm chances, while some monsoonal moisture continues to flow into the Southwest. ...Guidance Evaluation/Predictability Assessment... Recent deterministic and ensemble guidance remains in good agreement for the large scale setup and evolution for this weekend 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. Then while models agree that the West Coast troughing Saturday should quickly lift and deamplify somewhat as it tracks near the U.S./Canada border, there are some differences in energy evolution with impulses stemming from the original trough but also from Canada, which are all forecast to combine somehow to amplify troughing with its axis around the Great Lakes Tuesday-Wednesday. At least guidance has agreement on the large scale for this amplifying trough and surface low development in south-central Canada, but the details will impact frontal positions and placement of rainfall. The updated WPC forecast has trended a little faster with the low pressure/frontal system and the rainfall forecast in the north-central U.S. Despite the mesoscale differences, given the synoptic scale agreement, the WPC forecast blend was able to use the deterministic guidance early in the period and maintained a majority deterministic blend through the whole period, trading the UKMET for the EC ensemble mean days 6-7. ...Sensible Weather and Hazards... As the eastern part of a cold front finally begins to drop southward this weekend along the East Coast, it will provide a focus for anomalously high moisture to pool in the Southeast and lead to heavy rainfall and potentially flash flooding this weekend. A Slight Risk was added to the experimental day 4 Excessive Rainfall Outlook over coastal North Carolina and portions of South Carolina where rainfall and rain rates could be the heaviest. As the front gradually sinks south and dissipates, the best rain chances will shift toward the Gulf Coast and Florida, while clearing is expected for the Northeast and Mid-Atlantic behind the front. 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 this weekend. While moisture is high there, the possibly quicker storm motions and the uncertainty in the details of the shortwaves/frontal placement preclude any Slight Risk issuances there. Rainfall activity should shift toward the Great Lakes region and then the Northeast Monday-Tuesday, with scattered storms also possible along the trailing cold front in the Ohio Valley and even possibly back into central parts of the Plains and Mississippi Valley. Elsewhere, monsoonal moisture could persist even into next week with moist southwesterly flow/embedded shortwaves leading to mainly diurnally driven showers and storms over the Four Corners states. Thus the West Coast should be the most consistently dry area through 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 the southern Plains and Lower Mississippi Valley through at least early next week, while the Southeast may also experience this heat Saturday, though short-lived there ahead of the cold front bringing in slightly below normal temperatures. 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. The northern/central High Plains can also expect warmer than normal temperatures over the weekend but approaching closer to normal as the workweek begins. 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 the first half of next week progresses. Tate 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