Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Mon Jul 04 2022 Valid 12Z Thu Jul 07 2022 - 12Z Mon Jul 11 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 the upcoming weekend 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. The multiple days of high heat with little nighttime temperature relief will increase the potential impacts to sensitive and more vulnerable groups. Elsewhere across the CONUS, around the periphery of this upper ridge, troughing is expected in the northwest and northeast U.S., with periodic rounds of showers and thunderstorms likely moving through the flow. This could produce multiple episodes of stronger storms with the potential of locally heavy rainfall across the Ohio Valley to Mid-Atlantic and Southeast late this week and this weekend. ...Guidance Evaluation/Predictability Assessment... The latest deterministic and ensemble guidance remains in good agreement for the large scale setup and evolution from later this week through early next week across the CONUS. The upper ridge strengthens/builds and reasserts itself over the Southern Plains while troughing initially over the Pacific Northwest eventually works its way to dig a stronger trough over the Great Lakes and Northeast by this weekend. The embedded shortwaves and mesoscale details are yet to be resolved but there is growing consensus for a strong frontal passage and shortwave trough across the Ohio Valley and Mid-Atlantic Friday into Saturday. Some of the large model differences were with the approach and passage of shortwave energy into the Pacific Northwest Friday into Friday night. Here, the GFS was a faster outlier compared to the rest of the guidance. For the forecast blend, the days 3-5 were composed of the latest deterministic guidance while for days 6-7, higher weights of the GEFS and ECENS means were used for the typical model spread and uncertainty. ...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 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 within a broad area from the northern Plains/Midwest/Upper Great Lakes into the Ohio Valley and Appalachians to the East Coast. Areas of excessive rainfall/flash flooding are possible through the weekend, but considerable uncertainty with the placement of heavy rain amounts precludes delineating Slight Risk areas in the experimental day 4-5 Excessive Rainfall Outlook at this point. Scattered thunderstorms are also likely in the Southeast through the week, and the current forecast shows the eastern part of the main front sinking southward from meandering over the Mid-Atlantic into the Carolinas and eventually the Gulf Coast by next weekend, leading to more focused rainfall in the Southeast and clearing for the Northeast behind the front. At the same time a frontal system emerging into the northern Plains next weekend may provide added convective focus over that region once again. Meanwhile 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 upper 70s to low 80s for some areas) will increase the potential impacts from this prolonged heatwave to the more sensitive and vulnerable populations. Meanwhile, the West Coast could see cooler than normal conditions late week before moderating closer to average, 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. Tate/Taylor 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