Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM 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 with the potential of 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 troughing across the East Coast deepening a bit into the weekend and pushing a cold front southward. The trend has been toward a slightly faster front and thus QPF was farther south than the previous issuance around Friday-Saturday. Differences in smaller scale shortwaves remain within the trough, which have low predictability likely even into the short range and will be important for the details of rainfall amounts and placement. Meanwhile, troughing with its axis near the West Coast Friday should shift inland this weekend, but with some differences in the details of timing and energy distribution within the trough as it lifts northeast and then eastward early next week. Specifically, GFS runs have been somewhat faster to press energy east into the northern Plains compared to the ECMWF runs, while the CMC has been more along the lines of the GFS, but becomes flatter than preferred with its flow over the lower 48 by around Monday. Preferred a middle ground solution between the faster GFS and slower ECMWF. Model agreement seems to actually increase slightly day 7/Tuesday given the trough could slightly amplify over the Great Lakes region. For the WPC forecast blend, utilized a multi-model deterministic blend early in the period, with increasing proportions of the GEFS and ECENS means by the later period to about 40 percent to smooth out some model differences but maintaining GFS and ECMWF components to enhance features. ...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 across 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, as well as the likely fast-moving nature of the storms, precludes delineating Slight Risk areas in the experimental day 4-5 Excessive Rainfall Outlook at this point. 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. 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. 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