Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Sat Jun 10 2023 Valid 12Z Tue Jun 13 2023 - 12Z Sat Jun 17 2023 ...Heavy rain and strong storms expected from the Central Plains to the Mid-South, and summer heat builds from Texas to the Lower Mississippi Valley... ...Synoptic Overview... A well-defined closed upper low is forecast to be in place across the Great Lakes Tuesday as a shortwave drops south from the Hudson Bay region. This will spur surface cyclogenesis across the Ohio Valley early in the week with a trailing cold front exiting the East Coast Tuesday, and this will likely prolong a period of widespread showers and thunderstorms across the Northeast. A second closed upper low across the southwestern U.S. early in the week is expected to evolve into a trough and energies should reach the central/southern Plains by Wednesday, and that will likely support a wavy front. Slow system translation and return moisture/instability offers a threat for widespread showers and strong to severe storms from the southern Plains eastward across the Mid-South/South through the southern Appalachians for the remainder of the week with episodes of heavy rainfall possible, with building heat and humidity to the south of the frontal zone/convection from Texas to the Lower Mississippi Valley. ...Guidance Evaluation/Predictability Assessment... The WPC medium range guidance suite was primarily derived from a composite blend of reasonably well clustered guidance from the 06 UTC GFS and 00 UTC ECMWF/UKMET and supportive GEFS/ECMWF ensemble mean guidance along with 13 UTC National Blend of Models valid for early to mid next week in an overall pattern with seemingly above normal predictability. The 00 UTC Canadian alternately developed a more amplified upper trough/weather focus solution than most other guidance over the West mid-later next week versus allowing height falls to instead work more eastward over the Rockies into the High Plains. However, the 12 UTC Canadian has trended strongly toward the preferred composite solution that remains supported by latest 12 UTC guidance, bolstering forecast confidence. ...Weather/Hazards Highlights... The closed low over the Great Lakes and trailing surface cold front crossing the East Coast will advect copious moisture northward across the Northeast, and in combination with ample instability will likely produce storms with locally heavy rainfall, with a Marginal Risk of excessive rainfall in effect for Tuesday across eastern New England. However, the main focus for heavy rainfall is expected from the Central Plains to the Tennessee River Valley and the southern Appalachians for the Wednesday through Friday time period, where an axis of 1-3 inch rainfall totals with locally higher amounts is likely given potential for slow motions and training. Moisture advection from the Gulf of Mexico will intersect a quasi-stationary/warm frontal boundary and shortwave energy aloft will also aid in ascent. A few mesoscale convective complexes are likely to develop near this boundary and result in higher rainfall rates that could result in instances of flash flooding, and thus a Slight Risk of excessive rainfall is planned for both Day 4 and 5 from eastern Oklahoma to northern Georgia where deterministic and ensemble guidance along with the NBM continue to indicate a more concentrated signal for higher QPF. In terms of temperatures, the main story that will make headlines will be the building heat and humidity across much of Texas and into southern portions of the Deep South states, with the hottest weather of the season expected thus far. Widespread mid-upper 90s are likely from northern Florida to Louisiana, and 100-110 degrees for much of central and southern Texas away from the coastal areas. Some heat indices on the order of 110-120 degrees are possible across portions of Deep South Texas for the middle to end of the week with dewpoints well into the 70s. This heat wave will likely have some staying power beyond next Saturday based on the latest forecasts from the Climate Prediction Center. Elsewhere, expect pleasantly cooler conditions compared to average across much of the Southwest to the Central Rockies, and also for much of the eastern U.S. for the middle of the week, and then a gradual warming trend going into Thursday and beyond. Schichtel/Hamrick 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