Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM 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 early summer heat builds from Texas to Alabama*** ...Synoptic Overview... A well-defined 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 morning, and this will likely bring widespread showers and some storms across the Northeast states. A second closed upper low across the southwestern U.S. early in the week is expected to evolve into a trough and this should reach the central/southern Plains by Wednesday, and that will likely support a surface low along the front. The prospects for widespread showers and storms increases from Oklahoma eastward across the Mid-South and the southern Appalachians through the remainder of the week with episodes of heavy rainfall possible, and building heat and humidity south of the frontal zone. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite has consolidated well with the placement of the upper trough across the Southwest and the low across Great Lakes region and the corresponding surface features for the beginning of the week. The GFS remains slightly more amplified with the southern stream shortwave crossing the Gulf Coast region through Thursday, but still has good ensemble support. Across the Northwest, the CMC lingers the building upper trough/closed low compared to the more progressive GFS/ECMWF solutions. There is still strong model support for an upper ridge axis extending northward across Manitoba for the end of the week. The WPC forecast incorporated a nearly deterministic model blend through Wednesday, followed by slightly less weighting of the GFS and CMC, and more of the ensemble means going into Thursday and beyond. ...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. Moisture advection from the Gulf of Mexico will intersect a quasi-stationary 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 the deterministic guidance and NBM 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. 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