Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Jun 11 2023 Valid 12Z Wed Jun 14 2023 - 12Z Sun Jun 18 2023 ...Heavy rain and strong thunderstorm threat from the Central Plains/Mid-South to the Southeast overtop a building heatwave dome from Texas to the Lower Mississippi Valley this week... ...Synoptic Overview... The ongoing blocky pattern over the nation has proven slow to dislodge, but will transition this week. This is expected as upper trough energies split/exit the Southwest and with the gradual ejection of a New England closed low position as upper troughing works from western Canada and the northwestern U.S. to mid-continent, leading to several lead rounds of showers and thuderstorms across that broad area. A warming upper ridge will build over Texas and the Gulf Coast region. A wavy frontal drape overtop the ridge from the south-central U.S. to the Southeast along with ejecting southern stream upper level shortwave disturbances are likely to result in multiple rounds of heavy showers and thunderstorms from the Central Plains to the Southeast states as an excessive heat threat takes hold south of the front from Texas to the central Gulf Coast. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered guidance from the 06 UTC GFS and 00 UTC Canadian/UKMET and supportive GEFS/ECMWF ensemble mean guidance along with 13 UTC National Blend of Models for the coming week in an overall pattern with seemingly above normal to average predictability. The 00 UTC ECMWF seems an outlier progressive run from the northeast Pacific to mid-continent compared to prior ECMWF runs and the aforementioned composite blend, but the latest 12 UTC run has trended a bit back into the fold. ...Weather/Hazards Highlights... The main focus for heavy rainfall is expected across portions of the South/Southeast mid-later week, where an axis of 1-2+ inch rainfall totals with locally higher amounts is likely given potential for slow cell motions and convective training. There has been a recent trend over the past 24 hours for the axis of heaviest rainfall to be more towards the south compared to earlier forecasts, and also a slight reduction in the magnitude of the expected rainfall. 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 some instances of flash flooding, and thus a Slight Risk of excessive rainfall is planned for the new Day 4 ERO from northern Alabama to central South Carolina where deterministic and ensemble guidance, along with the NBM, has the most concentrated signal for higher QPF. Elsewhere, some showers are likely from the Great Lakes to the Northeast as moisture pivots around the upper level low. For the Day 5 period, patchy areas of heavy rain are possible across portions of the central and northern Plains, but the lack of model agreement precludes anything higher than a Marginal Risk at this time. In terms of temperatures, the main story that will make headlines will be the persistent 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 especially 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 slightly 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