Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Jun 11 2023 Valid 12Z Wed Jun 14 2023 - 12Z Sun Jun 18 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 rather blocky upper level pattern appears likely to remain the case going into the middle to end of the week, with a broad upper low anchored in place across southeastern Canada and New England, a trough over western Canada into the northwestern U.S., and a building upper ridge over the Gulf Coast region and into central Texas. A wavy frontal boundary situated over the south-central U.S. along with weak upper level shortwave disturbances is likely to result in multiple rounds of heavy showers and thunderstorms from the Central Plains to the Southeast states, and a heat wave south of the boundary from central Texas to the central Gulf Coast. By next weekend, another cold front is likely to drop south across the Pacific Northwest and northern Rockies with additional rounds of showers and storms. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite initially has a very good overall depiction of the synoptic scale pattern across the continental U.S. on Wednesday, and therefore a multi-deterministic model blend suffices as a starting point in the forecast process. The GFS and to a slightly lesser degree the GEFS mean are more amplified with the shortwave tracking across the Deep South going into Thursday, and this solution has the axis of heaviest rainfall south of the multi-model consensus. More notable differences become apparent with the next shortwave trough building in across the northwestern U.S., with the GFS being considerably stronger and the CMC quite weak with it, so the ECMWF serves as a good middle ground solution going into next weekend across this region. There are also considerable differences regarding the extent of a ridge axis across the central U.S. by next Sunday, and overall forecast confidence is below average by that time with significant forecast changes possible in the days ahead. The ensemble means accounted for about half of the forecast blend by next weekend. ...Weather/Hazards Highlights... The main focus for heavy rainfall is expected across portions of the Deep South and into South Carolina for the Wednesday and Thursday time period, 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. 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