Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 AM EDT Fri Jun 16 2023 Valid 12Z Mon Jun 19 2023 - 12Z Fri Jun 23 2023 ...Hazardous heat is likely across Texas and Louisiana with some record temperatures possible through next week... ...Overview... A fairly stagnant pattern will once again be in place for the medium range period next week, with an upper ridge/high near the Rio Grande that will produce excessive heat across the south-central CONUS. The upper ridge should extend to the northeast near the Midwest/Great Lakes, while an upper low looks to be atop the Southeast/Tennessee Valley for a potential Rex Block pattern. The upper low along with ample moisture is likely to produce heavy to excessive rain across those regions. Meanwhile in the West, persistent troughing will lead to cooler than normal temperatures and some precipitation in the Northwest shifting into the Plains with time. ...Guidance Evaluation/Predictability Assessment... The 12/18Z model cycle was in generally good agreement with the overall pattern described above. A deterministic model blend was able to be used early in the period with the 12Z ECMWF/UKMET/CMC and 18Z GFS, as there was good agreement with the anomalously deep upper low across the Northwest, the ridge to the east, and the developing low in the Southeast. The guidance diverges somewhat with these features by midweek and beyond. In the East, the interface of the ridge and the upper low show some differences that affect the path of the upper low by the latter part of next week. The 12/18Z model guidance at least generally indicated a drift north of the upper low with time. However, the newer 00Z model cycle shows some divergence in the potential path of the upper low. The 00Z GFS and ECMWF both came in with the upper low splitting off farther south, all the way to the central Gulf Coast Thursday-Friday, while ridging still builds in to its north, but farther south of the previous model consensus. Meanwhile the 00Z CMC actually had a farther north track than its previous run and brings the upper low into the Ohio Valley Thursday-Friday, totally out of phase with the 00Z GFS/ECMWF. The blend of 12/18Z models and ensembles that the WPC forecast used is at least sort of in between these types of solutions with the low's and high's path, so this may be a reasonable place to be for now, but this indicates the continued uncertainty with the pattern. At least model guidance from multiple cycles agrees that upper ridging will be in place across the south-central U.S. through the period. The Western low/trough begins with good agreement but GFS runs seemed to be outliers in the evolution by later next week, bringing ridging into the Central Great Basin or so as some of the energy from the troughing split into the Pacific. The 00Z GFS and GEFS mean do not show this pattern anymore. A non-NCEP blend of the 12/18Z models seemed to work well across the West. ...Weather/Hazards Highlights... The upper low meandering over the Tennessee Valley or so, along with a frontal system and ample Gulf moisture, will provide support for multiple rounds of rain and thunderstorms for the southeastern quadrant of the country through much of next week. Slight Risks of excessive rainfall are in place for both Monday and Tuesday in the Southeast to southern Appalachians (with a slight shift eastward Tuesday) given the potential for widespread 1-3" totals per day with locally higher amounts. Portions of this region have or will likely have by that time wet antecedent conditions given recent or upcoming episodes of heavy rainfall. In fact the Slight Risk EROs were expanded into the Florida Panhandle with one reason being the extreme rainfall amounts unfolding currently overnight across Pensacola and the vicinity. As next week progresses, the track of the upper low will affect where rainfall spreads. This forecast cycle ended up with less QPF for the Mid-Atlantic region compared to the previous forecast given model trends, maintaining the higher QPF farther south across the Carolinas and south through the latter part of next week, but this is still an uncertain aspect of the forecast. Meanwhile temperatures across the region are likely to be below average with the closed low overhead, influence of clouds/precipitation, and cold-air damming setting up along the Appalachians. As another upper low comes into the Northwest and pivots next week, precipitation chances will increase there near and behind a frontal system Monday-Tuesday. Given the below normal heights and thus temperatures, the highest elevations could see some snow. Farther east into the Northern Rockies/High Plains, there may be some instability to work with and produce storms, leading to some possibly locally heavy rain over areas that could be sensitive due to well above normal rainfall over the past few weeks. Into Tuesday and beyond, rain and thunderstorm chances should be increasingly common the Plains as the low pressure/frontal system stalls and focuses moisture. The troughing in the West will also lead to well below normal temperatures particularly for highs. High temperatures of 15-25F below average in the Great Basin could challenge daily records for cool highs on Monday and Tuesday, with lows around 10F below normal. Temperatures look to gradually moderate closer to, but still remaining below, normal for the latter half of the week across the West. The excessive heat will continue into next week across much of Texas and into portions of Louisiana and New Mexico, with widespread 100s and even some 110s across central and southern portions of Texas, and 90s covering much of the Lower Mississippi Valley and the Central Plains. Many daily record high temperatures are possible, and some monthly records may even be challenged. Heat indices as high as 115-120 degrees are possible for parts of southern and southeastern Texas where the greatest combination of high dewpoints and temperatures will likely exist. As mean ridging expands further north, additional much above average temperatures are forecast for portions of the Northern Plains, particularly on Monday-Tuesday, with highs reaching into the mid- to upper 90s. Then the Upper Midwest can expect 80s to low 90s for highs for the latter half of the week, about 10F above average. 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