Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM 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, with an eventual heavy rainfall threat possible here as well. ...Guidance Evaluation/Predictability Assessment... Model guidance has continued to generally be in good agreement with the overall pattern during most of the medium-range period the last few days, with the main features accounted for and more subtle differences in their strength/placement. An omega-like blocky northern stream pattern continues with mean troughing over the East, riding over the central CONUS, and a trough over the West. Meanwhile, an energetic southern stream remains in place over the South, with a ridge centered over eastern portions of the Southwest and Texas, and an upper-level low developing over the Southeast, expected to remain in the region through at least the middle of the forecast period. Over time the ridging over the central CONUS extends northward into southeastern Canada, setting up a bit of a Rex Block pattern for the East. The western upper-level trough tends to retrograde to the west as heights rise a bit over the western Plains towards the end of the period. The 00Z CMC tended to be a bit of an outlier compared to the other guidance, particularly with respect to the evolution of the pattern over the central and eastern CONUS. The upper-level low over the Southeast early in the period is stronger while the southern stream ridge, and the general mean northern stream ridging over the central CONUS, tends to be a bit weaker through the mid- to late period. The 00Z UKMET is similarly a bit weaker with the northern extent of this ridge, but more in line with the latest runs of the GFS/ECMWF. In terms of predictability, there also looks to be a bit less run-to-run consistency in the GFS and ECMWF with respect to the pattern in this same region compared to the last couple of days, with the strength of the cutoff low and ridge varying a bit in prior runs, though the latest 00Z/06Z GFS and 00Z ECMWF are at least consistent with each other. Uncertainty in the pattern increases at the end of the period, with the 00Z/06Z GFS depicting the cutoff low over the Southeastern U.S. beginning to retrograde westward along the Gulf Coast as the energy splits in in the 00Z ECMWF, with some retrograding westward similar to the GFS but with an upper-level low retained northeastward over the Mid-Atlantic. The predictability of the pattern in this region remains low as well, with earlier runs of the GFS showing the cutoff low deepening over the Midwest/Great Lakes while weakening in the ECMWF. The 00Z CMC remains a larger outlier compared to the most recent guidance, with a much deeper cutoff low hovering over the eastern CONUS. A general model blend is used for the updated WPC forecast, with more weight given to the 00Z ECMWF/06Z GFS over the 00Z UKMET, and the 00Z CMC in particular, given increasing divergence from the clustering towards the 00Z ECMWF/06Z GFS solution. The 00Z ECens mean tended to more closely agree with the deterministic guidance in the strength of the features over the central/eastern CONUS in the mid-period, thus a contribution from the 00Z ECens is added following the removal of the forecast time-limited UKMET. The 00Z GEFS mean similarly retains a bit more detail in the late-period, and a contribution is added while removing the 00Z CMC given the increasing divergence in the pattern over the central/eastern CONUS compared to the 00Z ECMWF/06Z GFS. The initial forecast blend is similar to the prior WPC forecast through the early to mid- forecast period. However, the diverging trends in the guidance led to a weaker upper-low in the East while retaining more upper-level ridging to the north in a Rex Block pattern compared to the prior forecast. ...Weather/Hazards Highlights... The upper low meandering over the Tennessee Valley or so, along with a meandering 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. Continual adjustments have been made to the Slight Risk areas given this prior and anticipated rainfall, including westward extensions over portions of the Florida Panhandle that have recently experienced extreme rainfall, as well as to the east into northern Florida on Tuesday as forecast rainfall amounts consolidate over the region and overlap expected rainfall on Monday. As next week progresses, the track of the upper low will affect where rainfall spreads, but the trend has been to focus on the Southeast, with the potential to expand a bit westward towards the Lower Mississippi Valley as well as into South Florida. 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 over portions of the the northern and central Plains as the low pressure/frontal system stalls and focuses moisture, with the greatest potential for some locally heavy rainfall Wednesday-Thursday. 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. The heat is expected to gradually shift westward with time, focusing on western Texas towards the end of next week. 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. As a Rex Block pattern sets up over the northern tier, the Upper Midwest into the interior Northeast can expect 80s to low 90s for highs for the latter half of the week, about 10F above average. Putnam/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