Extended Forecast Discussion NWS Weather Prediction Center College Park MD 403 PM EDT Sat Jun 17 2023 Valid 12Z Tue Jun 20 2023 - 12Z Sat Jun 24 2023 ...Watching the potential of heavy rain expanding over parts of the Southeast as a prolonged heat wave continues across the southern Plains through next week... ...Overview... A similarly blocky pattern is forecast to continue through much of next week across the mainland U.S. This pattern will feature (1) a deep positively-tilted trough across the northwestern U.S., (2) another longwave trough down the East Coast together with an expansive cool surface anticyclone, (3) a warm ridge to remain anchored over Texas, and (4) an upper low that will likely bring heavy rain into the Southeast and parts of the Mid-Atlantic with models trending toward high rainfall amounts across the southern Mid-Atlantic into the southern Appalachians. Meanwhile, the potential for heavy rain exists from the central High Plains to the northern Plains as a front is forecast to become stationary in the vicinity midweek. ...Guidance Evaluation/Predictability Assessment... Despite the prevailing blocky and amplified synoptic pattern across the U.S., models and ensembles have displayed relatively minimal spread through the medium-range period. The most uncertain feature continues to be the potential for a closed upper low to form over the Southeast which may linger further in the vicinity if the feature ends up stronger than predicted. This upper low will have important implications to where and how much heavy rain will impact the Southeast. Models have been trending toward developing a better-defined upper low, leading to a deeper and stronger influx of moisture toward the southern Mid-Atlantic and southern Appalachians, with a stronger low pressure wave forming along a coastal front. An amplified upper ridge building east across the Great Lakes toward New England will also help to close off the upper low over the Southeast while keeping the low in place. This situation will need to be further monitored as it does not take much for the dynamics to bump up QPFs there. ...Weather/Hazards Highlights... The upper low meandering over the Southeast/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 Days 4 and 5 (Tuesday and Wednesday) in the Southeast to southern Appalachians 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. The Slight has been split into two, with the northern portion associated with the development of a wave along the coastal frontal in conjuction with a stronger influx of Atlantic moisture ahead of a stronger upper low over the Southeast. The southern portion of the Slight is associated with a stationary front near the eastern Gulf Coast. It appears that the biggest threat of the heavy rain in the Southeast will be through midweek next week before gradual decrease in QPFs sets in for later next week as the upper low is forecast to weaken. However, the recent model trend for a stronger upper low could imply a better-defined low to linger in the vicinity further into next week, which may help keep the rainfall in place across the Southeast and then perhaps spread farther up the East Coast by next weekend. As another upper low comes into the Northwest and lifts toward southern Canada next week, some precipitation is possible along and behind a front on Tuesday. Given the below normal heights and temperatures, the highest elevations of the Cascades and northern Rockies could see some snow. Farther east, rain and thunderstorm chances should be expanding over portions of the the northern Plains and down toward the central High Plains as the low pressure/frontal system stalls and focuses moisture, with the greatest potential for some locally heavy rainfall Wednesday-Thursday. A Slight Risk has been introduced near and just east of the forecasted frontal position. Behind the front, widespread well below normal temperatures are expected on Tuesday, with a few record cool temperatures possible. A slow moderating trend in temperatures is forecast thereafter but they will still be below normal for the latter half of the week across much of the West. Meanwhile, 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 with heat indices well over 100F elsewhere. 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 upper ridging expands farther north, highs in the 90s are likely across much of the Plains and Mississippi Valley. This will be about 10-20F above average for the northern Plains Tuesday. 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. Kong/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