Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Jun 24 2023 Valid 12Z Tue Jun 27 2023 - 12Z Sat Jul 01 2023 ...Prolonged heat wave across Texas and southeastern New Mexico will expand northeast toward the mid-Mississippi Valley next week... ...Excessive rainfall possible across the northern Plains and Northeast early-mid next week... ...Overview... A potent and persistent upper ridge will nudge eastward from Texas to the lower Mississippi Valley through the end of next week while mean upper low/troughing holds over the East as a positively-tilted trough over the West lifts generally toward the northern Plains. A northeastward expansion of the excessive heat is expected underneath the ridge from the southern Plains to the mid-Mississippi Valley and portions of the central Plains next week. WPC and CPC continue to jointly produce "Key Messages" for this heatwave. Meanwhile, low pressure waves underneath the upper troughs will focus showers and thunderstorms over the Northeast and the north-central U.S. ...Guidance Evaluation/Predictability Assessment... Model and ensemble solutions overall remain well clustered next week, bolstering forecast confidence. The more uncertain part of the forecast has to do with the timing of the ejection of the upper trough from the Rockies into the northern U.S. The ECMWF and CMC had been on the fast side of the guidance with a better-defined low pressure system tracking toward the Great Lakes by about next weekend, whereas the GFS had been slow with the system up to the 06Z run. However, the 12Z GFS suddenly flipped to a solution much more agreeable with the faster guidance. The WPC medium range product suite was primarily derived from a composite blend of GFS/ECMWF/Canadian models together with their ensemble means Tuesday-Thursday to provide maximum detail consistent with a pattern with seemingly above normal predictability. More of the ensemble mean solutions were included for Days 6 and 7 to handle to higher uncertainty with the timing of the trough ejection into the northern U.S. late next week. This solution maintains good WPC product continuity except across the northern tier on Day 7. ...Weather/Hazards Highlights... The next low pressure system of concern will move across the Great Lakes as the medium-range forecast period begins on Tuesday. Ample moisture will likely support widespread precipitation for the entire northeastern U.S. into the Mid-Atlantic. Locally heavy rainfall will be possible with potential training and embedded thunderstorms associated with this system that should again prove to be slow to move on. A WPC Excessive Rainfall Outlook (ERO) "Marginal Risk" area is maintained over the Northeast in a slow-to-evolve flow pattern in an area that had experienced antecedent rainfall. Embedded "Slight Risk" areas remain possible in future ERO updates when confidence in the location of heavier rainfall totals increases. Models generally agree that the trough will make a slow eastward progression through the end of next week with the heaviest rains probably slide off New England by Friday but with lingering residual scattered activities into the weekend. Meanwhile, the ejection of upper trough energies out from the West will bring scattered showers and storm chances, with activity likely to be on the increase and spread downstream to the Northern Rockies and northern Plains, and heavy rainfall may be a concern. There is a planned "Marginal Risk" of Excessive Rainfall for parts of this region Tuesday and Wednesday to address potential training/repeat cell runoff concerns. Precipitation will spread into the upper Midwest and Ohio and Tennessee Valleys mid-to-late next week as the system progresses to the east. In term of temperatures, excessive heat will continue to plague the south-central U.S. through at least next week with a mid-week expansion north and east across portions of the central Plains and into the mid- and lower Mississippi Valley. The heat will be oppressive with widespread high temperatures exceeding 100 degrees and heat indices as high as 120 degrees. Heat indices may even exceed 120 degrees in some locations in South Texas. Several temperature records are in danger of being broken during this significant heat wave. The Desert Southwest will also experience temperatures well over 100 degrees next week, but that is not really anomalous for this time of year. Elsewhere in the country, near normal high temperatures are forecast with below normal highs likely in areas of precipitation each day. Kong/Schichtel 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, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw