Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Wed Jun 21 2023 Valid 12Z Sat Jun 24 2023 - 12Z Wed Jun 28 2023 ...Prolonged heat wave continues across Texas and southeastern New Mexico and will expand northeast next week... ...Heavy rain and flooding are possible across the north-central U.S. into Saturday with a low pressure system... ...Overview... The medium range period will begin Saturday with a couple of upper troughs/lows helping to produce rain and thunderstorms, one over the north-central U.S. and another centered near the Lower Great Lakes region. These features look to combine later in the period to one main trough over the Eastern U.S., while additional troughing/closed low moves from the eastern Pacific into the West. Meanwhile, in between these troughs, an upper high centered over Texas will continue to bring persistent excessive heat to much of the southern Plains, with some expansion at times into the Lower Mississippi Valley and Oklahoma/Kansas. ...Guidance Evaluation/Predictability Assessment... The latest model guidance was in good enough agreement for a general blend of the deterministic solutions through day 5. Models show excellent agreement through at least the middle of next week that the stubborn upper level high should hang around the south-central U.S. with maybe some slight adjustment westward. There are some questions in the details as the initial upper low over the Lower Great Lakes becomes absorbed into the compact low across the north-central U.S. early next week. By Tuesday, the GFS and CMC are notably faster with the broader resulting trough over the East Coast than the ECMWF. Ensembles may support a slower progression, closer to the ECMWF. There is overall good agreement through even day 7 on the upper low/shortwave dropping down the West Coast and into California, with some differences in the details which may take until the short range period to resolve. WPC used a blend of the ensemble means with the ECMWF for the latter half of the period. Today's WPC forecast remains consistent and very similar to the overnight forecast. ...Weather/Hazards Highlights... As the upper troughing in the East lifts north and weakens, it will still provide some support for rain and thunderstorms but likely with reduced widespread heavy amounts compared to the short term. Scattered thunderstorms could produce localized heavy rain rates given the instability in place. There may be a somewhat focused axis of rainfall across the northern Mid-Atlantic region into the Interior Northeast this weekend, so Marginal Risks are in place there for Saturday and Sunday, with areas of low Flash Flood Guidance of particular concern. Farther west, a consolidating low pressure system at the surface and coupled jets (right entrance/left exit region combination) aloft will likely produce heavy rainfall (likely around 2-4" with locally higher amounts) across the north-central U.S. on Saturday. A broad Slight Risk of excessive rainfall is focused on North Dakota and northern/central Minnesota for this potential, with a Marginal Risk for lingering rainfall centered on the Arrowhead of Minnesota on Sunday. Shower and storm chances will likely also extend southward across the Missouri and Mississippi Valleys this weekend ahead of the frontal system with SPC highlighting an area for potential severe weather as well. Along a dissipating warm front and ahead of the approaching cold front, there could be an additional area of enhanced organized convection on Sunday. High rain rates and sufficient instability should create at least a localized flash flood threat and so a marginal risk is in place for this area on the Day 5/Sunday Excessive Rainfall Outlook. Additional rounds of rain and storms are likely into the East as this system continue to progress eastward early next week. The next system into the West will also allow scattered showers and storm chances to return across parts of the Northwest and into the northern Rockies/High Plains next Monday-Tuesday. In terms of temperatures, there is really no end in sight for the excessive heat that has plagued particularly south and western Texas/southeastern New Mexico in recent days. In fact, for the weekend and into next week, temperatures over 100F and heat indices much higher will extend farther east into the Dallas-Fort Worth and Houston metros by Saturday and then into the Lower Mississippi Valley as well. Several record temperatures could be set or tied. The Desert Southwest starts to see temperatures well over 100F by the weekend and next week too, though not really anomalous for this typically hottest time of the year there. As mean upper ridging expands farther north, highs in the 90s are likely across much of the Plains and Mississippi Valley, with current forecasts showing temperatures exceeding 100F into Oklahoma/Kansas/Arkansas by next Wednesday. Meanwhile the West should see cooler than normal temperatures particularly for highs with the bouts of upper troughing aloft. The East looks to be within a few degrees of average for early summer. Santorelli/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