Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Jun 20 2023 Valid 12Z Fri Jun 23 2023 - 12Z Tue Jun 27 2023 ...Prolonged heat wave continues across Texas and southeastern New Mexico throughout this week with some spread into the Lower Mississippi Valley early next week... ...Heavy rain and flooding are possible across the north-central U.S. late this week with a low pressure system... ...Overview... The medium range period will begin Friday with an upper low finally lifting north out of the Tennessee Valley, though with some remaining showers and thunderstorms across the East with a low pressure/frontal system at the surface. Farther west, another upper low is forecast to press across the West into the northern Plains through the weekend and then into the Great Lakes region, spreading rain with it that could be heavy and cause flash flooding concerns particularly in the northern Plains. Meanwhile, a strong upper high will continue to meander around the Rio Grande through the week and into next week, causing persistent heat across much of Texas and southeastern New Mexico, and high heat indices are likely to spread into the Lower Mississippi Valley as well by early next week. ...Guidance Evaluation/Predictability Assessment... For late week, model guidance continues to show fairly good agreement with the overall initially blocky synoptic pattern, with troughing across the West including a small upper low closing off atop the northern High Plains by early Saturday, ridging extending from Texas northward, and the upper low/trough gradually lifting out of the east-central CONUS. For the early part of the forecast period, utilized a blend of the 12/18Z deterministic guidance favoring the ECMWF and GFS. By early next week, the north-central U.S. upper and surface lows look to continue their trek eastward and move atop the Upper Great Lakes region for renewed troughing across the East, with reasonable model agreement given the lead time. The 12Z ECMWF was on the slow side with the trough by Day 7/Tuesday but the newer 00Z models seem to have sped up in general. More differences in terms of placement and timing arise with an upper trough/low in the eastern Pacific Sunday-Monday that could push into the West by Tuesday. The 18Z GFS was on the fast side, so started to favor the 12Z GFS that was better aligned with the axis of the ensemble means. On the other hand the 12Z ECMWF slowed farther west than the other guidance, including the newer 00Z guidance suite. Meanwhile models show good agreement that the upper high over western Texas/northern Mexico will continue to hold on, while ridging may poke northward into the Four Corners states and beyond, building ahead of the eastern Pacific to western U.S. trough. With the increasing model spread especially in the West, started to favor the ensemble mean guidance to about half by days 6-7. ...Weather/Hazards Highlights... As the upper low meandering over the Southeast/Tennessee Valley in the short range weakens and lifts north, it will still provide some support for rain and thunderstorms but likely with reduced widespread amounts compared to the short term. The upper and surface lows lifting north should allow for rain chances to spread north into the Mid-Atlantic and Northeast as well. For Friday, a Marginal Risk of excessive rainfall is in place across the Southeast and Mid-Atlantic for lingering convection over what is likely to be wet ground after multiple days of rain in some areas. The instability in place could support localized heavy rain rates. By Saturday for the Day 5 ERO, there may be a somewhat focused axis of rainfall across the northern Mid-Atlantic region into the Interior Northeast, so a Marginal Risk is in place for those regions, and areas of low Flash Flood Guidance could be of particular concern. Scattered thunderstorms may remain farther south too. A wet pattern looks to persist across the East through early next week as additional troughing comes in. Farther west, support starts to become favorable for heavy rain in the north-central U.S. as a coupled upper jet (right entrance/left exit region combination) sets up around Montana on Friday and into the northern Plains by Saturday, while at the surface a low pressure/frontal system consolidates. Broad Slight Risks of excessive rainfall are in place for this potential, across the northern High Plains on Friday and for the northern Plains/northern Minnesota on Saturday. Ensemble members from the GEFS/ECens/CMC model suites combined indicate about a 60 percent chance of the 24 hour rainfall exceeding the 1 year Average Recurrence Interval (ARI) on Friday in eastern Montana, which is quite high for this lead time. As the low picks up speed and moves east early next week, areas west of the Great Lakes should dry out, except that there may be a renewed round of rain across the Northwest early next week. Meanwhile, the excessive heat will continue through late week, with widespread 100s across central and southern portions of Texas and southern New Mexico, and 90s with heat indices well over 100F elsewhere in the south-central CONUS. The hazardous heat looks to once again become more widespread into next week, stretching into the Lower Mississippi Valley as well. Many daily record high temperatures are possible, and some monthly records may even be challenged. The Desert Southwest starts to see temperatures well over 100F by early 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, while 80s and low 90s in the Upper Midwest/Great Lakes region late this week will be 10-15F above normal. Meanwhile the West should see cooler than normal temperatures particularly for highs with the bouts of upper troughing aloft, while cooler than average temperatures will also be present in the southeastern U.S. late this week given the cloudiness/rain chances and the upper low. 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