Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM EDT Sat May 18 2024 Valid 12Z Tue May 21 2024 - 12Z Sat May 25 2024 ...Overview... Multiple progressive upper shortwaves/possible closed lows and surface low pressure/frontal systems will traverse the West to northern/central Plains and Midwest during the medium range period. For Tuesday- Wednesday, a potent shortwave will send a modestly deep surface low into the Upper Midwest and potential for at least locally heavy rain and thunderstorms across the region. Its attendant cold front will push across the East and partway into the South midweek and beyond, with rain and storms along and ahead of it and cooler temperatures behind. Heat will continue to be hazardous for at least southern Texas though. Another couple rounds of energy may push through the Northwest eastward late week, but with much more uncertainty. ...Guidance/Predictability Assessment... Models show reasonable agreement with the first shortwave that kicks out into the Upper Midwest Tuesday-Wednesday, though with some uncertainty in the timing and depth of the surface low with the potential merging of upper-level energies. The next upper low tracking through the Northwest Wednesday and eastward late week starts with slightly timing better agreement compared to runs from a day ago, as 06Z and new 12Z GFS runs are more agreeable in showing this feature separate from upstream energy, similar to the non- NCEP guidance. There is still some spread in the deterministic and AI models though. By Saturday, the 00Z deterministic ECMWF seemed to become an outlier in producing ridging over the north- central U.S. as it is faster to shear out the shortwave energy. Other models including EC-based AI models prefer troughing if not the upper low remaining closed there, and the 12Z ECMWF trended toward this, though north with its center into south-central Canada. Then upstream, models generally show yet another closed low dropping south into the Northwest for the end of the week. The 00Z ECMWF was west of other operational and AI models with its position by Saturday. Another feature of possible concern but on the smaller scale is a southern stream shortwave moving through the southern Plains to Mid-South to Mid-Atlantic Thursday-Saturday, which has implications for QPF among other sensible weather, so this will continue to be monitored. Thus the WPC forecast utilized a blend of the deterministic guidance early in the period, but introduced and increased the proportion of ensemble means to over half by the end of the period given the model spread. This maintained reasonable continuity with the previous forecast, though slowed the central U.S. surface low and front by Saturday with most models trending that way. ...Weather/Hazards Highlights... An upper shortwave and leading cold front, with deepening surface low, will push into the Upper Midwest Tuesday/Wednesday, helping to fuel showers and thunderstorms across this region. Ample moisture will be in place while the low track and upper level flow provide significant lift for heavy rain on Tuesday. Thus will embed a Slight Risk in the Day 4/Tuesday Excessive Rainfall Outlook across parts of the Upper Midwest. Notable instability will be present on the warm side of the low, which will also provide support for likely severe thunderstorms over the Middle Mississippi Valley according to the Storm Prediction Center. But, a limiting factor for flash flooding farther north will be the lack of instability on the backside of the low, despite model guidance showing heavy amounts across northern Minnesota for example. A broader Marginal Risk extends around and south just ahead of the cold front. After Tuesday, expect northeastward progression of the surface low to push the trailing cold front and accompanying rain/storms farther east and south with some more potential for heavy rainfall in the eastern and southern U.S. for Wednesday-Thursday. The front is likely to slow or stall west-east in a moist and unstable environment, and with multiple rounds of convection with possible heavy rain rates, will embed a Slight Risk in the ERO for Day 5/Wednesday from around Texarkana into the Mid-South. Also expanded the Marginal Risk into west- central Texas with possible dryline convection with heavy rain rates. Approach/arrival of another couple of systems as currently advertised into the Northwest by midweek and next weekend would produce somewhat more organized precipitation there, and potential for heavy snow in the mountains, with snow levels dependent on the depth of the upper low(s). Expect South Texas to see multiple days of hazardous heat during the period with highs persistently running 10-15F above normal with max heat index values possibly reaching at least 110F. Highs near 100 degrees could stretch farther north across the southern High Plains at times as well. Some daily records for highs/warm lows will be possible. Above normal highs will also track east across the Midwest and Northeast into Wednesday but should moderate by Thursday-Friday as the cold front pushes through the region. The forecast pattern will favor below average highs over the Northwest to northern Plains for most of next week. Tate/Santorelli 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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