Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 AM EDT Sun May 19 2024 Valid 12Z Wed May 22 2024 - 12Z Sun May 26 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. By Wednesday, a potent shortwave and deep surface low will be moving through the Upper Midwest while its attendant cold front will push across the East and partway into the South beyond midweek, with rain and storms along and ahead of it and cooler temperatures behind. Some recurring threat for heavy rainfall along the western side of this front from parts of Texas into the Lower Mississippi Valley as the boundary stalls and lifts back north as a warm front. Farther south, heat will continue to be hazardous for at least southern Texas though. Another couple rounds of energy may push through the Northwest and eastward late week and next weekend, but with more uncertainty. ...Guidance/Predictability Assessment... Models show reasonable agreement with the first shortwave in the Upper Midwest at the start of the period on Wednesday, although the past few runs of the GFS have been slightly faster with the system as it moves into southeast Canada. Still, a general model blend seems to serve as a good starting point. The next upper low should arrive into the Northwest by Wednesday as well with plenty of lingering uncertainty late week as it moves into the Upper Midwest. Models continue to struggle with its evolution and possible combination of energy with another shortwave across central Canada at the same time. The GFS and ECMWF (and now the new 00z CMC) show consolidation of this energy into one big low over south-central Canada next Saturday. However, the ECMWF- initialized ML models show more support for stream separation. So lots of questions on the evolution and the ensemble means are very washed out with this energy. The WPC forecast for tonight used a blend of the deterministic solutions the first half of the period amidst better agreement. By late week and especially next weekend, continued to maintain more of the deterministic solutions than usual along with the ensemble means to help smooth out the detail differences but still maintain system definition. This late period blend also serves well for yet another upper low that drops into the Northwest next Saturday. ...Weather/Hazards Highlights... The cold front associated with a deep surface low in place across the Upper Midwest on Wednesday will be the main focus for showers and thunderstorms from the southern Plains into the Ohio Valley on Wednesday. The Day 4/Wednesday ERO shows a broad and elongated marginal risk stretching from the southern Plains to the Ohio Valley. There is an embedded slight risk from northeast Texas into the lower Ohio Valley as the western portion of the boundary stalls and eventually begins to lift north as a warm front. This will continue the heavy rainfall threat into Thursday as well across parts of the southern Plains to lower Mississippi Valley - a region that has been very late as of late. Given the overlap too with Day 4, a slight risk was introduced from northeast Texas into southeast Oklahoma and central Arkansas with a broader marginal risk surrounding. 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 into the Northeast/Mid-Atlantic Wednesday-Thursday but should moderate thereafter 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. 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