Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM 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. There is 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. 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 and surface low 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, but models have struggled with its ejection eastward into the north-central U.S. late week. WPC's preference has been for an upper low or at least strong shortwave to persist in the Dakotas into Friday, consistent with the majority of the deterministic/ensemble/AI guidance. For the 00Z/06Z cycle, that eliminated the GFS runs as they sheared out the shortwave and had the associated surface low farther northeast of other guidance, even though the GFS runs from a day ago showed the feature. The new 12Z GFS seems in slightly better alignment though may be a little fast still. Given the model waffling, there is fairly low confidence in the details, especially in precipitation forced by the features. Then upstream, guidance generally has shown an upper low dropping south into the Northwest late week into the weekend, other than a few rogue runs with weaker energy (like the 06Z GFS). A model (excluding the GFS)/ensemble blend worked to handle this feature. Thus the WPC forecast used a blend of the deterministic guidance to start the period, but removed the GFS by Friday due to its north-central U.S. and then northwestern U.S. issues, in favor of the GEFS and EC ensemble means. Increased the proportion of the ensemble means to just over half by the end of the period. ...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 from northeast Texas into southern Illinois and vicinity as the western portion of the boundary stalls and eventually begins to lift north as a warm front in a moist and unstable environment. A similar setup is expected on Thursday, so a Marginal to embedded Slight Risk remains in place for Day 5/Thursday. Rainfall totals may decrease a bit by Thursday but still could cause flash flooding issues after a wet Wednesday. Some areas (with the exception of the southern half of Arkansas) have wet antecedent conditions because of multiple heavy rainfall episodes in the previous few weeks, so these may be particularly vulnerable. Higher elevations of the northern Rockies can expect several inches of accumulating snow Wednesday-Thursday underneath an upper low, with snow levels depending on the low's depth. Rain in lower elevations of Montana may be locally heavy. Then rain will push into the Plains and Mississippi Valley for late week into the weekend. Amounts and positioning of heaviest rain are quite uncertain but there may be a general focus in parts of the Upper/Middle Mississippi Valley, with scattered convection elsewhere. 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. 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