Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Fri May 14 2021 Valid 12Z Mon May 17 2021 - 12Z Fri May 21 2021 ...Heavy rain threat for the Southern Plains/Lower Mississippi Valley next week... ...Guidance Evaluation and Preferences... Latest models and ensembles continue to support the threat of a multi-day heavy rain event across the south-central U.S. well into next week. Subtle changes within the associated upper trough in recent model runs have generally resulted in a northward shift in the frontal boundary forecast to slowly lift northward as a warm front, which in turn resulted in a northward shift in the heavy rain axis. The heaviest rains/convection remain generally south of this front. The 00Z ECMWF represented the southern-most guidance regarding the heavy rain axis in contrast with the latest 12Z run which moved the frontal boundary significantly farther to the north later next week. The GFS has been faster than the 00Z ECMWF and has been carrying more energy downstream to extend the rain axis into the Ohio Valley. A consensus of the 00Z ECMWF/EC mean, 06Z GFS/GEFS, together with the 00Z CMC was used to handle this system. Toward the latter part of next week, the 12Z ECMWF yesterday started to depict a much more amplified trough digging into the West Coast. The CMC and now the GFS have followed suit into agreeing with this scenario. On the other hand, both the ECMWF and CMC have backed off from forming a closed low over New England later next week in agreement with the more progressive GFS. The WPC medium-range forecast package was based on the consensus of the 00Z ECMWF/00Z EC mean, the 06Z GFS/GEFS mean, as well as some contribution from the 00Z CMC and CMC mean. This yielded a solution close to WPC continuity but with a northward shift in the frontal position across the Plains, and a more progressive and deeper upper trough moving into the West later next week. ...Weather Highlights/Hazards... A favored model and ensemble composite solution indicates that deep layered moisture from the Gulf of Mexico will return into an emerging and slow moving south-central U.S. frontal system next week. This will occur in downstream response to the slow ejection and passage of a southern stream upper-level trough/low energies forecast to exit the southern Rockies. Expect a multi-day period with a threat of significant rainfall along with periods of increased instability and strong to severe convection. Locations and magnitude of heaviest rainfall will likely vary on a day-to-day basis and some specifics may take until well into the short range to be resolved, but the guidance signal for a wet pattern over the south-central U.S. has been consistent. Meanwhile, the Midwest eastward into the Mid-Atlantic may see one or more convective systems aided by a wavy front and uncertain northern shortwave energy aloft to focus moisture and instability. Well south, late period moisture/vorticity influx underneath a building Southeast upper ridge may support some organized heavy downpours over southern Florida. The favored GEFS/ECMWF ensemble mean solution over the West mid-later next week would support mainly moderate precipitation from the Pacific Northwest out across the north-central Intermountain West/Rockies to include some mountain snows. Ahead of the Pacific front that moves into the West next week, lead maximum temperatures 10-20F above normal will also progress from the Northwest U.S. eastward though the northern Rockies/Plains early next week. A cooling trend behind the front may bring 5-10F below normal temperatures. Kong/Schichtel 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml