Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM 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... The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered larger scale guidance of the 18 UTC GFS/GEFS, 12 UTC ECMWF/ECMWF ensemble mean/Canadian along with the 01 UTC National Blend of Models. Smaller scale system differences and convective rainfall focus details are much less certain. System differences increase more significantly mid-later next week, especially with the GFS digging much less upper troughing down over the West than the ECMWF/Canadian. Leaned toward the more run to run consistent GEFS/ECMWF ensemble means that similarly develop a moderately deep upper trough over the West at these long time frames. Latest 00 UTC models and ensembles tend to maintain these aforementioned guidance differences. ...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. A cooling trend behind the front may bring 5-10F below normal temperatures. 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