Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 AM EDT Sat May 15 2021 Valid 12Z Tue May 18 2021 - 12Z Sat May 22 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 mid-larger scale guidance of the 18 UTC GFS/GEFS, 12 UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of Models. Smaller scale system differences and convective rainfall focus details remain much less certain. Latest 00 UTC models and ensembles are overall in line with this favored composite plan. ...Weather Highlights/Hazards... 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 to lift over the Plains. 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 mainly over the south-central U.S. has been consistent. Meanwhile over-top, the Midwest eastward 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 underneath a building Southeast upper ridge may support some heavy downpours for southern Florida and into the Gulf of Mexico. The 12/18 UTC GFS runs seemed too aggressive developing an organized low, but the 00 UTC run backed off. Mid-later next week digging of amplified upper troughing over the West will support widespread moderate precipitation from the Pacific Northwest through the north-central Intermountain West/Rockies, including some mountain snows. Ahead of the Pacific front that moves into the West, maximum temperatures 10-20F above normal will progress from the Northwest U.S. though the northern Rockies/Plains early next week. A post-frontal cooling trend may bring 10-20F below normal temperatures into the Intermountain West. 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