Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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... Models and ensembles this morning continue to advertise the likelihood of a heavy rain event that should be well under way across the central to southern Plains during the middle of next week. The latest model solutions tend to focus an axis of heaviest rain across eastern Texas and eastern Oklahoma on Wednesday before shifting southward closer to the Texas Gulf coast region on Thursday. There is general agreement that the associated upper trough and the surface front will weaken as they lift northward through the Plains for the remainder of next week. The weaker dynamics should tend to lower the threat of widespread heavy rain toward next weekend but deep-layered southerly flow will likely keep the channel of Gulf moisture open to support at least scattered convection with areas of heavy rain across the central and southern Plains. In addition, the convection appears to become more diurnally-driven toward next weekend and may tend to edge westward closer toward the central to southern High Plains as a deep upper trough approaches the western U.S. Meanwhile, there has been a strong model signal for a deep trough/closed low to dig into the western U.S. during the latter half of next week but the details of its eventual evolution remains subject to change. The latest model consensus has trended toward a slightly less amplified system overall, which allows a faster progression of a frontal wave to move eastward into the northern Plains late next week, together with moderate precipitation across the northern Rockies to northern High Plains. 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. 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