Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM EDT Wed Apr 15 2020 Valid 12Z Sat Apr 18 2020 - 12Z Wed Apr 22 2020 ...Heavy rainfall/runoff and strong thunderstorm threats on tap again for the South next week... ...Overview and Guidance/Predictability Assessment and Weather/Hazard Highlightsā€ ...Saturday into early Monday... The 00 UTC ECMWF has trended more in line with recent GFS runs in bringing a more amplified northern stream upper trough eastward over the northern Mid-Atlantic Sat whose approach spawns deep coastal low development off New England into Sun. This bolsters forecast confidence in a swath of modest precipitation across the region and a subsequent maritime threat. Meanwhile, complex shortwave trough/low energies over the eastern Pacific are expected to track inland across the Southwest and southern Rockies this weekend in southern stream flow to produce swaths of mainly modest precipitation. Guidance has come into better agreement recently with the energy as it reaches the South Sun-Mon. where it should begin to tap a return of downstream Gulf of Mexico moisture to fuel showers and thunderstorms back into the forecast. Models and wet conditions suggest potential for heavy rainfall/runoff issues with this system, mainly from eastern TX through the Southeast. WPC products during this period were primarily derived from a composite blend of reasonably well clustered medium-larger scale guidance from the latest GFS/ECMWF and GEFS/ECMWF ensemble means, the National Blend of Models and WPC continuity. ...Later Monday through next midweek... Behind the aforementioned weekend Southern tier system, upper level ridging will follow into the Western and eventually Central U.S. while another possible closed low/amplified trough drifts slowly eastward towards the West Coast early next week. Energy rotating around the base of this trough should send light-moderate precipitation with another frontal boundary into the Southwest U.S./Southern Rockies which reaches the Southern Plains by next Tue-Wed. This may set the stage for a heavy rainfall/runoff threat for the South and Mid-South into Thu as guidance shows potential to tap deep lead moisture and instability from the Gulf of Mexico under favorable upper jet flow/dynamics. SPC suggests potential for severe weather. Prefer a blend of the well clustered ensemble means with smaller contributions from the ECMWF which offers a deterministic solution closest to the means. The last several runs of the GFS also are slowly trending in the direction of this somewhat less progressive solution. This bolsters forecast confidence in the event, even as timing/detail issues remain. Santorelli/Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Sat-Sun, Apr 18-Apr 19. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Wed, Apr 22. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Northern Plains, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, and the Upper Mississippi Valley. - Much below normal temperatures across portions of the Central Plains and the Middle Mississippi Valley, Sat, Apr 18. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Wed, Apr 20-Apr 22. - Heavy precipitation across portions of mainland Alaska, Sat-Sun, Apr 18-Apr 19. - Much above normal temperatures across portions of mainland Alaska, Sat-Mon, Apr 18-Apr 20. 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