Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1208 PM EDT Sat Apr 27 2019 Valid 12Z Tue Apr 30 2019 - 12Z Sat May 04 2019 ...Heavy rain and the potential for flooding and flash flooding are expected for parts of the Southern Plains and Lower/Middle Mississippi Valley... ...Overview and Assessment of Model/Ensemble Guidance and Predictability... A large upper low located in central Canada on day 3/Tue will retrograde westward through the medium range period, encroaching upon an upper ridge set up in Alaska and into the Arctic. Uncertainty remains regarding the orientation of the low and vort maxes within it, as well as the amount of energy that could stream underneath the aforementioned ridge. The 00Z deterministic ECMWF is the most bullish with that energy by day 6/Fri, which suppressed ridging in the northern Pacific. This was unsupported by other models and ensembles, so leaned away from that solution for now. Across the CONUS, a general pattern of upper troughing will remain across the West through midweek, while a subtropical ridge persists over the Southeast. A shortwave is expected to strengthen and move through the Midwest by Thursday. Additional energy may push through the southern stream across the Southern Plains/Lower Mississippi Valley at that point too, with the 00Z and 06Z GFS runs more bullish with this feature than the EC. Nevertheless, expect the upper trough to push eastward late in the week with these features, leading to high pressure at the surface for the central and eastern parts of the country by Fri/Sat. Model disagreement increases by day 6 and especially day 7/Sat for an upper low in the eastern Pacific approaching the West Coast; the 00Z EC keeps the low in the southern stream, while particularly the 00Z GFS combines the northern and southern streams and leads to a more progressive trough in the West. At this point, a compromise seems reasonable, but leaned toward the EC mean a bit as the GFS has more run-to-run variability and is often too fast. Overall, a multi-model mainly deterministic blend seemed like the best solution early in the period, gradually transitioning to favoring the EC and GEFS means by late in the week. ...Weather Highlights/Threats... With the troughing in the West early in the period, temperatures will be cold enough to support snow mainly in the Northern/Central Rockies, though late season light snows are possible in the Northern Plains. The main threat in the medium range, however, is for heavy rainfall developing in the center of the country as moisture streams in ahead of the trough and combines with a slow-moving/wavy front. Multi-day totals are expected to be several inches for the Southern Plains and the Lower/Middle Mississippi Valley as multiple rounds of rain and thunderstorms (potentially severe) will occur. Flooding remains possible as particularly the Mississippi Valley has had flooding concerns for weeks, and with the convective nature of the precipitation, flash flooding is possible as well. There is some potential for heavy rain to spread over the Ohio Valley and Great Lakes region at times as well. The front is expected to push eastward ahead of the trough by Fri/Sat, helping lessen the heavy rain threat. Tate 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 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