Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1152 AM EDT Sun Apr 28 2019 Valid 12Z Wed May 01 2019 - 12Z Sun May 05 2019 ...Strong to Severe Thunderstorms with Heavy Rains and Flash Flooding/Flooding Potential Will Threaten the Southern Plains and Lower/Middle Mississippi Valley Wednesday into Thursday... ...16Z Update... The overnight model guidance maintained its pattern of showing troughing over the western U.S. midweek with shortwaves moving into the central U.S. on day 4/Thu, and a general model blend worked well there. By Fri and into the weekend, model spread has been high regarding an upper low tracking across the eastern Pacific toward California. Fortunately, the 00Z and 06Z GFS/GEFS have now come more in line with the EC, namely, this southern stream low remains split from the northern stream flow throughout the period, whereas older GFS runs combined the streams and moved the merged trough quickly across the West. The GFS suite remains quicker than the EC with the upper low track, but has been trending slower, and there is much better agreement than previously. However, questions remain downstream across the CONUS by the weekend--the 00Z EC/EC mean trended toward more zonal flow in the central and eastern CONUS, whereas the GFS/GEFS indicated a somewhat more amplified pattern with ridging in the central U.S. and troughing in the East. Tried to keep some amplification to the flow by using some WPC continuity from the night shift by Sat/Sun. Widespread thunderstorms remain likely in the central U.S., continuing from the short range period into Wed/Thu. See the previous discussion for more details. Tate ...Previous Discussion... ...Overview and Assessment of Model/Ensemble Guidance and Predictability... A large closed upper circulation will retrogress and settle over western Canada this week. This occurs as a series of northern stream troughs, height falls, and precipitation swaths dig into the Northwest then eastward over the U.S. northern tier. There is potential for some degree of phasing with a southern stream flow of impulses/height falls working underneath during this period from the eastern Pacific inland over the U.S. southern tier. The GFS/GEFS and to a lesser extent the FV3 have tended to be on the progressive side of the full envelope of solutions with recent ECMWF runs and to a lesser extent ECMWF ensembles and the UKMET/Canadian lie on the slower side of the forecast envelope. The WPC medium range product suite was primarily derived from composite solutions including the National Blend of Models increasingly toward ECMWF ensemble mean guidance over time. This is with consideration of the overall blocky flow upstream over the Pacific, uncertainty with embedded feature timing and phasing in two main stream flows, and the inherently slow nature of closed lows to eject into the flow. ...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/Central Plains. A resultant lead and wavy frontal system will progress mid-late week over the central then eastern U.S. Deepening inflow/moisture from the Gulf of Mexico consistent with ultimate frontal wave developments will feed into convectively active warm sector meso-boundaries as well as a stark baroclinic zone overtop. Strong to severe thunderstorms with heavy rains and flash flooding/flooding potential will focus/threaten the Southern Plains and Lower/Middle Mississippi Valley Wednesday into Thursday. 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. The bulk of activity will progress across the East and South into Friday before shifting offshore by next weekend as new upstream northern and southern stream trough energies/height falls and modest precipitation likely works back into the West in continued split flow. 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 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