Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Jun 02 2019 Valid 12Z Wed Jun 05 2019 - 12Z Sun Jun 09 2019 ...A heavy convection/rainfall and flood/flash flooding threat to spread slowly northeastward next week from the south-central Plains to the lower and mid Mississippi Valley, Tennessee Valley and lower and mid Ohio Valley... ...Overview and Weather/Threats Highlights with Guidance/Uncertainty Assessment... Split flow remains evident over North America. In an active southern stream an amplified upper-level trough will be kicked slowly from the Southwest to Mid South next week together with tropical moisture currently consolidating over the southern Gulf of Mexico. This would fuel a potentially heavy pre-event and system proximity rainfall threat to spread from southern Texas to the Ohio Valley. Models and ensembles are in decent agreement on the slow progression of the upper-level trough and associated and moisture/instability pooling surface low. This pattern by itself would represent a heavy rain threat. The more uncertain part of the forecast continues to be with a potential tropical system that may consolidate over the Bay of Campeche and how much of the associated moisture will be drawn northward into the southern Plains and lower MS Valley. Recent GFS runs jave taken the system toward the Mexican mountains and deposits much of the moisture in Mexico before drawing the remaining moisture northward into the Deep South. Recent ECMWF runs tend to track the system near or just off northern Mexico, then toward up into the West Gulf Coast. Recent Canadian models represent the most convective solution with the eastern-most extreme track, taking the system faster and stronger along the Texas coast next Wednesday and across Louisiana next Thursday. With the consideration of the upper-level trough moving slowly eastward across the Plains next week and the convective nature of the tropical system, the eastern solutions indicated by at least the ECMWF and cannot be ruled out, but early indications from the NHC seem to favor system dissipation over Mexico. Meanwhile, potentially amplified northern stream flow offers a mean upper trough over the Northwest and lead upper trough well downstream over New England, sandwiching warming upper ridging. The most uncertain part of the forecast concerns amplitude and progression of these moderate precipitation focusing and cooling troughs late next week. The ECMWF and to a lesser extent ECMWF ensembles have been forecasting a more amplified and slower progressing pattern than other guidance. WPC favors a solution slightly on the more amplified side of the full envelope of solutions given overall blocky hemispheric flow pattern characteristics. This better favors some June snows for the northern Rockies. Overall, the WPC medium range product suite was mainly derived from a 18/00 UTC GFS/GEFS mean and 12 UTC ECMWF/ECMWF ensemble mean blend. Blend weightings slightly favor amplitude and shift more in favor of the ensemble means days 5-7. In this pattern there remains a growing signal in support of a heavy rainfall threat next week from the south-central Plains to the mid-lower Mississippi Valley, Tennessee Valley and lower-mid Ohio Valley in a region already inundated with heavy rainfall and flooding issues. Kong/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