Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1127 AM EDT Mon Jun 03 2019 Valid 12Z Thu Jun 06 2019 - 12Z Mon Jun 10 2019 ...Excessive Rain and Flash Flood/Flooding Threat to slowly spread Thursday to next Monday from the south-central Plains and Mid-Lower Mississippi Valley to the Tennessee/Ohio Valleys and Southeast/Appalachians/Mid-Atlantic... 1530 UTC Update... Models have remained relatively consistent through the medium range, and the forecast was not changed significantly in this update. A blend of the 00Z ECMWF/06Z GFS was used to incorporate the latest deterministic guidance through much of the forecast period, with a gradual increase in weighting of ECENS/GEFS ensemble means during the mid to latter portion of the period. Forecast confidence is slightly above average at the large scales with the overwhelming majority of solutions showing an upper ridge axis rebuilding off the Pacific Northwest coast by early next week, with an amplified upper trough axis shifting into the Northern Plains, and from the Ohio Valley north toward Hudson Bay. The main points of contention among the guidance are the specific track/timing of slow-moving southern stream shortwave energy moving from the Southern Plains to the Mississippi/Ohio Valleys Thu-Sun, and its interaction with whatever is left from the system currently in the Bay of Campeche as it moves north. Further, while solutions are similar in terms of large scale timing of the amplified western trough, detail differences remain as to the specifics of individual shortwaves traversing the larger trough and their impacts on where the strongest height falls occur and when. Given no clear outliers, the aforementioned model blend should smooth out some of these differences and represent a consensus at this time. See previous discussion below for further details on the excessive rain/flash flooding threat expected across much of the central/eastern U.S. during the medium range. Ryan Previous Discussion (issued at 0540 UTC)... ...Guidance/Predictability and Weather/Hazards Highlights... An amplified upper-level trough/height falls and surface system in a split southern stream will track from the south-central Plains to the Appalachians. Deep moisture return from the Gulf of Mexico will feed into the system to fuel a widespread threat of heavy rain and flash flooding/flooding. There is a strong pattern and guidance signal in support of an excessive rain and flash flood/flooding event to slowly spread Thursday-next Monday from the south-central Plains/Mid-Lower Mississippi Valley to the Tennessee/Ohio Valleys and Southeast/Appalachians/Mid-Atlantic. This threat will include a risk of repeat/training of convective cells considering the slow system progression as common in guidance. Cumulative WPC day 4-7 qpf offers an expansive 2-5" mean areal rainfall that implies much higher, but less predictable local amounts. These are partly contingent on the less certain full extent of deepest tropical moisture to pool into the system and the National Hurricane Center continues to monitor a disturbance over the Bay of Campeche. Meanwhile, models continue to show development of a significant/anomalous cooling upper trough from the northern stream flow to move across the Northwest/West to the north-central Plains late week into early next week. While guidance seems to be slowly converging on a more clustered forecast, bolstering confidence, there still remains quite a bit of uncertainty with respect to the amplitude and evolution of this trough and heavier precipitation focus. The ECMWF and to a lesser extent ECMWF ensembles have been forecasting a more amplified and slower progression than most other guidance. WPC favors a solution slightly on the amplified side of the full guidance envelope given overall blocky hemispheric flow pattern characteristics. This would also favor June snows for the higher elevations of the northern Rockies in this pattern as highlighted in the WPC medium range winter weather outlook probabilities. Overall, the WPC medium range product suite was primarily derived from a composite of the 12 UTC GFS/FV3/ECMWF/UKMET and GEFS/ECMWF ensemble mean along with the National Blend of Models, shifting WPC blend weightings slightly in favor of the ECMWF/ECMWF ensemble mean over time. This solution maintains good WPC continuity. 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