Extended Forecast Discussion NWS Weather Prediction Center College Park MD 100 AM EDT Mon Jun 15 2020 Valid 12Z Thu Jun 18 2020 - 12Z Mon Jun 22 2020 ...Overview and Weather/Hazard Highlights... A mid/upper-level low initially across the Southeast early in the forecast period is forecast to slowly weaken and move northeast, away from the eastern U.S. over the weekend. As it weakens, the coverage and intensity of the associated precipitation should gradually diminish, with the threat of heavy rains likely coming to an end late in the short range forecast period. Clouds and some precipitation will continue to result in modestly below average temperatures across portions of the Southeast and Mid-Atlantic through Thu, however (highs 5 to 10 deg F below average). In the wake of this feature, a broad region of weakly positive height anomalies will expand across much of the CONUS southern tier, from an upper-level ridge initially centered across the Southern Plains. In association with the ridge, above average temperatures are expected from the Plains/Midwest to the Great Lakes and Northeast Thu through the weekend, gradually becoming predominantly focused in the Northeast over time. A relatively active northern stream will traverse the northern periphery of the ridge across Canada and the CONUS northern tier. Shortwave energy is forecast to quickly deepen across the Canadian prairies as it exits the Rockies, with most guidance now showing the development of an upper low along the lines of what the ECMWF showed yesterday. Greater confidence in the development of this closed upper low slows down the progression of the feature quite a bit, but the trailing surface front is still expected to linger across the Central Plains and Midwest for several days, where it will serve as a focus for the development of shower and thunderstorms, with at least locally heavy rains possible. As the frontal system begins to move farther east by early next week, showers and thunderstorms will also spread east across the Great Lakes and into portions of the eastern U.S. by next Mon. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF/UKMET was heavily used during days 3-4 (Thu-Fri), with weight shifted more toward ECENS/GEFS ensemble means during days 5-7 (Sat-Mon). The ECMWF/UKMET were closely aligned with the overall ensemble consensus during that time frame. The GFS was slower than the overall consensus with lifting the upper low out of the eastern U.S. Additionally, the GFS has struggled with the northern stream upper low, and while the 18Z run trended significantly toward the ECMWF, would like to see more run-to-run stability in the GFS before including it in the preference. Finally, the GFS was faster than the ensemble consensus bringing another shortwave into western Canada and the Pacific Northwest by late in the forecast period. These considerations drove the preference toward the ECMWF/UKMET initially in the medium range, and the shift to more ensemble mean weighting later in the forecast period. Ryan 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