Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1228 AM EDT Thu Jun 18 2020 Valid 12Z Sun Jun 21 2020 - 12Z Thu Jun 25 2020 ...Overview and Weather/Hazard Highlights... A slow moving trough/upper-level low centered over the Mid-Atlantic on Sun is forecast to gradually weaken as it drifts northeastward through early next week, eventually merging into stronger northern stream westerlies. Upstream, another slow moving upper low in the northern stream will drift east from northern Manitoba to northern Ontario through early next week with associated height falls and a cold front lingering over the Upper Midwest to central Great Plains. Ahead of this trough/cold front, high temperatures are forecast to remain 10 to 15 deg F above average from the Great Lakes to the northern New England through much of next week. The trailing end of this frontal boundary is expected to stall across the Central/Southern Plains and linger through early next week with some reinforcement, and serving as a focus for shower and thunderstorm development. As the trough/upper low moves farther east, shower and thunderstorm activity should shift farther south and east, from the Southern Plains and Lower Mississippi Valley to the Ohio Valley then the Appalachians and eventually the Eastern Seaboard by the middle of next week. The Southern Plains may see the greatest potential for locally heavy rain through much of next week as a broad upper-level weakness persists overhead. In the West, upper-level ridging should gradually strengthen and drift northeast over the Desert Southwest through early next week, before weakening some by the middle of next week. Shortwave energy and a cold front from a Gulf of Alaska low will push across Washington state through Sunday, with another front approaching later in the week. Given that the Sunday system will be heading into broad anticyclonic flow, and will be weakening as it moves ashore, precipitation across the Northwest should be relatively light, and primarily across the Olympics and northern Cascades. Additionally, as heights rise across the West next week, warming temperatures are also expected, with highs forecast to be 5 to 15 deg above average Sun to Thu across much of the Great Basin and eventually the interior Northwest. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF along the the ECENS/GEFS ensemble means was used as a basis for the WPC forecast throughout the extended period. The deterministic ECMWF was weighted heavily during days 3-4 (Sun-Mon), with increased/majority weight toward the ensemble means from day 5 (Tue) through day 7 (Thu). The largest differences in the guidance were noted from the Rockies into the Northern Plains toward the middle of next week. Models have quite different handling of Pacific shortwave energy moving into western Canada, with the GFS keeping a stronger ridge across the Southwest, and deflecting this shortwave farther north across Canada, and the ECMWF weakening the ridge, allowing the incoming shortwave to deepen as it crosses central Canada and begins to phase with the broader system farther east. Ensemble means were somewhat more agreeable with a bit of a compromise solution, and thus the logic for trending heavily toward the means by the second half of the forecast period. Regardless, there does seem to be better consensus that the pattern at the large scale should evolve toward some degree of ridge axis across the Rockies, and some degree of troughing across eastern Canada and the Great Lakes/Midwest. The implications of this perhaps being a return toward wetter conditions across the eastern third of the CONUS, and a continuation of fairly wet conditions across the south central U.S. by that time and beyond. 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