Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM EDT Thu Jun 18 2020 Valid 12Z Sun Jun 21 2020 - 12Z Thu Jun 25 2020 ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from the 00 UTC ECMWF/ECMWF ensemble mean bridging short to medium range time scales preferences Sunday into early Monday. This mainly addresses a slower ejection of the lingering Mid-Atlantic to offshore New England system in an otherwise reasonable guidance composite. Forecast spread increases significantly though later Monday through next Thursday across the nation in a period seemingly best suited for the blending of more consistent and compatible latest GEFS/ECMWF ensemble means and the 13 UTC National Blend of Models. ...Overview and Weather/Hazard Highlights... It remains the case that 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 and smaller border systems within the northern stream will drift east across southern Canada with associated height falls and a cold front working across the Midwest and 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. The trailing end of this frontal boundary is expected to stall across the warmed Central/Southern Plains and linger through early next week with some reinforcement to serve 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 the Eastern Seaboard by the middle of next week. The s-central Plains and Lower MS Valley may see the greatest potential for locally heavy rain and runoff issues under additional influence of a broad upper-level weakness. 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. Schichtel/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 Hazards: - Heavy rain across portions of the Central/Southern Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Tennessee Valley, Sun-Wed, Jun 21-Jun 24. - Heavy rain across portions of the Southern Plains, Tue-Thu, Jun 23-Jun 25. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Mid-Atlantic, and the Northern Plains. - Flooding likely across portions of the Mid-Atlantic and the Northern Rockies. - Much above normal temperatures across portions of the Central Great Basin, California, and the Northern Great Basin, Tue-Thu, Jun 23-Jun 25. - Much above normal temperatures across portions of the Northeast, Sun-Tue, Jun 21-Jun 23. 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