Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1249 AM EDT Sun Jun 14 2020 Valid 12Z Wed Jun 17 2020 - 12Z Sun Jun 21 2020 ...Heavy rain possible over parts of the Carolinas and southern Mid-Atlantic... ...Overview and Weather/Hazard Highlights... A mid/upper-level low initially across the Southeast midweek is expected to slowly weaken and move northeast, away from the eastern U.S. The system will produce numerous showers and thunderstorms from the central Appalachians east across the southern Mid-Atlantic region. Clouds and precipitation will result in below average temperatures Wed-Thu as well, with high temperatures forecast to be 5 to 10 deg F below normal. Guidance continues to show a signal for at least locally heavy rains across portions of Southeast Virginia and eastern North Carolina Wed-Thu before activity becomes more sparse later in the week as the upper-level system weakens and moves east. In the wake of this feature, a broad region of weakly positive height anomalies will extend across much of the CONUS southern tier, with an upper-level ridge centered across the Southern Plains expected to gradually expand. This ridge will promote well above average temperatures across much of the central U.S., particularly the Central/Northern Plains Wed-Thu where high temperatures of 5-15 deg F above normal are possible. These temperature anomalies are forecast to spread east across the Great Lakes and Northeast later in the week. A relatively active moderately progressive northern stream will traverse the northern periphery of the ridge. Shortwave energy will eject from a broad upper trough initially over the Northwest, and move east across the northern Rockies/Plains and Midwest. By later in the week, guidance shows consensus that the northwestern trough should progress eastward, with associated height falls reaching the north central U.S. by next weekend. The relatively slow progression of this feature and the growing southern U.S. ridge will cause a frontal boundary to stall from the Central Plains to the Midwest. While moisture will initially be relatively sparse on Wed. Convection associated with this boundary across the central U.S. should gradually become more widespread through time as the larger western trough moves east, and moisture return across the central U.S. improves. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF along with the ECENS/GEFS ensemble means was used through the extended forecast period. Majority weight was placed on the ECMWF during days 3-4 (Wed-Thu), with more weight on the ensemble means during days 5-7 (Fri-Sun). Model variability with respect to the eastern upper low has reduced significantly over the past couple days, and forecast confidence with respect to that feature is now near or perhaps slightly above average. The GFS remained a bit north/west of the overall consensus with this feature, while the ECMWF was closer to consensus, and thus was effectively blended with the means. With the northern stream, guidance has struggled with the timing of shortwave feature, and also with the intensity of the larger trough as it move east. The ECMWF seemed a bit aggressive with deepening an upper low associated with this trough across the north central U.S. late in the week. Thus, the reasoning for going heavily toward the ensemble means by late 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