Extended Forecast Discussion NWS Weather Prediction Center College Park MD 504 PM EDT Tue Aug 25 2020 Valid 12Z Fri Aug 28 2020 - 12Z Tue Sep 01 2020 ...Laura heavy rain and strong convection threat with lead tropical moisture from Mid-South/Midwest to Appalachians/Mid-Atlantic/Northeast Fri-Sat... ...Overview and Guidance/Predictability Assessment... Latest ensembles still show consensus that a large scale flow amplification is likely to occur, from Asia eastward across the Pacific to North America. This process continues to appear to begin with the extratropical transition of Typhoon Bavi across northeast China Thu-Fri, which triggers a downstream wave train and flow amplification that propagates eastward across the North Pacific, reaching North America over the weekend. The manifestation of this process should initially be development of a relatively deep upper-level trough across western Canada/CONUS, which then spreads eastward into the central U.S. by next Tue, along with a strong early fall-like cold front. Farther east, Laura (forecast to be weakened to a tropical depression) is expected to move from the lower Mississippi into the Tennessee Valley on Fri, and across the Appalachians/Mid-Atlantic on Sat. Please refer to the latest advisories issued by the National Hurricane Center for the latest information on Laura. An amplifying shortwave moving from the Midwest to the Northeast over the weekend will eventually absorb the remnants of Laura as the system becomes extratropical. The WPC medium range product suite used a composite blend of the GFS/ECMWF, GEFS/ECMWF ensemble means and National Blend of Models. Forecast clustering and predictability seen better than average other than Laura. NHC forecasts were inserted for Laura. This solution maintains great WPC continuity. ...Weather/Hazard Highlights... Heavy rain and flooding/runoff/wind impacts are possible near the track of Laura as it moves inland and weakens to a tropical depression. A lower MS/TN/lower OH Valleys threat will shift to the Appalachians/Mid-Atlantic Fri-Sat. An additional strong to severe thunderstorm and rainfall threat will be also driven by northern stream energies with a wavy front overtop from the Midwest to the Northeast, enhanced by an advance tropical moisture feed. The deep upper-level trough digging into the western U.S. over the weekend, and into the north central U.S. early next week will be accompanied by a strong early fall-like cold front. Ahead of the trough and cold front, relatively widespread showers and thunderstorms are expected from the Central/Northern Plains to the Mississippi Valley Sun-Tue, with areas of heavy rain possible. High temperatures 10 to 20 deg F below average will spread into the northern Rockies Sun, and into the north central U.S. Mon-Tue. Favored northern Rockies spots may have their first freezing temperatures in some time Mon-Tue mornings. Highs in the 60s and 70s will be common across the Central/Northern Plains and Upper Midwest Mon-Tue. Ryan/Schichtel Hazards: - Heavy rain possible across the interior Deep South, portions of the Tennessee and Ohio Valleys, the lower Great Lakes, the Appalachians, and from the Mid-Atlantic to much of the Northeast, Fri-Sat, Aug 28-Aug 29. - Heavy rain possible from the upper Midwest down into the Mid-Mississippi Valley and the nearby central Plains, Sun-Mon, Aug 30-Aug 31. - Heavy rain possible from the central Plains northeastward across the Midwest into the lower Great Lakes, Tue, Sep 1. - Severe weather possible across the Midwest into portions of the lower Great Lakes and northwestern Pennsylvania, Fri, Aug 28. - Severe weather possible across the northern Mid-Atlantic and nearby central Appalachians into central New England, Sat, Aug 29. - Flooding possible west of the lower Mississippi Valley and into southeastern Texas. - Excessive heat across portions of far southern Texas, Fri-Sat, Aug 28-Aug 29 and Mon-Tue, Aug 31-Sep 1. - Excessive heat across portions of southern Texas, Fri, Aug 28. - Much below normal temperatures across portions of northwestern Wyoming, Tue, Sep 1. 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