Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 AM EDT Wed Aug 26 2020 Valid 12Z Sat Aug 29 2020 - 12Z Wed Sep 02 2020 ...Heavy rains associated with Laura possible from the Central Appalachians to the Mid-Atlantic on Saturday... ...Overview and Guidance/Predictability Assessment... Ensembles continue to show consensus that a large scale flow amplification is likely to occur, from Asia eastward across the Pacific to North America. This process begins with the extratropical transition of Typhoon Bavi across northeast China Thu-Fri, which triggers a downstream wave train and flow amplification propagating 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 and the northwestern U.S., which then spreads eastward into the north central U.S. early next week, along with a strong early fall-like cold front. The ECMWF made a bit of a change to the evolution of this pattern with the 12Z run, separating a portion of the energy from the jet and developing a cut off upper-level low across the Great Basin next Tue-Wed. In many cases, such a change from previous runs would often be discounted, but looking at the ensembles, a sizable number of members in both the ECENS and GEFS ensemble systems also showed this trend. Further, the overall consensus of members, even those that do not cut off an upper low, have shifted to be slower with the eastward progression of the trough, with the GFS appearing to be the solution lagging this trend. Given the variability, confidence in this aspect of the forecast was a bit below average. Farther east, Laura (forecast to be weakened to a tropical depression) is expected to move across the Central 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. A frontal boundary trailing this extratropical system is forecast to stall/linger across the south central U.S. over the weekend and into early next week, before moving north as a warm front ahead of the incoming north central U.S. upper trough. The WPC medium range forecast was based on a blend of the 12Z ECMWF along with ECENS/GEFS ensemble means. The ECMWF was weighted most heavily during days 3-5 (Sat-Mon), with a transition to slightly more ensemble weighting days 6-7 (Tue-Wed). Since the ECMWF solutions of a cutoff upper low in the West early next week was considered a distinct possibility, a significant component of that solution was used through the end of the forecast period. NHC forecasts were inserted for Laura. ...Weather/Hazard Highlights... Heavy rain along with potential flooding impacts are possible across portions of the Central Appalachians and southern Mid-Atlantic on Sat as Laura quickly traverses those regions, along with an incoming Midwest/Great Lakes shortwave and incoming cold front. The cold front may focus additional potential for severe thunderstorms and heavy rain across portions of the Mid-Atlantic/Northeast on Sat as well. 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. Model/ensemble guidance shows a signal for more widespread heavy rain potential across portions of the Mid-Mississippi Valley Mon-Tue along a stalled frontal boundary. High temperatures 10 to 15 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. One potential caveat to note, if the ECMWF solution for a more significant western cutoff low materializes, this would limit the extent of cooler air spreading east into the central U.S., as the upper trough would be a bit more limited in magnitude across the north central U.S., compared to a scenario where all energy remains consolidated into one larger upper-level trough. 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