Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 AM EDT Tue Aug 25 2020 Valid 12Z Fri Aug 28 2020 - 12Z Tue Sep 01 2020 ...Heavy rain from Laura is expected to spread farther inland from the lower Mississippi Valley across the lower Ohio Valley and possibly into the southern Mid-Atlantic through 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, over the next several days. As described in recent days, the process appears 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. Several model solutions suggest that this process may result in fairly quick deepening of an extratropical cyclone off the northeast U.S. coast Sat night/Sun morning. The UKMET has consistently been the most bullish on that scenario, but a number of model solutions show some level of extratropical cyclone development. The WPC medium range forecast was based on a blend of the 18Z GFS/12Z UKMET and 12Z ECENS/18Z GEFS ensemble means. More weight was applied toward the deterministic solutions during days 3-4 (Fri-Sat), with a shift toward more weight on ensemble means during days 5-7 (Sun-Tue). Forecast confidence on the described large scale pattern evolution is slightly above average given good ensemble consensus. Spread with respect to the details increased through time, and increased use of ensemble means helped to smooth out some of these differences with lower predictability at longer time scales. ...Weather/Hazard Highlights... Heavy rain as well as flooding/runoff impacts will be possible along the track of Laura as it moves inland as a tropical depression. The lower Mississippi Valley and Tennessee Valley will see these threats Fri/Fri night, shifting east into the southern Appalachians and possibly southern portions of the Mid-Atlantic on Sat. Sat night into early Sun, as the merged extratropical low pressure system deepens off the Northeast coast, moderate to heavy rain, and possibly gusty winds (depending on the eventual depth of the low) will be possible across New England, especially for areas along the coast. 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-Mon, 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. The higher elevations of the northern Rockies may see 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 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