Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Aug 26 2020 Valid 12Z Sat Aug 29 2020 - 12Z Wed Sep 02 2020 ...Appalachians to Mid-Atlantic and Northeast Heavy Rain Threat Saturday... ...Central U.S. Heavy Rain Threat... ...Overview and Guidance/Predictability Assessment... 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 00 UTC ECMWF/UKMET and 00 UTC ECENS/06 UTC GEFS ensemble means Sat into Mon. The ECMWF/UKMET seemed a better fit with the NHC forecasts for speed and organization of Laura than faster recent GFS runs. Growing forecast spread and run to run variances then led to using just these ensemble means into days 6/7. ...Weather/Hazard Highlights... Heavy rain and runoff impacts are possible with Laura Saturday for the Central Appalachians/Mid-Atlantic then coastal New England. There is also a threat of severe weather and heavy rain with favorable northern stream upper support from the northern Mid-Atlantic to the Northeast as deep moisture feeds into a wavy cold front. 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 front, relatively widespread showers and thunderstorms are expected from the Central/Northern Plains to the Mississippi Valley into early next week, with areas of heavy rain possible. Guidance shows a signal for more widespread heavy rains for portions of the Mid-MS Valley and vicinity with enhanced frontal moisture feed and convergence. High temperatures 10 to 15 deg F below average should spread to 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 recent ECMWF and to a lesser extent Canadian solutions that dig a western U.S. closed trough materializes, this would limit the extent of cooler air into the central U.S., as the upper trough would be a bit more limited in magnitude over the north central U.S.. The deeper western trough scenario might also favor more monsoonal flow and rainfall return into the Southwest. Ryan/Schichtel 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