Extended Forecast Discussion NWS Weather Prediction Center College Park MD 218 AM EDT Tue Aug 06 2019 Valid 12Z Fri Aug 09 2019 - 12Z Tue Aug 13 2019 ...Overview and Guidance/Predictability Assessment... An upper low is expected to drift slowly eastward across James Bay and northeastern Canada through the medium range, with broad cyclonic flow south of the low across the eastern third of the CONUS. Several shortwaves will traverse this zone of cyclonic flow, the most pronounced of which should move a cold front to the Eastern Seaboard on Fri, with the trailing end of the front lingering from the Southern/Central Plains to the Southeast into the weekend. Farther west, an upper low/trough initially off the West Coast is forecast to push inland by Sat, moving into the northern Great Basin and northern Rockies by Sun, and the Northern Plains by Mon. Meanwhile, broad upper ridging is expected to remain centered across the south central U.S. An additional shortwave appears likely to approach the Pacific Northwest early next week, although models differ quite a bit by that time on the flow pattern across western/central Canada and the northwestern U.S. The 18Z GFS along with the 12Z ECMWF/CMC showed sufficient consensus to use a blend of these solutions as a basis for the forecast during days 3-4 (Fri-Sat). Starting on day 5 (Sun), models continue to show differences with the handling of the shortwave crossing the U.S. northern tier, and more generally with the character of the northern stream flow from the Gulf of Alaska into the northwestern U.S. The ECMWF shows more progressive flow by the weekend into early next week across these areas, while the GFS is somewhat less progressive, with a more amplified trough offshore of British Columbia, and a slower eastward progression of incoming additional shortwave energy as well as the shortwave crossing the northern Rockies. Based on teleconnections associated with a Rex block from the western Bering Sea into the Arctic Ocean, preferred to lean toward the somewhat more amplified and slower solution shown by the GFS by early next week. The 00Z GFS, which ran after completion of the WPC forecast, doubled down on this idea even further. Given these considerations, the forecast for days 5-7 (Sun-Tue) leaned gradually more heavily on the 18Z GFS/GEFS, with less weight on the 12Z ECMWF and ECENS. ...Weather Highlights/Threats... The central U.S. will be the focus for potentially multiple rounds of showers and thunderstorms with heavy rainfall. Models continue to show a signal for an area of organized convection with heavy rainfall across portions of the Mid-Mississippi Valley on Fri, along the trailing end of the frontal boundary moving off the Eastern Seaboard. The Pacific shortwave will initially bring areas of heavy rain to portions of the interior Pacific Northwest Fri-Sat before bringing showers and thunderstorms to the Northern/Central Plains and portions of the Midwest Sun-Mon. Convection appears likely to focus along a warm front, with models showing the potential for heavy rainfall from portions of the Central Plains east into the Midwest. Meanwhile, monsoonal moisture will keep diurnal showers and thunderstorms active across the Four Corners region and the southern/central Rockies through the forecast period, with a daily threat for locally heavy rainfall. The upper ridge will keep hot conditions in place for the Southern Plains, with high temperatures forecast to be 5 to 10 deg F above average through early next week. Cooler temperatures will accompany the Pacific shortwave inland, with highs forecast to be 5 to 15 deg below average from the Pacific Northwest to the northern Rockies Sat-Sun, spreading into the Northern Plains by 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