Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Fri Sep 11 2020 Valid 12Z Mon Sep 14 2020 - 12Z Fri Sep 18 2020 ...Guidance/Predictability Assessment... The WPC medium range product suite for early next week was derived from a composite blend of well clustered guidance of the 18 UTC GFS/GEFS mean, the 12 UTC ECMWF/ECMWF ensemble mean, and the 01 UTC National Blend of Models in a period of above average predictability. Forecast spread and uncertainty grows significantly days 5-7 (Wed-Fri) and the models were omitted from the WPC blend. The 18 UTC GFS and now the 00 UTC GFS run have trended more in line with consensus versus the 12 UTC run, but still seem too progressive considering well supported upstream amplification over the Pacific/Alaska. Recent ECMWF runs have trended strongly toward amplification over time, but not always in line with the ensemble means. ...Overview and Weather/Hazard Highlights... A complex evolution over the northeastern Pacific should yield a fairly deep trough/closed low and moderating front slated to work into the Northwest mid-late next week in a wet late summer pattern. Overtop, progressive northern stream flow over southern Canada and the northern U.S. should amplify into mid-later next week in response to upstream amplification aided by energy approach of former Haishen into Alaska. This should favor an upper level trough and ample cold frontal surge outward/down from the n-central U.S states. Meanwhile, warming western and southeastern U.S./western Atlantic ridges may combine to produce a broad southern tier mean ridge, albeit with an embedded weakness over the southern High Plains. Underneath, tropical development being monitored by the NHC may play an enhanced role over the northern Gulf of Mexico and Gulf Coast as moisture feeds into a lingering front stalled draped over the region. Deep moisture and slow system translation suggests a heavy rainfall threat may work inland later next week over the MS/TN Valleys as per the aforementioned northern stream flow amplification and frontal surge interaction. 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