Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Wed Sep 01 2021 Valid 12Z Sat Sep 04 2021 - 12Z Wed Sep 08 2021 ...Guidance Evaluation and Preferences... Models and ensembles continue to show fairly good agreement on the large scale upper flow pattern through much of the medium range period for much of the nation, but still struggle with smaller scale individual systems, especially by early next week. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/ECMWF ensemble mean and the 01 UTC National Blend of Models. This maintains reasonably good WPC continuity for a pattern with near average forecast predictability. This forecast baseline remains in line with newer 00 UTC guidance composites despite lingering embedded system timing variances. ...Pattern Overview with Weather/Hazard Highlights... A series of energetic northern stream upper troughs will force several wavy frontal systems into the Northwest and out though the north-central U.S. and increasingly southward into next week across the central and eastern U.S. with main upper trough reinforcement/amplification. Conditions seem best for some locally enhanced lead system convection from the Great Lakes to the Mid-South and central Plains Saturday, with more modest potential downstream more into the southern and eastern U.S. Sunday into Monday. The next trough to move into the north-central U.S. should bring another chance for organized rainfall across parts of the Midwest and Upper Great Lakes by next Monday into Tuesday, with scattered trailing frontal activity then again settling down over the southern and eastern states into next midweek in response to upper trough amplification. The breakthrough approach of northeast Pacific systems into western Canada and the Northwest U.S. may foster some moderate rain potential into northwest WA this weekend. Farther south, lingering deepened moisture may fuel some local downpours as enhanced by terrain over the southern Rockies into Saturday around the western periphery of a potent south-central U.S. upper ridge. The hot ridge shifts slowly westward over the Southwest/West next week forcing a modest monsoonal channel back into southern NM/AZ/CA to the east of an upper troughing off CA. This ridge transition may also offer some room for moisture/convection to lift across the western Gulf of Mexico in about a week. NHC has collaborated bringing a tropical wave into the Bay of Campeche to monitor for any development. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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