Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Aug 31 2021 Valid 12Z Fri Sep 03 2021 - 12Z Tue Sep 07 2021 ...Guidance Evaluation and Preferences... Models and ensembles offer a reasonably similar larger scale upper flow pattern evolution through much of the medium range, but offer more rampant embedded system and local weather focus variance from run to run that may be best addressed with increased reliance of more steady ensemble mean guidance, especially from the weekend into next week. Accordingly, the WPC medium range product suite was primarily derived from a GFS/ECMWF and ensemble mean composite day 3/Fri before switching to primarily the ECMWF ensemble mean and the National Blend of Models onward for days 4-7. This maintains best WPC continuity. ...Pattern Overview with Weather/Hazard Highlights... Ida is forecast by WPC to become a well organized post-tropical cyclone that will be lifting/exiting offshore New England Friday. Uncertain interaction from northern stream energies may alter Ida's track and development as a maritime hazard as collaborated with OPC. Ida is expected to be absorbed into a well organized northern stream low over the Canadian Maritimes whose wrapping rains may prove slow to exit northern New England into Saturday given closed low/upper trough support and proximity. A moderating trailing front will settle down into the Southeast. Upstream, a continued series of 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 into the east-central U.S. with main upper trough reinforcement/amplification. Conditions seem best for some locally enhanced convection from the central Plains to Upper Midwest Friday, with more modest potential downstream into the east-central U.S. in the wake of Ida. Meanwhile, lingering deepened moisture from Nora may fuel some locally heavy downpours as enhanced by terrain over the southern Rockies Friday into Saturday around the western periphery of a potent south-central U.S. upper ridge. The ridge shifts slowly westward over the Southwest forcing a modest monsoonal channel back into southern NM/AZ/CA to the east of an upper trough off CA into next week. Meanwhile, the breakthrough approach of northeast Pacific systems into western Canada and the Northwest U.S. will foster advent of a periodically wet pattern into western WA from this weekend into next week. 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