Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EDT Tue Aug 31 2021 Valid 12Z Fri Sep 03 2021 - 12Z Tue Sep 07 2021 ...Guidance Evaluation and Preferences... The models and ensembles continue to show fairly good agreement on the large scale upper flow pattern through much of the medium range period, but struggle with smaller scale embedded details of individual systems, especially by early next week. The most notable area of uncertainty continues to be surrounding what is left of Tropical Depression Ida by Friday and Saturday (likely a post-Tropical system). The ECMWF has been consistently on the slower side of the guidance to lift the system out of the Northeast, while the GFS continues to be quite a bit faster. The CMC, along with the ensemble means, at this point, seem to offer the best middle ground solution which is also consistent with the latest track guidance from the WPC tropical advisories for Ida. Including Ida, and across the rest of the CONUS, a general model blend on days 3-4 between the ECMWF/CMC (and a minor percentage of the GFS) worked well as a starting point for the WPC medium range. For days 5-7, incorporated increasingly more weighting of the ECENS to help mitigate some of the detail differences surrounding energy into eastern Canada and the upper Great Lakes. This approach fit reasonably well with previous shift continuity as well. ...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. 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. 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. Meanwhile, lingering deepened moisture from was was previous tropical cyclone 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 this weekend. Santorelli/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