Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM EDT Sun Aug 29 2021 Valid 12Z Wed Sep 01 2021 - 12Z Sun Sep 05 2021 ...Ida Heavy Rainfall/Flooding Threat from the Central Gulf Coast Today to the Mid-Atlantic/Northeast Midweek... ...Nora lead moisture to Fuel a Southwest U.S. Heavy Rain Threat mid-later week... ...Emerging Locally Heavy Convective Rainfall Pattern for the Northern Plains/Upper Midwest Mid-later week... ...Guidance Evaluation and Preferences... Impending hazardous high impacts are expected from Major Hurricane Ida's looming landfall into Louisiana as well as Hurricane Nora that is forecast to track northward up the Gulf of California and into northwest Mexico. Please refer to products from the National Hurricane Center and the local offices for the latest regarding these dangerous systems. The WPC medium range product suite was primarily derived from a composite of best clustered guidance from the 12 UTC ECMWF/ECMWF ensemble mean and 18 UTC GEFS mean. Of the models, the 12 UTC ECMWF has the best ensemble support for both northern stream system transitions and convection potential and also fits close to the latest Ida and Nora forecasts from the National Hurricane Center. The pattern offers better than average forecast predictability, albeit with some lingering model system timing variance that seems to have decreased marginally with advent of latest 00 UTC model runs. ...Pattern Overview with Weather/Hazard Highlights... Dangerous Hurricane Ida is expected to strengthen further over the Gulf of Mexico as it moves steadily towards the northwest with an expected landfall along the Louisiana coast today as a major hurricane. By the start of the medium range period Wednesday and lingering into Thursday, a steadily weakening/exiting but still very wet Ida is forecast to spread a heavy rainfall threat from the upper Ohio Valley/Central Appalachians to the Mid-Atlantic and southern portions of the Northeast as leading deep moisture interacts with a slow moving front draped over the system. Cell training and terrain will significantly enhance the runoff threat within a well defined precipitation axis. The latest track guidance for Hurricane Nora brings it northward into the Gulf of California and eventually into northwest Mexico, with the expectation that it will weaken rather quickly as it interacts with the Mexican terrain. Leading deep tropical moisture ahead of Nora will likely fuel a Southwest U.S. heavy rain threat by the middle of the week. The guidance continues to show some uncertainty in the track and certainly a track for longer over the Gulf waters could bring a more substantial heavy rainfall threat into the Southwest. By later next week, leading moisture may also bring moderate to locally heavy, and terrain enhanced rains into the central Rockies as well. Elsewhere, ample northern stream upper troughing will meanwhile dig a wavy and modestly unsettling/cooling front down from the Northwest. Subsequent system progressions and genesis mid-later week out across the Rockies to the north-central U.S. will increasingly focus an enhanced convection/rainfall pattern that should focus locally heavy convection/rainfall as enhanced by embedded impulses and frontal/wave reinforcements. Moisture/instability pooling and height falls will likely support some locally strong to severe convection and heavy downpours. 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