Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Sun Aug 29 2021 Valid 12Z Wed Sep 01 2021 - 12Z Sun Sep 05 2021 ...Ida Heavy Rainfall/Flooding Threat continuing into the Mid-Atlantic/Northeast on Thursday... ...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... The latest guidance continues to struggle with timing of Ida as she begins her extratropical cyclone transition and exits the Northeast Coast Wednesday into Thursday. The 12z UKMET/00z ECMWF remain on the slower side of the envelope to exit the coast, while the 06z/12z GFS and CMC are faster. The ensemble means generally fall somewhere in the middle, also consistent with the official NHC track, at least through day 5. Elsewhere, no major differences to note. There are some uncertainties with structure/timing of an upper low in central Canada which impacts details of QPF and frontal structure/timing across the north-central U.S. into next weekend. This cycle of the WPC forecast relied more heavily on the deterministic models the first half of the period with more emphasis towards the 00z ECMWF which was closest to the latest Ida track from NHC. After day 5, the forecast trended quickly towards the ensemble means which helps mitigate the detail differences across the northern tier states. This approach maintained good continuity with the previous WPC forecast as well. ...Pattern Overview with Weather/Hazard Highlights... Dangerous Hurricane Ida made landfall this afternoon over southeast Louisiana. By the start of the medium range period on Wednesday, Ida should be transitioning to a post/extra tropical cyclone as it moves across the Mid-Atlantic off the coast on Thursday. Regardless, a heavy rainfall threat from Ida will extend from the eastern 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 and flood threat within a well defined precipitation axis. The latest track guidance for Tropical Storm Nora brings it northward along the western Mexico coast, and into Northwest Mexico and will likely weaken rather quickly as it interacts with the Mexican terrain. Leading deep tropical moisture ahead of Nora will continue to fuel a Southwest U.S. heavy rain threat into the start of the period on Wednesday. There remains some uncertainties regarding how fast Nora degenerates and that certainly would have implications for total rainfall across the Southwest. Moisture from Nora may also bring moderate to locally heavy, and terrain enhanced, rains into parts of 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 across the Dakotas and into the Upper Mississippi Valley Thursday and Friday. 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