Extended Forecast Discussion NWS Weather Prediction Center College Park MD 514 PM EDT Fri Aug 27 2021 Valid 12Z Mon Aug 30 2021 - 12Z Fri Sep 03 2021 ...Hurricane Ida will bring a heavy rainfall/flooding threat from the central Gulf Coast to the Mid-Atlantic through Thursday... ...Tropical Storm Nora, forecast to become a Hurricane, to fuel a Southwest U.S. heavy rain threat mid-later next week... ...Guidance Evaluation and Preferences... The latest guidance remains reasonably clustered across the board for days 3-5 so a majority blend of the ECMWF/GFS serves as a good starting point for the WPC forecast. By the second half of next week, guidance begins to differ mainly in the details and timing of digging energy across the Western U.S. and progression of the leading frontal boundary. The 00z ECMWF is noticeably faster with that leading shortwave days 6-7 than the GFS, but the ensemble means are a little more agreeable and give a good middle ground solution at this this point. Thus, the WPC forecast blend leans more on the ECENS/GEFS ensemble means by later in the period. The big story however during this period continues to be impending hazardous impacts from Hurricane Ida in the Gulf of Mexico and Tropical Storm Nora, forecast to track up the Gulf of California. The WPC forecast for these systems closely follows the latest track guidance from the National Hurricane Center, though the ECMWF seems to be the best proxy for Ida at least. Please refer to products from the National Hurricane Center and the local offices to obtain the latest information regarding both of these dangerous tropical systems. ...Pattern Overview with Weather/Hazard Highlights... Hurricane Ida is expected to strengthen further as it moves steadily towards the northwest with an expected landfall along the Louisiana coast late Sunday or very early Monday as a major hurricane. By Monday morning (the start of the extended range), Ida should be bringing gusty winds and very heavy rainfall to parts of the central Gulf Coast region with total rainfall (Sunday-Tuesday) in excess of 6 inches (with locally much higher amounts) likely from southeast Louisiana into Mississippi. After Monday, Ida is forecast to make a sharp turn towards the east-northeast spreading the heavy rainfall threat into the Tennessee/Lower Ohio Valleys on Tuesday/Wednesday. Considerable flash, urban, small stream, and riverine flooding is likely associated with Ida. Moisture feeding increasingly inland with time may also then interact with a slowing front draped over the system, offering a lead heavy rainfall focus into parts of the central Appalachians and Mid-Atlantic after Wednesday. Tropical Storm Nora is also forecast to become a Hurricane with the current track guidance from NHC bringing it northward into the Gulf of California and eventually into far northeast Mexico. Leading deep tropical moisture ahead of Nora will likely fuel a Southwest U.S. heavy rain threat next 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, a northern stream upper trough and lower atmospheric frontal system progression across the Northeast quarter of the country early next week will support a period with locally enhanced rain. Additional upper troughing well upstream will meanwhile dig a wavy but only modestly unsettling/cooling front down from the Northwest. Subsequent system progressions and genesis mid-later next week out across the Rockies to the north-central U.S. will increasingly focus an enhanced convection/rainfall pattern that could focus locally heavy convection/rainfall. Santorelli/Schichtel Hazards: - Heavy rain across portions of the northern Plains as well as from the central Gulf Coast northward through the Tennessee Valley into the Ohio Valley, Mon-Tue, Aug 30-Aug 31. - Heavy rain from portions of the Tennessee Valley, across the central/southern Appalachians, and into the Mid-Atlantic states; and from the northern Plains to the upper Midwest; as well as much of Arizona, Wed-Thu, Sep 1-Sep 2. - Heavy rain across southwestern Colorado, Thu-Fri, Sep 2-Sep 3. - Heavy rain across portions of the upper Midwest, Fri, Sep 3. - Flooding possible across portions of the central Gulf Coast states. - Flooding likely across portions of central Wisconsin. - High winds across much of the central Gulf Coast and into the lower Mississippi Valley, Mon, Aug 30. - Much above normal temperatures across much of the central High Plains, Mon-Wed, Aug 30-Sep 1. - Much above normal temperatures for a good portions of North Carolina and into South Carolina, Mon-Tue, Aug 30-Aug 31. - High significant wave heights for coastal portions of Louisiana into Mississippi, Mon, Aug 30. 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