Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Sat Aug 28 2021 Valid 12Z Tue Aug 31 2021 - 12Z Sat Sep 04 2021 ...Major Hurricane Ida to bring a Heavy Rainfall/Flooding Threat from the central Gulf Coast to the Mid-Atlantic... ...Hurricane Nora likely will fuel a lead Southwest U.S. Heavy Rain Threat mid-later next week... ...Guidance Evaluation and Preferences... The big story during the medium range period continues to be impending hazardous impacts from both Hurricane Ida in the Gulf of Mexico, as well as Hurricane Nora, forecast to track northward up the Gulf of California. Please refer to products from the National Hurricane Center and the local offices for the latest regarding these systems. For Ida, the guidance is in reasonably good agreement early next week but larger uncertainties arise after day 5 on whatever is left of Ida at that time (likely a remnant low). Most of the guidance shows the remnants of Ida becoming absorbed into the northern stream trough, but also suggest a southern component of the leftover energy sliding south/southeastward along the East coast with some models (like the 00z ECMWF) stronger than others in a reflection of a surface low. Prefer a sort of middle ground solution maintaining a wave along the frontal boundary as it exits the Mid-Atlantic. Elsewhere, guidance shows good agreement days 3-5 both with a digging trough over the Western U.S. and a northern stream trough sliding across the Northeast. After day 5, there's uncertainty with evolution of a closed low into central Canada in terms of timing, which translates to some smaller scale differences down into the northern CONUS but a blend towards the ensemble means should help mitigate those more difficult to resolve details at this time. The WPC medium range product suite was primarily derived with a blend of the ECMWF, GFS, and CMC for days 3-5, with increasing ensemble mean support thereafter. For Ida and Nora impacts, the WPC progs reflect the latest track guidance from the NHC. Overall, this approach maintains good continuity with previous WPC forecast. ...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 late Sunday as a major hurricane. Ida will bring powerful and gusty winds and very heavy rainfall to the central Gulf Coast region. At the start of the medium range period on Tuesday, a gradually weakening but still very wet Ida is forecast to make a sharp turn to the east-northeast spreading the heavy rainfall threat into the Tennessee/Ohio Valleys Tuesday and into Wednesday. Considerable flash, urban, small stream, and riverine flooding is likely with Ida. Deep moisture feeding inland with time will also interact with a slow moving front draped over the system, offering a lead heavy rainfall focus into the central Appalachians and Mid-Atlantic into Wednesday and Thursday. The latest track guidance for Hurricane Nora brings it northward into the Gulf of California and eventually into far 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 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, ample northern stream upper troughing 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 as enhanced by embedded impulses and frontal/wave reinforcements. Moisture/instability pooling may support some locally strong to severe convection and heavy downpours. 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