Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1125 AM EDT Sat Sep 29 2018 Valid 12Z Tue Oct 02 2018 - 12Z Sat Oct 06 2018 ...Heavy rains associated with Rosa will exit the Southwest and Rockies next Tuesday... ...Additional heavy rain threat from the mid/upper Mississippi Valley to Great Lakes later in the week... ...Hurricane Rosa... Please refer to the National Hurricane Center advisory for the latest information on Rosa and your local NWS office for local watches and warnings. Rosa is expected to rapidly weaken as it lifts inland across the Baja California peninsula then Arizona early next week, but will bring significant moisture to the Southwest. Combined with an incoming Pacific front into California, a locally heavy multi-day rainfall event straddling the short term into the medium range period is likely for portions of Southern California eastward across the Southwest and northeastward to the central Great Basin/Rockies. The potential exists for significant/life-threatening hydrological impacts including heavy rainfall and flash flooding, debris flow in the deserts, and local runoff issues as enhanced by terrain. ...Pattern Overview and Weather Highlights... It remains the case that a highly amplified flow pattern should persist from the North Pacific across North America with a blocking ridge centered over Alaska drifting northward into the Arctic, mean troughing pushing into the U.S. West Coast as well as through Hudson Bay, and persistent/resilient subtropical ridging anchored over the Southeast. Additionally, the extratropical transition of Typhoon Trami off Japan in the short term should support continued upper flow amplification downstream over the North Pacific and Alaska, suggesting a continuation of the highly amplified/blocked North American flow regime into the end of next week. In this pattern, ample troughing near the West Coast will act to steer Hurricane Rosa northeastward toward the Baja California peninsula before further weakening occurs while lifting toward the Southwest states and central Great Basin as per the latest NHC track. The overall flow pattern also favors southward transport of polar air from the northern Rockies east to the Great Lakes via embedded upper level shortwaves and more summer-like conditions from the Gulf Coast to the Southeast and mid-Atlantic states. Expect lingering rains for the Gulf Coast and Southeast into midweek within a moistened easterly low-level fetch. A persistent strong frontal zone in between these air masses will focus periods of enhanced precipitation per deepened moisture. Colder than average temperatures will spread/dig from the northern Rockies to the Plains and Upper Midwest/Great Lakes next week with maximum temperatures as cold as 10-25 deg F below average possible (40s and 50s). Waves along a strong and reinforcing north-central U.S. frontal boundary may produce heavy rainfall for the Upper Midwest/Great Lakes down into the s-central Plains and mid-MS Valley. Ejecting moisture/energy from the remnants of Rosa may act to enhance rainfall rather efficiently along the front, continuing the heavy rainfall threat through the week. Sufficiently cold air in place north of the frontal boundary will also support some northern Rockies to the northern Plains snows as reinforcing troughing dives southward through western Canada to the east of the Alaskan upper ridge. ...Guidance Evaluation/Preferences... The WPC medium range product suite was primarily derived from a blend of the reasonably well-clustered latest GFS/GEFS mean and ECMWF/EC-ENS mean given timing differences in the west with the lead and trailing trough. By later next week, the trend has been for much more ridging in the east which supports the continuation of above average temperatures and a slower southward progression of the surface front. Fracasso/Schichtel WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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