Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Fri Sep 28 2018 Valid 12Z Mon Oct 01 2018 - 12Z Fri Oct 05 2018 ...Excessive rains early next week for the Southwest associated with Hurricane Rosa... ...Below average temperatures from the northern Rockies to the Upper Midwest... ...Pattern Overview and Weather Highlights/Threats... A highly amplified flow pattern is expected to persist from the North Pacific across North America through medium range time scales, with a blocking ridge centered over Alaska, mean troughing off/along the U.S. West Coast and Hudson Bay, and persistent subtropical ridging anchored over the Southeast. Additionally, the extratropical transition of Typhoon Trami near eastern Asia this weekend 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. In this pattern, ample troughing near the West Coast should act to steer Hurricane Rosa northeastward toward Baja California early next week per the latest National Hurricane Center track, with a weakening Rosa then lifting into the Southwest. There remains a significant threat for deepened tropical moisture to work into the Southwest and south-central Great Basin/Rockies by early next week. Despite a weakening tropical system, potential exists for heavy rainfall and local runoff issues as enhanced by terrain. The overall flow pattern also favors southward transport of polar air from the northern Rockies east to the Great Lakes then Northeast and more summer-like conditions from the Gulf Coast to the Southeast. 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 prevail from the northern Rockies to the Upper Midwest next week with maximum temperatures as cold as 10-25 deg F below average are 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 into early-mid next week. Ejecting moisture/energy from the remnants of Rosa may act to enhance rainfall efficiency. Sufficiently cold air in place north of the frontal boundary may also support some northern Rockies to the northern High Plains snows. ...Guidance Evaluation/Preferences... The WPC medium range product suite was mainly derived from an ensemble mean composite blend after consideration of recent run to run model variance. Leaned WPC blend weighting in favor of the 18/00 UTC GEFS means that offer a Rosa and driving upstream pattern solution most in line with latest NHC track guidance. The most recent NHC track forecast trended progressive days 3-5. Otherwise across the entire domain and through longer time frames, the reasonably well clustered ensemble means blend also seems to mitigate model to model timing/emphasis issues with Rosa's ejecting extratropical remnants, northern stream embedded systems/fronts running overtop and uncertain stream interactions. This overall forecast strategy acts to maintain good WPC continuity. 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