Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Sep 29 2018 Valid 12Z Tue Oct 02 2018 - 12Z Sat Oct 06 2018 ...Heavy rains early week for the Southwest and central Great Basin/Rockies with Rosa... ...Heavy rains mid-later week from the mid-upper Mississippi Valley to Great Lakes... ...Early season snows from the northern Cascades to the northern Rockies/Plains... ...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/Utah early next week, but will bring significant moisture to the Southwest. Combined with a pre-Rosa surge through the Gulf of California and an incoming Pacific front into California, this sets the stage for a locally heavy multi-day rainfall event for portions of Southern California eastward across the Southwest and northeastward to the central Great Basin/Rockies. This system has the potential for significant hydrological impacts including heavy rainfall 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 off/along the U.S. West Coast as well as 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 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 to the Southwest 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 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 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 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 down into the s-central Plains and mid-MS Vallley. Ejecting moisture/energy from the remnants of Rosa may act to enhance rainfall rather efficiency along the front. Sufficiently cold air in place north of the frontal boundary may 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 GEFS/NAEFS/ECMWF ensemble means given model run to run uncertainties. The blend was manually adjusted to then better fit the latest National Hurricane Center track and strength forecast for Rosa and downstream moisture influx. 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