Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Fri Sep 28 2018 Valid 12Z Mon Oct 01 2018 - 12Z Fri Oct 05 2018 ...Heavy rain likely early next week for the Southwest associated with Hurricane Rosa... ...Wet pattern for the Upper Midwest and western/central Great Lakes... ...Hurricane Rosa... As always, please refer to the latest National Hurricane Center advisory for the latest information on Rosa and your local NWS office for local statements (including watches and warnings). Rosa is expected to weaken to a tropical storm as it lifts toward Baja California and southern Arizona early next week but will still 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 may set the stage for a widespread modest and locally heavy multi-day rainfall event for portions of Southern California eastward and northeastward to the Four Corners region. While the location of the heaviest rainfall axis (or axes) will likely change over the next few days, it remains prudent to stay abreast of the latest forecast information as 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... A highly amplified flow pattern is expected to 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 should act to steer Hurricane Rosa northeastward toward Baja California early next week as a tropical storm per the latest National Hurricane Center track, with a weakening Rosa then lifting into the Southwest. 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 rather efficiency along the front. Sufficiently cold air in place north of the frontal boundary may also support some northern Rockies to the northern High Plains snows (i.e., Montana) 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 mainly derived from a blend of the deterministic models to start with a preference toward the 06Z GFS and 00Z parallel GFS with respect to Rosa. The 00Z ECMWF/ECENS mean and 06Z GFS/GEFS mean then offered a reasonable blended solution for the Wed-Fri period as the troughing in the west lifts east-northeastward. The strength of the Southeastern ridging will play a part in how far east the troughing may progress and has been a point of contention within the ensembles for several days. The trend seems to be to hold onto the ridging longer than previous forecasts which would keep the frontal boundary closer to the U.S./Canadian border east of the Mississippi. However, the uncertain upstream flow over the Northeastern Pacific will also modulate the flow along 50N including upper ridge building between Hawaii and California. For now, opted to split the difference between the quicker ECMWF/GFS and slower ensemble means given the uncertainty. 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