Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Wed Sep 26 2018 Valid 12Z Sat Sep 29 2018 - 12Z Wed Oct 03 2018 ...Potentially heavy rain next week for the Southwest due to Hurricane Rosa... ...Extended period of below average temperatures expected from the northern Rockies to the Upper Midwest... ...Pattern Overview... Highly amplified flow pattern expected to persist from the North Pacific across North America during the medium range, with a blocking ridge centered over Alaska, mean troughing off/along the U.S. West Coast and just west of Hudson Bay, and a persistent subtropical ridge anchored across the southeastern states. This pattern will favor southward transport of polar air into the CONUS northern tier, from the northern Rockies east to the Great Lakes, with continued late summer-like conditions from the Gulf Coast to the Southeast, and a persistent strong frontal zone in between these air masses. The extratropical transition of Typhoon Trami across east Asia over the weekend should support continued/further upper flow amplification downstream, suggesting a continuation of the highly amplified/blocked flow regime across North America through and beyond the medium range period. ...Guidance Evaluation/Preferences... Consensus blend of the latest 00Z/06Z guidance offered a reasonable starting point to the forecast for the first few days (this weekend). A number of shortwaves are expected to traverse the northern stream flow, and model solutions show reasonably good clustering with respect to these during days 3-5 (Sat-Mon) with some expected timing differences. Additionally, a cutoff upper low is expected to reach the West Coast Saturday eventually opening into a wave as it interacts with northern stream energy across the northern Rockies, and quickly ejecting eastward into the zonal flow between anomaly centers east of 100W. Greater uncertainty is introduced into the forecast during days 5-7 surrounding the amplification of additional upper-level energy off the West Coast, with potential formation of another cutoff low near Northern California early Tuesday. The evolution/depth of this troughing will help determine the fate of current Hurricane Rosa in the East Pacific. The National Hurricane Center shows a track westward over the next few days but then turning northward this weekend and east of north next week per the latest advisory. Ensemble consensus for Tues/Wed shows Rosa weakening as it traverses cooler water but lifting northeastward toward Baja California. The ECMWF and its ensembles shifted in their 00Z run toward the GFS/GEFS solution showing this forecast recurvature toward NW Mexico. ...Sensible Weather Highlights/Threats... Well-below average temperatures will prevail across areas from the northern Rockies to the Upper Midwest through much of the medium range as cold polar air is transported southward into the north-central U.S. High temperatures as cold as 10-20 deg F below average will be possible (40s and 50s). A couple waves of low pressure along the strong central U.S. frontal boundary will result in potentially widespread precipitation for portions of the central/northern plains and Midwest. The most coherent signal for potentially heavy rainfall in the guidance is early next week from the central Plains/upper Mississippi Valley to the Great Lakes. Sufficiently cold air in place north of the frontal boundary may support precipitation falling as snow for some areas from the northern Rockies to the northern High Plains. Expect showers and thunderstorms with locally heavy rainfall across the Gulf Coast and Southeast through much of the period as a moist air mass remains in place and a slowly weakening frontal boundary fails to clear the region. Moisture is expected to spread into portions of the southwestern U.S. by early next week depending on the eventual track of Rosa. Despite a weakening tropical system, the potential exists for modest and perhaps locally heavy rainfall in southeastern California into Arizona into the Four Corners region. Rainfall amounts will be adjusted over the next several days as the atmospheric players come into better focus (i.e., incoming upper trough and relative precipitable water/IVT values), but the WPC deterministic QPF will remain somewhat conservative for now given a five or six-day lead time. Fracasso/Ryan 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