Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EDT Thu Sep 27 2018 Valid 12Z Sun Sep 30 2018 - 12Z Thu Oct 04 2018 ...Heavy rain possible next week across the Southwest associated with 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. Troughing near the West Coast will likely act to steer Hurricane Rosa northeastward toward the Baja California Peninsula early next week. Additionally, 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... A blend of the ECMWF/GFS/CMC served as a reasonable starting point for the forecast during days 3-5 (Sun-Tue). A number of weak upper-level shortwaves are expected to traverse the northern tier during the period, and deterministic solutions showed reasonably good consensus surrounding these features. Additionally, a cutoff upper low is expected to reach the West Coast Sat night, eventually opening into a wave as it interacts with northern stream energy across the northern Rockies, and quickly ejecting eastward. An additional cutoff upper low is forecast by all guidance to approach the West Coast next Mon-Tue. Model consensus with respect to this feature is also reasonably good, at least to day 5. As this feature digs southward off/along the California coast, this should steer Hurricane Rosa in a general northeastward direction toward the Baja California Peninsula, per the latest forecast from the National Hurricane Center. During days 6-7, spread begins to increase with respect to the degree of amplification that occurs with the western upper low/trough - differences that seem to largely come down to the degree of interaction with northern stream energy. The ECMWF digs additional northern stream energy south into the western trough, resulting in further amplification and a much slower progression relative to the GFS, which keeps the northern stream separate and moves the trough from California into the basin more quickly. The degree to which this occurs will play a significant role in the eventual track of Rosa and in which areas may see the threat for heavy precipitation. Whether this interaction of streams occurs has effects that propagate downstream to the central/eastern U.S. with a more amplified/slower western trough resulting in higher heights/stronger ridging for those areas. Ensemble means seem to favor something more along the lines of the slower/more amplified solution, and this idea is also supported by the general amplification of upper-level flow across much of the northern hemisphere. Thus, trended away from the GFS and toward heavier ECENS/GEFS mean weighting during days 6-7. ...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 10-25 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 produce potentially heavy rainfall for portions of the Midwest/Great Lakes late this weekend into early next week. 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 areas of heavy rainfall in southeastern California into Arizona and 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) and forecast confidence increases, but the WPC deterministic QPF will remain at least somewhat conservative for now given that the event is still several days out. 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