Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1143 AM EDT Thu Sep 27 2018 Valid 12Z Sun Sep 30 2018 - 12Z Thu Oct 04 2018 ...Locally heavy rain 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 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 per the latest National Hurricane Center track. Additionally, the extratropical transition of Typhoon Trami across eastern Asia over the weekend should support continued/further upper flow amplification downstream over the North Pacific and Alaska, suggesting a continuation of the highly amplified/blocked flow regime across North America through and beyond the medium range period. ...Guidance Evaluation/Preferences... A deterministic blend sufficed for the first few days of the forecast (Sun-Tue) with the most agreement east of 105W. A number of weak upper-level shortwaves are expected to traverse the northern tier during the period, and deterministic solutions showed some timing/amplitude differences in their recent runs but a blended solution was still preferred with these features given the shifting of the guidance. Additionally, a closed 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 should quickly eject eastward into the faster WSW flow east of the Rockies. An additional closed 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, but the deterministic models have been quicker than the ensemble means suggest, likely related to stronger/slower members pulling the mean trough axis back westward. A nearly even ensemble/deterministic blend has been a decent position over the last few days with a slow trend eastward in the ensemble means and a slow trend westward in the deterministic models. As this feature digs southward off/along the California coast, it should help steer Hurricane Rosa in a general northeastward direction toward the Baja California Peninsula though in a weakening state. During days 6-7 (Wed-Thu), 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 out of SW Canada and upstream Gulf of Alaskan flow. With uncertainty in several parts of the flow, opted to rely mostly on the ensemble means for the forecast which which have been much more stable (both the GEFS and ECENS), but have definitely trended toward stronger ridging in the Southeast and deeper troughing in the west. This will have an affect on the track of Rosa and which areas may see the threat for heavy precipitation. ...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 slowly weakens. 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 somewhat conservative for now given that the event is still several days out. 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