Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 AM EDT Wed Sep 26 2018 Valid 12Z Sat Sep 29 2018 - 12Z Wed Oct 03 2018 ...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 more 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... Forecast confidence is relatively good for roughly the first half of the period, and a ECMWF/GFS/UKMET blend served as a good consensus solution/forecast starting point. A number of shortwaves are expected to traverse the northern stream flow, and model solutions show reasonably good consensus with respect to these during days 3-5 (Sat-Mon). Additionally, a cutoff upper low is expected to reach the West Coast during that time frame, eventually opening into a wave as it interacts with northern stream energy across the northern Rockies, and quickly ejecting eastward. This idea is also generally well-represented by the consensus solution described above. Greater uncertainty is introduced into the forecast during days 6-7 surrounding the amplification of additional upper-level energy off the West Coast, with potential formation of another cutoff low. The ECMWF has come around to a slower/more cutoff solution (with additional support from the FV3) compared to previous runs, while the GFS has trended in the opposite direction. Given the support for continued or even increased hemispheric flow amplification, continue to favor a more amplified trough or cutoff low along/off the West Coast early-mid next week, with a slower eastward progression. The ECENS/GEFS means show this idea well, and weighting of those solutions was quickly increased during the day 6-7 (Tue-Wed) time frame. One important consideration is that the evolution of this feature will likely play a significant role in the eventual track of Tropical Storm (forecast become hurricane) Rosa. The amplification of an upper trough/low off the California coast would favor a northward movement for Rosa by the weekend or early next week, but the major question is to what degree this occurs, and also to what degree moisture from the system is able to move into the southwestern U.S., potentially increasing precipitation for that region. Confidence remains quite low in this aspect of the forecast; please refer to products issued by the National Hurricane Center for the latest official information on Rosa. ...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 may spread into portions of the southwestern U.S. by next week depending on the eventual track of Rosa. Heavy rain is possible if the system takes a more northerly track ahead of the trough to the north but confidence is low at this time as to where that may occur (if at all). 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