Extended Forecast Discussion NWS Weather Prediction Center College Park MD 114 AM EDT Sun Sep 23 2018 Valid 12Z Wed Sep 26 2018 - 12Z Sun Sep 30 2018 ...Pattern Overview and Guidance Evaluation/Preferences... A highly amplified flow pattern is expected to develop during the short range and persist into the medium range period from the north Pacific across North America. A ridge axis off the North America west coast will gradually be undercut by amplifying wave energy along the eastern periphery of the ridge, along with a cutoff upper low over the north central Pacific drifting toward the U.S. west coast. Downstream of the ridge, a series of upper shortwaves and subsequent pushes of low-level cold air will gradually lower heights from the northern plains to the Upper Midwest and Great Lakes through the period. Meanwhile, upper ridging will remain in place from the southern plains to the Southeast, setting up the potential for rather strong thermal gradients across a central U.S. frontal boundary by next weekend. Model consensus remained above average for the first half of the medium range, and a multi-model deterministic blend (including the ECMWF/GFS/CMC) was used as a basis for the WPC forecast starting out the period on day 3 (Wed) and continuing into day 5 (Fri). Models showed relatively minor timing/intensity differences with respect to a shortwave/surface front crossing the eastern U.S. during the time frame, as well as with a second clipper-like low pressure system diving southeastward into the northern tier. The more significant differences continue to arise by days 6-7 with respect to the amplitude and evolution of relatively vigorous shortwave energy attempting to dive south toward the Pacific Northwest. Additional complications are introduced by the upper low off the West Coast, with the 12Z ECMWF making a major change and bringing this feature to the Northwest and absorbing it into the northern stream much more quickly than previous runs or most other deterministic solutions. Additional variance in the intensity of the persistent northeast Pacific/Alaska ridge axis and the resultant effects on downstream flow across North America result in a rather quick escalation in forecast uncertainty during the day 6-7 (Sat-Sun) time frame. Thus, boosted weighting of ensemble means (ECENS and NAEFS) substantially during the later medium range to comprise a majority of the forecast blend. ...Sensible Weather Highlights... A cold front associated with the first in a series of shortwaves crossing the northern tier during the period will eventually become stationary, bringing locally heavy convection to portions of the southern/southeastern states Wed-Thu. The second clipper-like system crossing the northern plains/Midwest Wed-Thu may will bring areas of rain along with chilly temperatures. With a trend in the guidance toward keeping the lowest heights across central Canada late in the period, this would likely result in a quasi-stationary west-east frontal boundary from the central Rockies into the plains/Midwest by late in the week, potentially characterized by a rather strong north-south thermal gradient across it, and potentially producing areas of precipitation along/north of the boundary. The north central U.S. will be the focus for below average temperatures during the medium range as passage of successive cold fronts gradually brings colder temperatures. High temperatures of 10 to 20 deg F below average are possible Thu-Fri, with temperatures potentially 20-25 deg below average by Sat-Sun. Meanwhile, the Southwest and Southeast will see continued temperatures 5 to 10 deg above average. 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