Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Sep 18 2018 Valid 12Z Fri Sep 21 2018 - 12Z Tue Sep 25 2018 ***Pattern Overview and Model Guidance*** Upper ridging near the Aleutians and the south-central U.S. will tend to favor large scale troughing centered near the Pacific Northwest coast this weekend. A lead storm system will lift into southeastern Canada late this week and settle its cold front across the Mid-Atlantic region and Ohio Valley as a western front pushes through the interior. The National Hurricane Center is monitoring an area of disturbed weather south of Baja California that has the potential for tropical development. This feature is progged to lift northward and spread rainfall into portions of southern Arizona and New Mexico later this week and enhance rainfall across the southern plains. The deterministic models and their respective ensemble means were in relatively good agreement synoptically at the beginning of the forecast period Friday, so mainly a blend of the ECMWF/GFS/CMC was used through Saturday. The forecast becomes more uncertain for the western U.S. regarding the evolution of the upper level trough axis and the resulting surface low development over the Intermountain West. The 12Z ECMWF was considerably more progressive than the model consensus, including the EC mean, so the second half of the forecast incorporated more GFS along with greater percentages of the EC and GEFS means. ***Sensible Weather and Hazards*** The next heavy rainfall event during the medium range period is expected across the south-central plains for the upcoming weekend as tropical moisture surges northward from Mexico and the western Gulf. The potential exists for a few inches of rain across portions of Texas and Oklahoma, and lesser amounts extending eastward across the Midwest and the Ohio Valley region in the vicinity of a frontal boundary. Another area of moderate to locally heavy rainfall will be possible across the Upper Midwest towards the beginning of next week as a surface low develops over the northern plains. Expect temperatures to be well above average on Friday across much of the eastern U.S. ahead of the cold front, with some places close to 15 degrees above normal. More seasonal readings are forecast to return for the weekend followed by more above normal values over the central and southern U.S. by next Tuesday. Cooler than normal conditions are likely for the northern tier of the nation from the northern Rockies to Minnesota with this region under more of an influence from Canadian airmasses. D. Hamrick 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