Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1011 AM EDT Fri Sep 21 2018 Valid 12Z Mon Sep 24 2018 - 12Z Fri Sep 28 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Upper-level ridging is expected to strengthen over the northeast Pacific and form a closed 500 mb high. Downstream of the amplifying ridge, an upper-level trough will also become amplified from the northern plains and Midwest to the Great Lakes as the passage of multiple shortwaves progressively lowers heights. A low pressure system accompanying the first shortwave will cross the eastern northern plains/upper MS Valley Mon-Tue, with another following in its wake by Wed 26 Sep-Thu 27 Sep. Model/ensemble consensus was above average with respect to the large scale pattern evolution. Therefore, a multi-model deterministic blend of the 00z GFS/ECMWF/Canadian Global was used as a basis for the WPC forecast during days 3-5 (Mon-Wed). The overnight trend was to move the low pressure a bit slower than prior forecasts that current runs show to be in Manitoba Wed 26 Sep and near the MN border with Canada 12z Thu 27 Sep. The 06z GFS/GEFS mean amplified the 500 mb wave and forecast sfc low pressure further south Fri 28 Sep vs. continuity and the ECMWF/Canadian models/ensemble means, so these solutions were weighted less. The 00z ECMWF ensemble mean was blended into the forecast days 6-7 as spread starts to develop over the eastern Pacific. There is good multi-model agreement on a rex block developing with low pressure south of the aforementioned closed high. Typical timing differences on the low's track lead to a blend of the models and 00z ECMWF ensemble mean. ...Sensible Weather Highlights... A surge of moisture from the Gulf of Mexico ahead of the cold front will support areas of locally heavy rain across much of the lower Mississippi and Ohio and TN Valleys Mon-Wed, with showers and storms extending northeast across the Great Lakes into the northeast and Mid-Atlantic as well. The front will move slowly into the east with showers and storms Thu and Fri focused on the Appalachians across the southern mid Atlantic, central Gulf coast, and Florida. An area of showers develops over the eastern northern Plains to the upper Great Lakes Thu 27 Sep -Fri 28 Sep. In terms of temperatures, the a substantially cooler air mass moves across the front range of the northern Rockies across the northern Plains as the upper trough amplifies. High temperatures in the wake of the first cold front Mon-Tue will be 5 to 10 deg F below average, with temperatures perhaps dropping to 10-15 deg below average behind the second cold front later in the week. As high pressure builds down the plains, cold air penetrates further south,reaching the southern Plains by next Fri. Meanwhile, areas across the Southwest, southern Great Basin, and southeast are forecast to remain 5 to 10 deg above average. Petersen 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