Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EDT Fri Sep 21 2018 Valid 12Z Mon Sep 24 2018 - 12Z Fri Sep 28 2018 ...Pattern Overview... Upper-level ridging is expected to strengthen off the west coast of North America during the medium range, as supported by a consensus of model/ensemble solutions. Downstream of the amplifying ridge, an upper-level trough will also become increasingly 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 Midwest Mon-Tue, with another (reminiscent of a winter-like clipper system) quickly following in its wake by Wed-Thu. ...Guidance Evaluation/Preferences... Model/ensemble consensus was above average with respect to the large scale pattern evolution described above. Therefore, a multi-model deterministic blend (including the GFS/ECMWF/UKMET) was used as a basis for the WPC forecast during days 3-5 (Mon-Wed). The most significant difference during this time period involved some solutions (the CMC/UKMET as well as a number of ECMWF ensemble members) hanging energy back across the Great Basin/Rockies for a longer period of time relative to the other guidance. With respect to the northern stream, confidence was relatively high, however. Models have shown some run-to-run variability with respect to the timing and amplitude of the second significant shortwave crossing the northern tier days 5-6 (Wed-Thu), but in general the trend has been toward more amplification with this wave and a somewhat stronger surface low pressure system. Given the run-to-run variability with smaller-scale features by later in the period, weighting of ensemble means (ECENS and NAEFS) was gradually increased from day 5 onward, with means comprising a majority of the blend by days 6-7. ...Sensible Weather Highlights... A weakening surface warm front along with another approaching cold front will support areas of potentially heavy rain across much of the Mississippi and Ohio Valleys Mon-Wed, with some potential for heavy rains across portions of the Mid-Atlantic as well. The front will linger from the southern plains to the Southeast through late in the week, with locally heavy convection possible through Friday. In terms of temperatures, the most significant story will be the arrival of a substantially cooler air mass from the northern Rockies to the Midwest as the upper trough amplifies through the week. 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 10-20 deg below average behind the second cold front later in the week. Models show a range of solutions with respect to how strong the Canadian surface high will be by late this week, and thus how anomalously cold the air mass will be across the northern Rockies/plains. Meanwhile, areas across the Southwest, Great Basin, and West Coast will remain 5 to 10 deg above average with the building upper ridge just offshore. 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