Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EDT Wed Sep 11 2019 Valid 12Z Sat Sep 14 2019 - 12Z Wed Sep 18 2019 ...Overview and Guidance Evaluation/Preferences... An upper-level ridge is forecast to expand across the central and eastern U.S. through much of the medium range, centered across the Mid-Mississippi and Ohio Valleys. Meanwhile, a relatively vigorous shortwave is expected to move exit the Great Lakes into eastern Canada on Fri, with a large upper-level trough expected to amplify across the western U.S. from Sun onward. Farther south, a tropical disturbance is expected to move westward across the Gulf of Mexico Sat-Mon, with a number of solutions developing an area of low pressure along the wave. The latest outlook from the National Hurricane Center indicates a medium chance of tropical development with this feature through the next several days. A blend of the 12Z ECMWF/CMC/18Z GFS was used as a basis for the forecast during days 3-4 (Sat-Sun). A blend of these solutions should favorably depict features a series of shortwaves/surface frontal systems crossing the CONUS during that time period. Model consensus has improved quite a bit over recent days with respect to the evolution of a large western U.S. trough early next week, with the GFS now on board with the idea which was first shown by the CMC a few days ago. Conceptually, this continues to make sense given an amplified pattern in place upstream across the North Pacific. Despite the improvement shown in the GFS, it continues to show some degree of fluctuation on the eventual amplitude of the upper trough by Tue-Wed, and is generally a bit less deep than the ECMWF/CMC. Given the model trends across recent days, preferred to lean more toward the ECMWF/CMC along with increasing ensemble means (ECENS/NAEFS) through time during days 5-7 (Mon-Wed). Given trends shown in ensemble members, cannot rule out an adjustment toward an even slower/deeper upper trough solution across the West in future forecast cycles. ...Weather Highlights/Threats... Numerous showers and thunderstorms are expected across Florida and the Gulf Coast states this weekend into early next week in association with the westward moving tropical disturbance. Several cycles of model runs have consistently focused the greatest chance of heavy rainfall along the immediate coastline from Louisiana east to Florida, where several inches of rainfall are possible, spread across multiple days. Moisture from this system may spread farther north into portions of the lower Mississippi Valley by early Mon-Tue. Widespread rains (and perhaps some snow at the highest elevations) are expected to reach the Pacific Northwest by Sat night into Sun as the upper trough begins to amplify. Precipitation should spread inland across the Great Basin and northern Rockies early next week. Southerly flow east of the upper trough may revive monsoonal showers and storms across the Four Corners region, with coverage of precipitation increasing this weekend through early next week. The broad upper ridge across the central and eastern U.S. will result in above average temperatures across a wide area. Highs are forecast to be 5 to 15 degrees for many areas from the Plains to the Eastern Seaboard this weekend into next week. The upper trough reaching the western U.S. Sun-Mon will bring much cooler temperatures, with highs 5 to 15 deg below average expected to overspread the Northwest and northern Great Basin. As mentioned above, with increasing precipitation across the Northwest, temperatures should be cold enough to support snow at the higher peaks. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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