Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Tue Oct 15 2019 Valid 12Z Fri Oct 18 2019 - 12Z Tue Oct 22 2019 ...Series of storm systems to bring heavy precipitation to the Pacific Northwest... ...Pattern Overview and Guidance/Uncertainty Assessment... Amplified and relatively progressive pattern is expected to continue across the CONUS during the medium range. A low pressure system is forecast to be pulling away from New England Fri morning, moving northeast into the Canadian Maritimes. Meanwhile, a series of rapidly moving shortwaves and frontal systems are forecast to affect the northwestern U.S., spaced approximately 24-36 hours apart into early next week. The first of the Pacific shortwaves will move into the central U.S. Fri-Sat, pushing a cold front across the Plains and the Midwest. By Sat-Sun, another significant Pacific trough is forecast to quickly amplify across the Great Basin and Rockies before reaching the central U.S. by Mon, with a number of model solutions suggesting development of a closed upper low and deep surface cyclone across the Upper Midwest, which should then lift northward into Ontario on Tue. A strengthening subtropical ridge off the Southeast U.S. coast early next week will contribute to a highly amplified flow setup, with a broad region of moist southerly flow across the Mississippi, Tennessee, and Ohio Valleys. The WPC forecast was initially based on a blend of the 12Z ECMWF/CMC/18Z GFS during days 3-5 (Fri-Sun). These solutions showed relatively small differences during that time period, and a blend fit the overall model consensus well. During days 6-7 (Mon-Tue), the forecast was based on a blend of the ECMWF along with the ECENS and NAEFS ensemble means. The 18Z GFS showed some significant differences from the ECMWF across the Midwest/Great Lakes during this time frame, likely due to a less closed upper-level system causing a leading surface to to move quickly northeast into the Great Lakes. The ECMWF fit much better within the center of the model/ensemble spread over the past few runs. ...Weather Highlights/Threats... Widespread and potentially heavy precipitation is expected across the Pacific Northwest through the weekend as the series of shortwaves and surface fronts move inland. Snow-levels are forecast to fall as colder air and lower height press inland. The heaviest precipitation is expected to fall across the coastal ranges as well as the Cascades, with multi-inch liquid equivalents possible. Farther inland, rain and mountain snows are likely across the northern Great Basin, northern Rockies, and eventually the central Rockies. As the central U.S. trough amplifies this weekend into early next week, moisture return from the Gulf of Mexico will increase. A warm front is expected to focus potentially heavy rainfall along the immediate Gulf Coast this weekend, before deeper moisture and widespread showers/thunderstorms spread north across the Mississippi, Ohio, and Tennessee Valleys Sun-Mon. Models suggest the potential for 1-2 inches of rain across a large area from the Gulf Coast to the Ohio Valley, with embedded areas of heavier rainfall. Relatively cool conditions are forecast across the eastern third of the CONUS on Fri, with highs 5 to 10 deg F below average for many locations. These areas should warm up by into the weekend, with temperatures reaching 5 to 10 deg above normal by Sun-Mon. Meanwhile, below average temperatures are forecast to spread across the western U.S. as upper troughing moves inland. High temperatures 5 to 15 deg below average are forecast to spread from the Northwest and Great Basin east to the central/northern Rockies as the main upper trough axis moves inland and amplifies. As the trough reaches the central U.S. next Mon-Tue, highs may be 5 to 10 deg below average across the central/northern Plains. 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