Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1140 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 deep low pressure system is forecast to pull away from New England Fri morning, moving 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 Upper 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 western 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 multi-model deterministic blend of the 00Z GFS/ECMWF/CMC/UKMET and 06Z GFS which showed good clustering. The UKMET and Canadian departed from the consensus over the central CONUS (UKMET was quicker) by Sat and over the Gulf (UKMET/Canadian were farther east with a potential area of low pressure) by Sun, so they were dropped from the blend. The GFS/ECMWF and their ensemble means were utilized for the Sun/Mon period with an increased weight on the ensemble means as the north-central system next Monday showed typical spatial spread (ECMWF to the NW, GFS to the SE, but both ensemble means in between). 00Z Canadian was actually closer to the ensemble mean consensus than the ECMWF or GFS runs, but slower/deeper. This was certainly in the realm of possibility and the Canadian has had at least a recent history of better handling deepening systems out of the West the past few weeks. ...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 heights 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 this weekend. 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 (possibly severe) spread northward across the Mississippi, Ohio, and Tennessee Valleys Sun-Mon. Models suggest the potential for widespread 1-2 inches of rain (areal average) 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 into the weekend, with max temperatures reaching 5 to 10 deg above average by Sun-Tue ahead of the cold front (and mild overnight temperatures as well). 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 eastward 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. Fracasso/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