Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Mon Oct 14 2019 Valid 12Z Thu Oct 17 2019 - 12Z Mon Oct 21 2019 ...Series of storm systems to bring heavy precipitation to the Pacific Northwest later this week... ...Pattern Overview and Guidance/Uncertainty Assessment... Amplified and relatively progressive pattern is expected to continue across the CONUS during the medium range. A deepening coastal storm is forecast to be ongoing along the New England coast Thu morning, moving northeast into the Canadian Maritimes by Thu night/Fri morning, with models now reasonably well-clustered on the forecast track. Meanwhile, a series of rapidly moving shortwaves and frontal systems are forecast to affect the northwestern U.S. from Thu onward, the first of which should be moving inland across the Rockies on Thu, with systems reaching the Pacific Northwest spaced approximately 24-36 hours apart through the rest of the week and into the weekend. 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. Solutions continue to differ on the timing of this last trough, as well as how far south it digs. Teleconnections associated with the strongest hemispheric 500 hPa anomaly centers during the medium range provide somewhat conflicting guidance. A strong and persistent upper high across central Russia supports a continuation of relatively progressive upper flow across the CONUS, but positive height anomalies in the northeast Pacific support stronger height falls into the southwestern U.S. (more like the 00Z GFS of 12Z CMC). Given these considerations a compromise seemed in order, and decided to lean toward solutions in the middle of the model/ensemble spread. Based on an evaluation of ensemble members, a blend of the 12Z ECMWF/GFS/CMC appeared to best represent a consensus for the track and intensity of the surface low as it moves along the New England coast and into the Canadian Maritimes Thu/Thu night, and this blend was followed across the CONUS during days 3-4 (Thu-Fri). The CMC was dropped from the blend starting on day 5 as it began to stray from consensus with the central/eastern U.S. shortwave and was among the deepest/slowest solutions with the strong Pacific trough across the West on Sun. The forecast during days 5-7 (Sat-Mon) reflected continued use of the 12Z ECMWF/GFS solutions, with a gradual trend toward greater weighting of ensemble means (ECENS/NAEFS) through time consistent with increasing guidance spread. ...Weather Highlights/Threats... Lingering rain is likely across northern New England on Thu as the deep low pressure system moves along the coast. Sufficient cold air may be in place for rain with mix with or change to snow at the highest peaks of the White Mountains. Widespread and potentially heavy precipitation is expected across the Pacific Northwest later this week 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. By the weekend, moisture return across the central U.S. ahead of the western trough could generate showers and thunderstorms across much of the Mississippi Valley. A weakening warm front may focus potentially heavy rainfall across portions of the Gulf Coast, from southeast Texas to the Florida Panhandle. As the warm front along with deeper moisture drift northward across Sun-Mon, heavy rain is possible across portion of the lower/mid-Mississippi Valley. Relatively cool conditions are forecast across the eastern third of the CONUS on Thu, with highs 5 to 10 deg F below average for many locations. These areas should warm up by Fri and 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. Highs are forecast to be 5 to 10 deg below normal across the Northwest on Thu. By the weekend, colder conditions should overspread much of the West as the main upper trough axis moves inland and amplifies, with highs forecast to be as much as 15 deg below normal for some areas. 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