Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Oct 16 2019 Valid 12Z Sat Oct 19 2019 - 12Z Wed Oct 23 2019 ...Heavy rain and mountain snow expected for portions of 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 series of rapidly moving shortwaves and frontal systems are forecast to affect the northwestern U.S., spaced approximately 24-36 hours apart through early next week. The first of the Pacific shortwaves will cross the central U.S. Sat, pushing a cold front into the Great Lakes, Mississippi Valley, and southern Plains. By Sun, another significant Pacific trough is forecast to quickly amplify across the Rockies before reaching the central U.S. by Mon. Most deterministic model solutions suggest development of a closed upper low and deep surface cyclone across the Upper Midwest/Great lakes by Mon, 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 12Z ECMWF/CMC/18Z GFS during days 3-5 (Sat-Mon). Differences among these solutions with respect to precisely how much and how quickly the upper low deepens across the Upper Midwest Sun-Mon result in differences on the track and intensity of the surface low. The CMC differed from consensus a bit by day 5, closing off the upper low farther south than consensus, and this solution was thus deemphasized at that time. Uncertainty increases significantly by days 6-7 (Tue-Wed), particularly with respect to a building upper ridge off the West Coast and how it impacts the active train of shortwave energy from the north Pacific to the northwestern U.S. Models have shown quite large timing and amplitude variations from run-to-run, which significantly reduces forecast confidence by that time. A blend of the 12Z ECENS/NAEFS ensemble means along with a continued minority component of the deterministic ECMWF was used during this time period. The ECMWF seemed to best reflect a trend seen among several deterministic solutions toward faster (and stronger) arrival of another shortwave in the Northwest on Tue, with further amplification across the Rockies by Wed. Ensemble means show so much spread, however, that it is difficult to discern any wave pattern in the height contours, further emphasizing that forecast confidence across the western U.S. by the middle of next week is quite low. ...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. Significant snows are possible across the Cascades as well as portions of the northern Rockies, with heavy rains possible at the lower elevations. As the central U.S. trough amplifies this weekend into early next week, moisture return from the Gulf of Mexico will increase. A frontal boundary and wave of low pressure are expected to focus potentially heavy rainfall along the immediate Gulf Coast and perhaps into portions of the Southeast this weekend, before deeper moisture and widespread showers/thunderstorms spread northward into the Mississippi, Ohio, and Tennessee Valleys ahead of the deepening central U.S. low pressure system early next week. By Mon-Tue as the system moves east, widespread and locally heavy rains will also be possible across portions of the Northeast. 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. 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