Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1147 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. This will then shift eastward into the Mid-Atlantic and Northeast by Tue and out to sea on Wed. The WPC forecast was initially based on a multi-model deterministic blend of the 00Z GFS/ECMWF/CMC/UKMET during days 3-4 (Sat-Sun). 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. In the Gulf, the surface low (potentially with tropical characteristics) is forecast to lift northeastward into coastal AL or the FL panhandle and eventually be absorbed into the front to its north. 06Z GFS was farther east and faster than the 00Z model consensus and was discounted. 00Z UKMET departed from the consensus over the Plains/Great Lakes/southern Canada by Sun into Mon with a slower shortwave progression owing to its emphasis on trailing rather than leading height falls. Uncertainty increases significantly by days 6-7 (Tue-Wed) off the Pac NW coast, 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 06Z GFS and 00Z ECMWF/Canadian and the ensemble means was used during this time period to minimize future timing/amplitude shifts. ...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, along with a chance of severe weather (see SPC's Day 4-8 outlook for a highlighted area on Sunday in eastern OK and northeastern TX). Advancing frontal boundary and Gulf low pressure are expected to focus potentially heavy rainfall 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 strengthening 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. Maine will be the last to see the rain exit into the Atlantic on Wednesday. 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 push 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. Pre-frontal warmth will be limited to about 5-10 degrees above average for maxes but 5-15 degrees above average for overnight mins. Some records may be possible across Texas on Sunday and over Florida next Mon/Tue. 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