Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EDT Thu Oct 17 2019 Valid 12Z Sun Oct 20 2019 - 12Z Thu Oct 24 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. An area of low pressure originating in the western Gulf of Mexico is forecast to across the southeastern U.S. Sun-Mon after merging with a frontal boundary. Meanwhile, a series of rapidly moving shortwaves and frontal systems are forecast to affect the northwestern U.S., spaced approximately 24-48 hours apart through the middle of next week. One such Pacific shortwave will cross the Great Lakes on Sun, with a trailing cold front weakening across the Mid-Mississippi Valley. Another significant Pacific trough is forecast to quickly amplify across the Rockies Sun before reaching the central U.S. on Mon. Most deterministic model solutions suggest development of a closed upper low and deep surface cyclone across the Upper Midwest by Mon, which should then lift northward into the Great Lakes and 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. Uncertainty increases significantly by Tue-Wed across the Pacific Northwest and the northeastern Pacific, 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. This uncertainty extends east into the Rockies and central U.S. by Wed-Thu. Model differences were quite significant even early in the period with respect to the low crossing the Southeast and moving. The UKMET and GFS have consistently shown more interaction with an additional area of upper-level energy currently across western Texas, causing the eventual surface low across the Southeast to be deeper and move faster. Meanwhile, the 12Z ECMWF/CMC showed no or limited interaction, respectively, with a slower and weaker surface low across the Southeast. Preferred to lean toward the 12Z ECMWF/CMC solution until interaction of these feature becomes more certain, as this also maintains a forecast close to continuity. These solutions also fit relatively well within the model/ensemble consensus elsewhere and a 12Z ECMWF/CMC blend was used for the WPC forecast during days 3-5 (Sun-Tue). The aforementioned blend leans toward a somewhat deeper and slower solution (relative to the GFS) with the low pressure system crossing the Midwest/Great Lakes Mon-Tue. By days 6-7 (Wed-Thu) as uncertainty quickly increased, preferred to stick relatively close to forecast continuity, which aligned fairly well with the 12Z CMC. Thus, used a blend of the 12Z CMC along with 12Z ECENS/NAEFS ensemble means by late in the forecast period. ...Weather Highlights/Threats... Potentially heavy precipitation is expected across the Pacific Northwest into next week as the series of shortwaves and surface fronts move inland. Snow levels should rise some as the upper trough axis shifts to the central/eastern U.S., but some significant snows will still be possible at the higher elevations of the Cascades and portions of the northern Rockies. 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 across the western U.S. as the upper trough moves inland. High temperatures 5 to 15 deg below average are forecast on Sun 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. 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