Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Oct 24 2018 Valid 12Z Sat Oct 27 2018 - 12Z Wed Oct 31 2018 ...Guidance/Predictability Assessment and Preferences... To commence the period on Saturday morning, lower heights tracking toward the southern Appalachians will lift an ongoing surface wave up toward the Delmarva Peninsula. Its intensity should have already capped out by 27/1200Z with the system filling upon approach toward the southern New England coast. Additional shortwave energy rounding the base of the mean trough should sweep across the Missouri Valley midday Sunday while potentially inducing additional areas of low pressure over the eastern U.S. on Monday/Tuesday. Eventually the core of the upper low should situate over the Canadian Maritimes by the middle of next week as suggested by most of the guidance. Shifting focus toward the west, a sprawling upper ridge will situate itself across the western U.S. through the weekend while well established height falls loom off the coast. This longwave trough is forecast to make its way downstream with the core of the energetics focusing over the Pacific Northwest/southwestern British Columbia late Sunday/early Monday. A full-latitude trough should progress eastward with a position in the vicinity of the Rockies by Day 7/October 31 while an expansive ridge centers itself along the 140W longitude. After days of struggling with the initial coastal low across the northeastern U.S., the guidance have come into much better agreement, albeit with a quicker trend noted. Overall, the entire synoptic-scale pattern has shifted toward more progression considering multi-day ensemble spaghetti plot comparisons. While the initial wave decays a bit across New England, additional height falls accompanying a clipper-like system will traverse the Missouri Valley and eventually the Ohio/Tennessee Valleys by late Sunday. Given the level of amplification suggested, a secondary coastal low may emerge off the Delmarva Peninsula early Tuesday with the 18Z GFS being the most aggressive. This definitely needs to be monitored given the magnitude of colder thicknesses sweeping through. Across the western/central U.S., models agree on shifting a full-latitude trough eastward with some suggestions of an upper low pinching over the southwestern U.S. as shown by the 18Z GFS as well as many 00Z models (GFS/ECMWF/CMC). One thing that appears certain is the establishment of an upper ridge well offshore over the Pacific. Overall preferences were heavily weighted on the GFS/ECMWF modeling systems with initial focus on the 18Z/12Z GFS and 12Z ECMWF. Daily reductions in the operational components of the blend were made which led to heavier ensemble use into next week. The lowest confidence portion of the forecast is with the secondary height falls sweeping through the Tennessee Valley and the potential development of coastal low pressure. ...Weather Highlights/Threats... Wet weather is in store for the northeastern sector of the country given the slow-moving wave of low pressure inhabiting the region. While rainfall will be the primary threat along I-95 and points eastward, wintry precipitation is possible back toward the higher terrain, particularly over interior New England this weekend. Some additional precipitation threats will accompany the next shot of Arctic air although moisture may be lacking with this system. Back across the Pacific Northwest, the steady period of onshore flow and perturbations aloft will keep precipitation in the forecast over the Olympics and Cascades into next week. An additional maxima is likely farther downstream into the Upper Intermountain West given favorable upslope flow. Regarding temperatures, the coolest conditions during the period will congregate over the eastern third of the country given lower heights accompanied by cloud cover and precipitation. Departures from climatology may reach the 10 to 15 degree range in places as highs struggle to get out of the 50s with even some 40s up into New England. Meanwhile, warm weather is to prevail over the western U.S., particularly over the southwestern U.S. and into the Central Great Basin. Upper ridging will raise the mercury into the 80s over many locations of California with even some low 90s possible across the desert locales. Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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