Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1146 AM EDT Fri Oct 19 2018 Valid 12Z Mon Oct 22 2018 - 12Z Fri Oct 26 2018 ...Guidance/Predictability Assessment and Weather Highlights/Threats... Latest models and ensembles are well clustered through midweek in a pattern with above normal forecast confidence. Prefer a composite guidance solution to smooth out lingering small scale variance. Prefer to lean more on ensemble guidance and continuity by days 6/7 as variance gradually grows. The pattern is highly amplified at higher latitudes with a mean mid-upper level trough and surface low/storm solution into Alaska down to the Northeast Pacific...including powerful surface lows crashing into coastal British Columbia. Lead height falls into western Canada and the Pacific Northwest along with an increasingly precipitation amount and chance of moderate precipitation. Downstream, highly amplified flow will be in place with a mean upper low spinning over the upper reaches of Nunavut. Shortwaves embedded in this flow are primed to intensify and eventually become negatively-tilted near New England. Many solutions show a rather deep surface cyclone over the Canadian Maritimes by midweek. Well south in split flow that initially sets up across North America, a weak and stretched out southern stream trough anchors from the Desert Southwest. This feature should gradually press eastward in response to a lowering of upstream heights. Downstream, toward the Gulf of Mexico, a lingering frontal zone and eventual wave of low pressure should impact the Gulf Coast states then Southeast with a threat for heavy repeat rainfall. Expect more modest precipitation on the backside up northward through the southern and central rocky states with progression of a main southern stream trough from CA/NW Mexico. A heavy rainfall threat will remain along the western/central Gulf Coasts with the axis moving eastward in time accompanying the weak area of low pressure. It remains to be seen how much instability will approach the coast but hefty QPF is noted in the guidance just offshore. The forecast itself tends to keep this axis right along the coast but the ultimate placement with be heavily driven by where the instability sets up. Across the Pacific Northwest, semi-persistent onshore flow will keep conditions wet over the Olympics and Cascades throughout the period. Pacific dominated air masses will preclude any notable winter weather threats. Across New England, some snow may fall over northern sections if the low pressure were to deepen enough to pull in colder air down from eastern Canada. Regarding temperatures, below average numbers will prevail over much of the central/eastern U.S. During the early to middle part of next week, readings could be around 10 to 15 degrees below climatology over the Southern Plains with persistent northerly flow and active precipitation. Over the northwestern U.S., air masses of Pacific origin will keep mild air in place through at least the middle of next week. Schichtel 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