Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1146 AM EDT Mon Oct 15 2018 Valid 12Z Thu Oct 18 2018 - 12Z Mon Oct 22 2018 ...Pattern Overview and Guidance/Predictability Assessment... The latest runs of operational models/ensembles still show good agreement and continuity for the longwave pattern over the lower 48 through the weekend into early next week. Upper ridging will rebuild over the Northwest U.S. and western Canada to maintain an eastern North America mean trough. Shortwaves embedded within the trough will support periodic frontal passages over the central-eastern states. Across southern latitudes, a short range Southwest upper low in the process of ejecting from the West at the start of the extended period will be replaced by another upper low consisting of leftover energy. Meanwhile upper ridging that covers the Gulf Coast states will gradually be suppressed as eastern troughing amplifies. An operational model consensus continues to represent individual features well for days 3-5 Thu-Sat. After that time model runs begin to show greater shortwave differences among each other and/or in consecutive runs, favoring a trend toward 50 percent total 06Z GEFS/00Z ECMWF ensemble mean input for the latter part of the forecast. Of particular note the 00Z GFS brings low pressure into the upper Great Lakes late Sun-early Mon whereas consensus keeps the best-defined surface low well north over Canada. The 06Z GFS differs from consensus for its specifics of the upper trough approaching the West Coast toward the end of the period. ...Weather Highlights/Threats... Below average temperatures will continue late this week into next Monday for the Southwest through the southern Plains (especially Texas) northeastward through the Ohio Valley into the Northeast/Mid-Atlantic. The most extreme anomalies should be over the southern High Plains on Thu with some locations 20-25F below normal. Otherwise expect some locations to see at least 10-15F below normal highs within the broad area of cool temperatures. On the warm side of the southern stationary front, Florida will see warm/hot temperatures that may tie or break record highs/warm lows at some locations. Upper ridging across the Pacific Northwest (WA/OR/ID into western Montana) will also lead to continued above average temperatures for that region--generally plus 5-15F anomalies for highs. Expect highest 5-day rainfall totals over portions of Texas and near the western Gulf Coast, in the vicinity of an inverted trough and to the northwest of a stationary boundary in the western Gulf. Some of this southern Plains moisture may extend northeastward along a cold front crossing the East during the weekend. The lingering upper low in the Southwest will support rain and possibly high elevation snow showers mostly focused on/along the higher terrain while a stationary front across Florida will keep daily shower/storm chances in the forecast. Over the Great Lakes/Northeast lake effect precipitation should be tapering off Thu. Then the late week/weekend frontal system will bring a period of mostly light rain followed by another episode of lake effect rain/snow. The next system in the series may bring an area of light precipitation to the Great Lakes around the start of next week. Rausch/Fracasso 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