Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Wed Oct 31 2018 Valid 12Z Sat Nov 03 2018 - 12Z Wed Nov 07 2018 ...Pattern Overview... A mean upper-level trough is expected to persist and reload across the central U.S. through the extended period. The trough will be reinforced by a series of shortwaves originating in the north Pacific, potentially interacting with additional shortwave energy of polar/arctic origin as they cross the Canadian Rockies before amplifying as they encounter the area of broad and persistent cyclonic flow across the central U.S. A series of frontal/low pressure systems will accompany these shortwaves across the CONUS, generally 1-2 days apart, resulting in a period of relatively active weather. ...Guidance/Predictability Assessment... Model consensus has improved a bit with respect to the low pressure system expected to cross the northeastern U.S. on day 3 (Sat), although spread remains a bit higher than average for day 3, as this system has continued to prove rather difficult for forecast models to get a handle on. A clustering of solutions around the ECENS/GEFS means was favored, which included the recent GFS/ECMWF/UKMET/Canadian. This blend yielded favorable results elsewhere as well, with the next weakening low pressure system exiting the Rockies into the Plains this weekend. By Monday, the next shortwave should be exiting from the Rockies into the Plains, with yet another surface low pressure system developing. Ensemble means continue to trend stronger and farther south, led by the deterministic solutions though some spin up an intense low pressure system across the Upper Midwest/Great Lakes. With spread in both timing and amplitude of this system, opted to rely on a blend of the 06Z GFS/GEFS and 00Z ECMWF/ECMWF ensemble mean with the previous forecast which yields a moderate central pressure in the 990s mb. This has the potential to be a deeper system with more wind/rain and maybe some snow on the NW side. ...Weather Highlights/Threats... A low pressure system and the associated rain will be on the way out at the beginning of the extended period on Saturday across much of the northeastern U.S. The Mississippi and Ohio Valleys will see the potential for rain to become more widespread late in the weekend into next week as a couple frontal systems traverse the region and tap into Gulf of Mexico moisture. An expanding areas of rain will spread northeastward ahead of the midweek system (Tue-Wed) with more than an inch or so of rain over a large area, particularly over the Ohio/Tennessee Valleys northward to Michigan. Off the coast, the lingering stationary front may push back toward the coast and bring another area of rain into the Northeast. Heavy rain and higher elevation snow will be possible across the Pacific Northwest through the weekend and into early next week as upper shortwaves/surface fronts traverse the northern periphery of the upper ridge. These features will also bring rounds of snow to the higher elevations of the northern and then central Rockies by next week as snow levels lower. Chilly conditions will be prevalent through the weekend across portions of the central and eastern U.S. beneath the persistent upper trough, where highs will generally be 5 to 10 deg F below average. Meanwhile, above average temperatures will persist across portions of the West, from California into the Great Basin, where high temperatures may reach 5 to 15 deg F above average into next week downstream of a persistent upper-level ridge offshore. Fracasso/Ryan 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