Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 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 GFS and the 12Z ECMWF UKMET. This blend yielded favorable results elsewhere as well, with the next weakening low pressure system exiting the Rockies into the plains on days 3-4. By day 5 (Mon), the next shortwave should be exiting from the Rockies into the plains, with yet another surface low pressure system developing. Model spread begins to increase for this system as it crosses the plains and especially by days 6-7 (Tue-Wed) as some solutions spin up an intense low pressure system across the Midwest/Great Lakes. Model solutions show spread with respect to both timing and amplitude of this system, but there is broad agreement than the wave will reach the plains and amplify to some degree during this time period. As a result, use of deterministic guidance was gradually tapered during the day 5-7 time frame, with use of ECENS and GEFS ensemble means gradually increased. ...Weather Highlights/Threats... A low pressure system and the associated rain will be the way out at the beginning of the extended period on Sat 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. 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 perhaps the 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. 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