Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Wed Oct 24 2018 Valid 12Z Sat Oct 27 2018 - 12Z Wed Oct 31 2018 ...Guidance/Predictability Assessment and Preferences... General agreement with large scale features will provide somewhat better than average confidence in the overall mean pattern evolution but embedded medium to smaller scale shortwave features will lead to uncertainty in some forecast details. Into the latest cycle of guidance recent trends are continuing for systems within/supported by the eastern U.S. mean trough aloft. Consensus maintains faster progression of the storm system affecting areas near the East Coast this weekend in response to amplifying trends for upstream energy digging into the eastern trough. Importance of smaller scale details aloft for the East Coast storm is highlighted by the significant divergence of individual ensemble member surface lows that arises after early Sat. The amplifying trend for the upstream energy is steadily increasing the potential for meaningful system development along the northern half of the East Coast around days 5-6 Mon-Tue. However there is a fair amount of spread among models/ensemble members regarding how the energy aloft will ultimately evolve, with the surface low track/strength quite sensitive to those specifics. Meanwhile evolution over the eastern Pacific and western U.S. will lead to a significant pattern change, with an initial eastern Pacific trough eventually amplifying/elongating over the West as a ridge steadily builds over the eastern Pacific. Interestingly, over the past couple days the operational GFS runs have led the operational ECMWF in reflecting the amplitude and sharpness of the upper trough while the among the means the GEFS has been trending toward the ECMWF mean that had an earlier reflection of the trough from the eastern Pacific into the West. As a whole consensus has been trending more amplified/elongated with the trough, and teleconnections relative to the upstream ridge's positive height anomaly center support the northeast-southwest oriented trough seen in most guidance by late in the period. From the teleconnection/multi-day mean perspective by day 7 Wed either an open trough (GFS, ensemble means) or embedded closed low (00Z ECMWF/CMC) are plausible. Thus at that time an even blend among model/ensemble solutions would be reasonable--indicating some separation in flow but not to the point of having a closed low. The evolving Pacific/western U.S. pattern will eventually favor a flattening of flow aloft over the eastern U.S. Around the end of the extended period the question for timing the departure of the eastern trough aloft will be the relative influence of the upstream pattern that will become increasingly favorable for the trough's progression, versus rate of weakening of strong Atlantic ridging that during most of the period favors the amplified eastern trough. The forecast update started with an operational model blend and minor detail enhancement early in the period followed by a gradual trend toward an even model/ensemble mean blend by day 7 Wed given western U.S. preferences and increasing detail uncertainty over the East. ...Weather Highlights/Threats... The initial system along the East Coast will bring an area of significant precipitation to the Northeast this weekend. Inland/higher elevation locations may see some snow from this event. Meanwhile a tight pressure gradient between the surface low and retreating high pressure to the north will lead to strong winds on the north side of the system. The next system should spread an area of rain across parts of the northern Plains/Midwest this weekend and then the Appalachians and East Coast states by the first half of next week. Details of system evolution will determine the degree of snow potential over the higher terrain. If surface development is sufficiently strong then there would be another episode of breezy to windy conditions over the Northeast Tue-Wed. A frontal system nearing the Pacific Northwest during the weekend and a period of onshore flow will bring a period of focused precipitation to the coastal ranges, Cascades, and eventually to a lesser extent northern Rockies. Energy rounding the strengthening eastern Pacific ridge aloft may support additional activity toward midweek but at this time it is uncertain whether it will extend far enough south to reach the Pacific Northwest. Expect mostly rain except for perhaps a little snow at highest elevations. Rain and higher elevation snow will likely reach the Four Corners states by Tue-Wed as the upper level trough progresses inland. Much of the western third of the lower 48 will be quite warm during Sat-Mon with some locations seeing one or more days of highs and/or lows 10-20F above normal. Arrival of upper troughing will then bring a rapid cooling trend with highs at least 5-10F below normal from the Great Basin/Southwest into central Rockies by next Wed. Much of the eastern U.S. will see below normal temperatures most of the period with only gradual moderation from west to east by next Tue-Wed. Best potential for highs 10F or more below normal will extend from the Ohio/Tennessee valleys into the Appalachians and Northeast. Clouds/wind will tend to keep morning lows closer to normal. Rausch 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