Extended Forecast Discussion NWS Weather Prediction Center College Park MD 519 PM EDT Wed Oct 20 2021 Valid 12Z Sat Oct 23 2021 - 12Z Wed Oct 27 2021 ...Multi-day Heavy Rainfall and Mountain Snow Threat over the West... ...Heavy Rainfall Threat over the Midwest... ...Overview... It remains highly likely that an active storm track over the eastern Pacific in association with an anomalous upper level trough will result in multiple rounds of impactful weather for the West Coast and extending well inland across the Intermountain West and Rockies. Lead shortwave impulses and weather focusing surface lows/fronts will eject eastward across the Plains and then the Midwest states and Northeast/Mid-Atlantic, with the main upper level trough/wavy front reaching the Plains by mid next week. Elsewhere across the eastern U.S., a surface high should be in place after the cold front exits the coast on Friday, followed by an upper ridge axis developing ahead of the central-western U.S. trough by the end of the forecast period. ...Guidance Evaluation/Preferences... Despite high likelihood of the impending heavy precipitation event over the West, it has become increasingly evident that run to run model continuity has become increasingly problematic with local focus with the protracted series of dynamic Pacific systems to slam into the West Coast and subsequently progress downstream over the central and eastern states. The GEFS/ECMWF ensembles show the forecast spread, but at least the means are in good agreement and provide good WPC product continuity. The 12 UTC model suite has overall trended toward these means, but in particular the GFS/Canadian. ...Weather/Hazards Highlights... Across the West Coast region, multiple rounds of enhanced rains are expected from Washington through especially California that will likely be heavy at times with moist onshore flow ahead of multiple Pacific storm systems. This will especially hold true for Sunday and into Monday as a deep and long fetch moisture plume shifts southward from the Pacific Northwest across California as part of an atmospheric river event. Some locations may get on the order of 4 to 8+ inches of rain over the entire forecast period, particularly across favored windward terrain which may result in some instances of runoff issues and/or flooding. Burn scars will also be especially vulnerable. Heavy snows are likely for the higher elevations of the Cascades and Sierra, with over a foot of accumulation possible. Snow levels are expected to lower Sunday into Monday as the core of the upper trough moves inland. Strong winds are also possible for the coast and along the mountain ridges. Some of this moisture is expected to reach the Intermountain West and the central/northern Rockies with significant snowfall for some of these areas, with a cold front reaching the Plains and MS Valley Tuesday into Wednesday that may present an emerging and expanding lead rainfall/convective focus. Across the central to eastern U.S., a less certain but wavy lead front lifting northward across portions of the Midwest/Mid-Atlantic this weekend and into next week is expected to produce scattered to numerous showers with the potential for a few convective complexes that offer potential for locally heavy downpours/runoff issues, especially over the Midwest. In terms of temperature highlights, it will feel more like late September across much of the central and southern Plains as the upper level ridge remains in place across this region, with highs running up to 15 degrees above normal ahead of the cold front. Cooler than normal temperatures are forecast for the West Coast, influenced by the upper trough and mainly overcast skies with highs decreasing to 10-20 degrees below normal. Hamrick/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of California, the Great Basin, the Pacific Northwest, and the Southwest, Sat-Tue, Oct 23-Oct 26. - Heavy precipitation across portions of the Central Rockies, the Great Basin, and the Northern Rockies, Mon, Oct 25. - Heavy rain across portions of the Central Plains, the Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Sun, Oct 24. - Flooding occurring or imminent across portions of the Southern Plains. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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