Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 AM EDT Thu Oct 21 2021 Valid 12Z Sun Oct 24 2021 - 12Z Thu Oct 28 2021 ...Multi-day Heavy Rainfall and Mountain Snow Threat over the West... ...Heavy Rainfall Threat over the Midwest... ...Overview... A quite 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 this weekend into early next week. Lead shortwave impulses and convection focusing surface lows/fronts will eject eastward across the Plains and then the Midwest states and Northeast/Mid-Atlantic to the western Atlantic, with the main upper level trough/wavy front reaching an increasingly wet central to east-central U.S. into next Wed/Thu. ...Guidance Evaluation/Preferences... Despite high likelihood overall of the impending heavy precipitation event over the West, run to run model continuity has been 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 U.S. well into next week. Variability in the latest few model runs seems be be slowly improving, mainly during the first half of this forecast period, but recent GEFS/ECMWF ensemble means along with the National Blend of Models remain much more run to run compatible through medium range time scales and the latest were primarily used to derive the WPC product suite. This also acts to maintain good WPC product continuity. ...Weather/Hazards Highlights... A lead and wavy lead front across portions of the Midwest/Mid-Atlantic then western 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 Sunday to the Mid-Atlantic/New England Monday with coastal low development. Forecast spread with this feature is gradually decreasing with recent guidance runs, bolstering forecast confidence to a degree. Meanwhile across the West Coast region, multiple rounds of enhanced rains are expected from the Pacific Northwest 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 multiple feet of max 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 inland, especially along 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 as shown in the WPC Winter Weather Outlook linked below. The main associated cold front reaching the Plains by Tuesday and onward across the east-central U.S. Wednesday/Thursday that is expected to present an emerging and expanding lead rainfall/convective focus. Additional upstream Pacific system approaches should meanwhile keep an active pattern with periods with enhanced precipitation chances going into the Pacific Northwest well into next week. 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. 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 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