Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EDT Wed Oct 20 2021 Valid 12Z Sat Oct 23 2021 - 12Z Wed Oct 27 2021 ...Multi-day heavy rainfall and mountain snow threat inland from the Pacific Northwest and especially California... ...Overview... 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. Shortwave impulses are expected to to eject eastward across the Plains and then the Midwest states, with the main upper level trough reaching the Plains by the middle of next week. 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... The models and ensembles are in decent synoptic scale agreement on Saturday with the deep trough over the West Coast region and the closed low over southeast Canada. Changes become apparent by Sunday with a lead shortwave ejecting eastward from the broader scale trough to the west. The 12Z CMC was an outlier solution and out of phase with both the timing and especially the magnitude of the system crossing the Plains and Midwest by Monday, so none of the 12Z CMC was used for this forecast period. The new 00Z CMC was not as extreme with this feature, although still on the strong side of the guidance across the eastern U.S. by Tuesday. For the systems reaching the West Coast, there remains some model disagreement regarding timing and placement of the surface low centers, with the GFS indicating a more southern track towards northern California. By next Wednesday, the 12Z ECMWF is faster with the timing of the main trough axis across the Plains, and the CMC/GFS favoring a slower solution with potential closed upper low development. The WPC fronts/pressures forecast was primarily derived from a GFS/ECMWF/UKMET blend through this weekend, and then GFS/ECMWF and their respective ensemble means for Monday through Wednesday. ...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. Heavy snows are likely for the higher elevations of the Cascades and Sierra, with over a foot of accumulation possible above pass level. Snow levels are expected to be lower by late Sunday and 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 by Tuesday. Across the eastern U.S., a lead warm front lifting northward across portions of the Midwest this weekend and into Monday is expected to produce scattered to numerous showers with the potential for a few mesoscale convective complexes. 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 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