Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Oct 19 2021 Valid 12Z Fri Oct 22 2021 - 12Z Tue Oct 26 2021 ...Multi-day heavy rainfall and mountain snow threat inland from the Pacific Northwest and especially California... ...Overview... A highly active and increasingly wet multi-day weather pattern will develop and shift southward along the West Coast late week into early next week as a series of strong eastern Pacific storm systems pivot around a rather pronounced trough over the northeast Pacific. This will result in a couple of cold frontal passages that will progress inland across the Intermountain West/Rockies with local heavy precipitation and gusty winds, eventually working into the Plains. Meanwhile across the eastern U.S., a cold front and upper level trough will be exiting the East Coast by Friday afternoon with a Canadian surface high settling southward across the Plains and Midwest. An upstream warm front extending across the Midwest by early next week will likely provide a focus for showers and storms that could produce some localized heavy convection/downpours. ...Guidance Evaluation/Preferences... Model and ensemble solutions overall seem well clustered days 3/4 (Fri-Sat), bolstering forecast confidence in a composite GFS/ECMWF/UKMET/Canadian model solution that is well supported by ensembles and the National Blend of Models. The larger scale flow evolution is also decently depicted by most guidance into the day 5-7 time frame and there is a well defined trend toward amplification that seems reasonable considering active upstream flow. However, there is growing forecast spread in this period, especially with the transfer of energy and eastern Pacific storm genesis. Accordingly, the WPC medium range product suite focuses by then increasingly on the still compatible GEFS/ECMWF ensemble means. For a particular storm system that may reach the West Coast on Sunday, the 00 UTC CMC and UKMET were considerably stronger with the surface low compared to the GFS and ECMWF, and there was actually a decent supporting cluster of GEFS/ECMWF ensemble solutions, but the 12 UTC UKMET/Canadian have backed off. WPC product continuity is well maintained with this strategy and a composite of latest 12 UTC guidance remains in line for threat messaging. ...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. Heavy snows are likely for higher elevations of the Cascades and Sierra. Snow levels are expected to be lower by late Sunday and into Monday with pattern progression. Strong winds are also possible. 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 focusing front working into 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 an uncertain thunderstorm focus. 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. 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 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