Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Tue Oct 19 2021 Valid 12Z Fri Oct 22 2021 - 12Z Tue Oct 26 2021 ...Heavy rainfall potential next weekend from central California to western Washington... ...Overview... An active weather pattern will be developing across the western U.S. by the end of the week as a series of strong storm systems off the West Coast 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 with heavy precipitation and gusty winds. 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. A warm front extending across the Midwest and Ohio Valley early next week will likely provide a focus for showers and storms. ...Guidance Evaluation/Preferences... Model guidance for the 00Z cycle is in decent agreement with the overall synoptic scale pattern through Saturday night, particularly with the timing of the cold front reaching the East Coast and the first Pacific cold front affecting the West Coast region, so a GFS/ECMWF/CMC model blend sufficed as a starting point in the forecast process. The 00Z CMC strayed from the model consensus by Sunday with a much stronger trough crossing the Plains that evolves into a closed low over the Midwest by Monday night with very little ensemble support, so the latter half of the forecast was primarily derived from a GFS/ECMWF/ECENS/GEFS blend along with some previous WPC continuity. For the storm system reaching the West Coast Saturday night, the GFS is a bit more amplified and faster, with height falls spreading across the Intermountain West sooner that the CMC/ECMWF. By the end of the forecast period, the CMC is stronger with a shortwave trough crossing the central Plains, whereas the ensemble means are suggesting more of an upper ridge axis here. For days 6 and 7, the WPC forecast incorporated more of the EC and GEFS means, while still maintaining some of the operational guidance. ...Weather/Hazards Highlights... Across the West Coast region, multiple rounds of rain are expected from northern California to western Washington, and some of these rain showers 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 early morning as a deep moisture plume moves inland across much of central and northern 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 windward terrain which may result in some instances of flooding. Snow is likely for the highest mountains of the Cascades and Sierra Nevada, mainly above pass level. Snow levels are expected to be lower by late Sunday and into Monday with the potential for over a foot of snow for the highest elevations. Strong winds are also possible near the coast. Some of this moisture is expected to reach the Intermountain West and the central/northern Rockies with significant snowfall for some of these areas. Across the eastern U.S., a warm front lifting northward across portions of the Midwest and Ohio Valley this weekend and into Monday is expected to produce scattered to numerous showers and perhaps some thunderstorms. In terms of temperatures, 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. Generally cooler than normal temperatures are forecast for the West Coast, influenced by the upper trough and mainly overcast skies with highs on the order of 10-20 degrees below normal. Temperatures should be generally within 5 degrees of climatological averages for the eastern U.S. through early next week. 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