Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Mon Oct 18 2021 Valid 12Z Thu Oct 21 2021 - 12Z Mon Oct 25 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 with a Canadian surface high settling southward across the Plains and Midwest. ...Guidance Evaluation/Preferences... Model guidance for the 00Z cycle is in decent agreement with the overall synoptic scale pattern through Friday 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/UKMET model blend sufficed as a starting point in the forecast process. The 12Z CMC strayed from the model consensus with an embedded closed low crossing the northern Rockies, but it was otherwise close to the other guidance. 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 showers are expected from northern California to western Washington, and some of these will likely be moderate to heavy at times with moist onshore flow ahead of multiple Pacific storm systems. Some locations may get on the order of 3 to 6 inches of rain over the entire forecast period, particularly across windward terrain. Snow is likely for the highest mountains of the Cascades and Sierra Nevada, mainly above pass level. There is some signal in the model guidance for an atmospheric river event next weekend ahead of an energetic Pacific storm system across the coastal ranges of northern California and southwest Oregon. Strong winds are also possible near the coast. By Sunday and into Monday, some of this moisture is expected to reach the Intermountain West and the central/northern Rockies. Across the eastern U.S., a broken swath of showers and perhaps some thunderstorms can be expected ahead of the advancing cold front, but the relative dearth of deep moisture ahead of it will serve to limit the heavy rainfall potential. In terms of temperatures, it will feel more like mid November across much of the Northern Plains in the wake of the departing storm system on Thursday, with high temperatures running about 5 to 15 degrees below average. There should be a gradual moderation in temperatures going through the end of the workweek and into the weekend as an upper-level ridge begins to build in, and then readings up to 10 degrees above average for the High Plains by next Sunday. Generally cooler than normal temperatures are forecast for the West Coast, influenced by the upper trough and mainly overcast skies. Ahead of the cold front, it will likely feel more like late September across the East Coast with highs on the order of 5-10 degrees above average for Thursday, although humidity levels should remain in check. Slightly below normal highs are then expected behind the cold front. 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