Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Sun Oct 17 2021 Valid 12Z Wed Oct 20 2021 - 12Z Sun Oct 24 2021 ...Overview... A fairly typical late October weather pattern is expected to be in place across the majority of the nation during the medium range forecast period. A surface low and trailing cold front will track from the Midwest to the Northeast, with the front exiting the East Coast by the end of the week. A rather active pattern will be in place across the eastern Pacific with a persistent upper level trough off the West Coast that will support several shortwaves rotating around it along with a couple of cold frontal passages and rounds of precipitation. Meanwhile, a ridge axis is expected to briefly build over the Intermountain West and then be positioned over the Plains by next weekend. ...Guidance Evaluation/Preferences... Model guidance for the 00Z cycle is in good agreement with the overall synoptic scale pattern through about Thursday, and the operational guidance agrees that the upper low tracking across the Plains and Midwest will likely become absorbed by northern stream flow over the Great Lakes region. Model differences remain across Ontario and Quebec with a closed low dropping south, with the 00Z CMC appearing to be a slower and slightly weaker solution, and the past few runs of the GFS remaining more progressive. Out West, the models remain in decent agreement on the overall large scale trough, with the typical differences in shortwave timing and magnitude, with the GFS more amplified with the shortwave reaching California Friday night. The WPC fronts and pressures were primarily derived from a multi-deterministic blend through Thursday, and then increasing contributions from the ensemble means by next weekend owing to increasing model spread and less weighting to the GFS. ...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 2 to 5 inches of rain over the entire forecast period, particularly across windward terrain. Snow is possible 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, with rainfall reaching as far south as the greater San Francisco metro area. Strong winds are also possible near the coast. 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 likely feel more like mid November across much of the Northern Plains in the wake of the departing storm system on Wednesday, with high temperatures running about 10 to 15 degrees below average. There should be a gradual moderation in temperatures going through the end of the week and into the weekend as an upper-level ridge begins to build in, and then readings up to 10 degrees above average for the western 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 from the Great Lakes and Ohio Valley to the East Coast with highs on the order of 5-10 degrees above average for the middle of the week, although humidity levels should remain in check. 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