Extended Forecast Discussion NWS Weather Prediction Center College Park MD 218 AM EDT Sat Oct 16 2021 Valid 12Z Tue Oct 19 2021 - 12Z Sat Oct 23 2021 ...Overview... A typical late October weather pattern is expected to be in place across the majority of the nation for next week. The main weather system of interest will be a closed upper low initially over the Rockies that will eject eastward over the northern Plains and Upper Midwest by the middle of the week, and this will sustain a cold front that should reach 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, with an upper level ridge over the Intermountain West that will tend to limit the eastward progression of these weather systems. ...Guidance Evaluation/Preferences... Model guidance for the 00Z cycle is in good agreement with the overall synoptic scale pattern through Wednesday, and the operational guidance has locked into the slower solution for the closed low tracking from the Rockies to the Plains. However, northern stream flow over central Canada is more complicated and questions remain as to whether the central U.S. low may try to phase with northern stream energy, or remain separate. Model differences are rather significant across the Hudson Bay region with the 12Z ECMWF appearing to be an outlier by drifting a strong closed low westward across Manitoba, and the GFS is much more progressive. This leads to lower forecast confidence downstream across the Northeast U.S. by later in the forecast period. Out West, the models remain in decent agreement on the overall large scale trough, with the typical differences in shortwave timing and magnitude. The 00Z CMC is indicating less in terms of amplitude by late Friday compared to the ECMWF/GFS. The WPC fronts and pressures were primarily derived from a CMC/GFS/ECMWF blend along with some previous WPC continuity, and increasing contributions from the ensemble means by Friday and Saturday owing to increasing model spread. Less of the 12Z ECMWF was used near the Canadian border over the northern Plains given the differences noted earlier. ...Weather/Hazards Highlights... The developing storm system over the Plains and Midwest will fuel the development of scattered showers and storms from Texas to the Ohio Valley for the middle of the week, and a chilly rain north of the surface low across the Dakotas and Minnesota. It will be cold enough for accumulating snow for the higher terrain of Wyoming and the Black Hills of South Dakota on Tuesday as the upper low and moisture slowly depart the region. 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 moisture rich onshore slow ahead of multiple Pacific storm systems. Some locations may get on the order of 2 to 4 inches of rain over the entire forecast period, particularly across windward terrain. In the temperature department, it will likely feel more like early-mid November across much of the Intermountain West and the central/northern Rockies in the wake of the departing storm system on Tuesday, with high temperatures running about 5 to 15 degrees below average. There should be a gradual moderation in temperatures going through the middle to end of the week as an upper level ridge begins to build in, and then readings up to 10 degrees above average for some of these same areas by next Saturday. Ahead of the cold front, it will likely feel more like late September across the Great Lakes and Ohio Valley with highs on the order of 5-15 degrees above average for the middle of the week, although not to the levels observed over this past week and not as humid. 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 Hazards: - Flooding possible across portions of the Southern Plains. - Flooding occurring or imminent across portions of the Southern Plains. - Flooding likely across portions of the Southern Plains. 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