Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sat Oct 16 2021 Valid 12Z Tue Oct 19 2021 - 12Z Sat Oct 23 2021 ...Overview... Fairly progressive upper-level flow is expected for the initial part of the medium range period, with a closed upper low initially over the Rockies/High Plains that will eject eastward over the northern Plains and Upper Midwest by the middle of the week, while pushing a cold front ahead that should reach the East Coast by the end of the week. Meanwhile, mean troughing over the eastern Pacific is likely to have several shortwaves rotating around it, which should lead to a couple of cold frontal passages and rounds of precipitation along the West Coast, while an amplifying upper ridge over the Intermountain West will tend to limit the eastward progression of these weather systems. ...Guidance Evaluation/Preferences... Model guidance remains in good agreement with the overall synoptic scale pattern through Wednesday with the aforementioned closed low tracking across the central U.S., as well as troughing across the eastern Pacific and a closed low centered over the Canadian Maritimes. However from Wednesday onward, complex northern stream flow across Canada has shown several differences between models and from run to run. While guidance is now more agreeable in the 00Z/06Z cycle for a closed low over east-central Canada by Thursday, its positioning shows differences--the UKMET and EC are southwest of the GFS runs and south of the CMC. These discrepancies remain on Friday, as the CMC retrogrades while GFS runs lift the core of the upper low northeastward. This feature affects the track of the closed low as it shrinks and its timing of joining up with the northern stream flow (GFS runs are slower with the southern shortwave/low and maintain separation longer, but the new 12Z GFS may be trending a bit more towards consensus of the other guidance). These discrepancies lead to lower forecast confidence across the Northeast U.S. by later in the forecast period. Across the eastern Pacific into the western U.S., the large-scale trough pattern is shown by all deterministic and ensemble guidance, though with typical differences in shortwave timing and magnitude. However, timing of a shortwave entering the Pacific Northwest early Wednesday is becoming more agreeable. By the latter part of the week, the 00Z CMC remains somewhat of a weaker amplitude outlier compared to the GFS runs, with the 00Z ECMWF sort of a middle ground solution. Details with this are quite uncertain especially since this troughing/energy tracks from data-poor regions of the Arctic/Russia/northern Pacific. For today's WPC mass fields, a multi-model deterministic blend of the 06Z GFS and 00Z ECMWF, UKMET, and CMC was utilized for the beginning of the medium range period. Then for the latter part of the period, transitioned toward using a higher proportion of ensemble means, particularly the EC mean given it gave better definition to the Canadian upper low that deterministic guidance has been trending towards. Overall this maintained good continuity with the previous forecast. ...Weather/Hazards Highlights... The developing storm system over the Plains and Midwest is expected to produce snow across higher elevations of the north-central Rockies and into the Black Hills, with some possibility of light snow over the northern High Plains as well, or a chilly rain. Rain should then spread across the Midwest and Great Lakes along the track of the surface low, while light rain showers are also expected along the cold front as it moves eastward from the Plains to the Eastern Seaboard. 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 4 inches of rain over the entire forecast period, particularly across windward terrain. In terms of temperatures, 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 10 to 20 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. Generally cooler than normal temperatures are forecast for the West Coast, influenced by the upper trough. 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. Tate/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