Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sun Oct 18 2020 Valid 12Z Wed Oct 21 2020 - 12Z Sun Oct 25 2020 ...Guidance Evaluation/Preferences... Guidance continues to offer a reasonably well clustered pattern evolution for much of the coming week. The pattern will be highlighted by a series of shortwaves moving through a broad upper trough that is forecast to remain the dominant feature over much of the lower 48, with lingering ridging over just offshore the Southeast. Both the GFS and ECMWF as well as their ensemble means agree quite well with the timing and evolution of an apparent surge of cold air down into the Rockies from western Canada, with potential upper trough development over the West by next weekend. The CMC begins to accelerate the forward speed of individual shortwaves, resulting in a pattern evolution incompatible with the GFS and ECMWF by Day 6. Towards the end of the week, moisture may be drawn northward from the Gulf of Mexico into the east-central Gulf states/Florida as a tropical low could develop over the western Caribbean Sea per the latest from the NHC. The WPC medium range product suite was derived mostly from an even blend of the 06Z GFS/GEFS mean with the 00Z ECMWF/ECMWF ensemble mean and the National Blend of Models. This yielded a pattern progression that is slightly slower over the eastern U.S. than previous WPC forecasts. ...Weather/Hazard Highlights... A series of wavy frontal systems with modest moisture will focus periods of moderate precipitation from the Northwest/Rockies and north-central U.S./Midwest to the Northeast. Post-frontal Canadian cold air surges will dig through the Northwest/Great Basin/central U.S. and eventually across the East. Temperature profiles favor periods of snow for the Cascades, the northern Rockies and the north-central states. The highest chance of heavy snows remains for the northern Rockies, particularly during the middle of the week and the weekend. Meanwhile, the heaviest rains may focus over the Upper Midwest and Great Lakes Wed/Thu with frontal wave genesis and passage. A swath of wintry precipitation will appear to move across the northern Plains to the upper Midwest during this time-frame as the developing frontal wave pulls down colder air from Canada. Farther south, moisture may increase over Florida and the east-central Gulf states late week as a tropical low is forecast to move slowly northward from the western Caribbean Sea. Over southern California, some meaningful rainfall could move onshore and especially for the wind-facing mountain slopes by the weekend as an upper trough is forecast to deepen. Kong/Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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