Extended Forecast Discussion NWS Weather Prediction Center College Park MD 234 AM EDT Wed Oct 21 2020 Valid 12Z Sat Oct 24 2020 - 12Z Wed Oct 28 2020 ...Arctic air surges into the U.S... ...Heavy Snow Threat Northern-Central Great Basin/Rockies and the North-Central U.S... ...Heavy Rain Threat for the Southern Plains/Mid-Mississippi Valley and Midwest/Great Lakes... ...Guidance Evaluation/Preferences... Average to above average confidence exists for the large scale pattern during the medium range period across the CONUS and a composite blend of the compatible GFS/ECMWF, GEFS/ECENS means and the National Blend of Models seems to offer a solid forecast basis days 3-7. The upper level pattern will favor troughing and possible closed low separation over the West as per guidance trends, with potent trough energies ejecting over the north-central US. Downstream, ridging will persist for the Southeast U.S. while a predominant storm track over the deeply cooled West and north-central U.S./Great Lakes will bring periods of significant wintry weather. Heavy rainfall will meanwhile develop underneath from the southern Plains/Mid-MS Valley through the Midwest/Great Lakes and into the Northeast. ...Weather/Hazard Highlights... Canadian high pressure ushering in much colder temperatures will dig sharply through thr central U.S., but will moderate as it reaches the East. Upstream, a strong trough and reinforcing energies will dive robustly across the West this weekend and next week. This will result in significant temperature changes and widespread wintery precipitation. A heavy snow threat will spread over the north-central Great Basin and the Rockies onward across the northern Plains and Upper Midwest/Great lakes with frontal wave genesis and another ample post-frontal cold high pressure surge. Downstream, deeper warm sector moisture sourced from the Gulf will interact with a leading cold front to produce widespread showers and thunderstorms, some of which could repeat/train and produce some heavy amounts from the southern Plains northeastward to the Midwest and Northeast Sun-Tue. Persistence of cold Canadian air will keep the core of most extreme below normal temperature anomalies centered over the north-central Rockies/High Plains during the period where readings up to 30-40F below normal are possible for multiple days, while only moderately less extreme anomalies push southward over the Plains in response to the individual shortwaves/surface systems and snow cover. Cold air will also dig over the West, with temperatures 10-25F below normal for the Interior Northwest/Great Basin as upper troughing amplifies. Farther east, the warm sector ahead of the first front will contain well above normal temperatures (up to plus 10-20F anomalies). Frontal passage will bring highs close to normal over the East, but morning lows will remain above normal over the southeast third of the lower 48 as the boundary lingers over the Florida Peninsula and adjoining waters. Meanwhile expect moisture and rainfall of varying intensity currently over and near Florida to reach the eastern Gulf of Mexico by the start of the period. This rain may lift northward across the Southeast in advance of the leading front that reaches the East. There is still fairly high uncertainty regarding the amount of moisture that ultimately reaches the Southeast and how it may interact with the front. There has also been a dwindling signal for additional moisture that could reach Florida in association with a western Caribbean Sea disturbance. 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