Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Oct 22 2020 Valid 12Z Sun Oct 25 2020 - 12Z Thu Oct 29 2020 ...Arctic air surges into the Western and Central U.S... ...Heavy Rain Threat for the Southern Plains/Mid-Mississippi Valley/Midwest/Northeast Sunday and Monday along with Upper Midwest Snows... ...Heavy Snow and Rain Threat for Southern and Central Portions of the Great Basin/Rockies/Plains Sunday-Wednesday... ...Guidance Evaluation/Preferences... The latest forecast guidance remains in above average agreement for the medium range period, particularly Sun into Mon where the WPC blend incorporated a composite GFS/ECMWF and GEFS/ECMWF ensemble means solution. For Tue-next Thu, the notable model differences lied with the ejection of a separated southern stream closed low across the Southwest U.S. where the GFS/GEFS exhibited its typical fast bias recently, but have trended significantly slower with the latest runs. Still, much lesser weights of the GFS/GEFS was used and instead increasingly replaced with the ECMWF ensemble mean. This solution supports a significant wintry episode for Southern and Central Portions of the Great Basin/Rockies/Plains Sun-Wed. The ECMWF ensemble mean also seems to offer best continuity within northern stream flow. The upper level pattern will favor troughing over the West as per guidance trends, with potent trough energies ejecting over the north-central US. 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. Rainfall will meanwhile develop underneath from the southern Plains/Mid-MS Valley through the Midwest/Great Lakes and into the Northeast mainly Sun/Mon. ...Weather/Hazard Highlights... Canadian high pressure ushering in much colder temperatures will dig sharply through the 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 wintry precipitation. A heavy snow/rain threat will spread over south-central portions of the Great Basin/Rockies/Plains and also over the Upper Midwest/Great lakes in lead flow with frontal wave genesis and another ample post-frontal cold high pressure surge Sun/Mon. Deeper warm sector moisture sourced from the Gulf will also 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/wave. 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