Extended Forecast Discussion NWS Weather Prediction Center College Park MD 229 PM EDT Tue Oct 20 2020 Valid 12Z Fri Oct 23 2020 - 12Z Tue Oct 27 2020 ...Cold air surges into the U.S... ...Heavy Snow Threat Northern/Central Rockies and North-Central U.S... ...Heavy Rain Potential for the Midwest/Great Lakes... ...Guidance Evaluation/Preferences... Average to above average confidence exists for the large scale pattern during the medium range period (Oct 23-27) across the CONUS. The upper level pattern will favor troughing over the northern Rockies into the north-central US that is reinforced later in the forecast period. Downstream, ridging will persist for the Southeast U.S. while the predominant storm track through the central U.S. to Great Lakes will bring periods of unsettled weather. The latest deterministic and ensemble guidance exhibited fairly agreeable solutions for the period and the WPC blend therefore incorporated the GFS/ECMWF/UKMET/CMC initially followed by a near equal blend of the GFS/GEFS/ECMWF/ECENS for later periods. This trended well with continuity. ...Weather/Hazard Highlights... Early on in the forecast period, a shortwave trough and associated low pressure will spread precipitation across the Midwest/Great Lakes regions Friday. In its wake, Canadian high pressure ushering in much colder temperatures will spread south/east but begin to moderate as it reaches the East Coast this weekend. Late this weekend into early next week, a much stronger shortwave trough originating from the Northeast Pacific will dive down across portions of the Northwest, Intermountain West, and northern Rockies. This will result in significant temperature changes and widespread winter precipitation. Heavy snow potential will exist from portions of the Rockies eastward into the Plains/Great Lakes/Upper Midwest. On the warm side of the cold front, deeper moisture sourced from the Gulf will interact with the front to produce widespread showers and thunderstorms, some of which could repeat/train and produce heavy amounts. Persistence of cold Canadian air will keep the core of most extreme below normal temperature anomalies centered over the northern Rockies/High Plains during the period. Parts of Montana may see readings up to 30-40F below normal for multiple days while only moderately less extreme anomalies push southward over the Plains in response to the individual shortwaves/surface systems expected early and late in the period. Expect some of this cold air to extend into the West as well, with temperatures 5-20F below normal over the Interior Northwest/northern Great Basin from the start of the period Thu, followed by expansion/southward push of minus 10-20F anomalies over the West 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 signal for additional moisture that could reach Florida in association with low pressure that could emerge from the western Caribbean Sea. However guidance has been diverse and inconsistent with this. Schichtel/Taylor 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