Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM 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... Models and ensembles continue to offer a reasonably similar depiction of the larger scale pattern evolution over the lower 48 and vicinity through medium range time scales and even the more organized embedded systems offer lower forecast spread in recent guidance. This bolsters forecast confidence, but the smaller scale interactions and weather focus still have room to improve into short range time scales. Overall, a composite blend of the most compatible GFS/ECMWF and GEFS/ECMWF ensemble means seems to provide a good forecast basis. ...Weather/Hazard Highlights... A potent ejecting n-central U.S. lead shortwave energy and associated low pressure will produce periods of moderate precipitation across the Midwest/Great Lakes Fri. Expect modest activity down along a trailing cold front that will usher in quite a cooling Canadian high pressure surge far southward through the central U.S. Fri that spreads in moderated form across the East this weekend. Meanwhile, a Northeast Pacific wave reaching the Northwest late this week will dig moisture down through the West this weekend and early next week, with the rain/higher elevation snow extending southward with time as upper troughing amplifies/sharpens and with cold frontal/high pressure surge. Toward the start of next week another Plains/Great Lakes wave may develop, spreading potential for heavy snow from the Rockies to parts of the northern half of the Plains into the Upper Midwest. Also expect increasingly organized/enhanced rains with the front trailing from the surface low into early next week from the southern Plains through the Midwest/Great Lakes and into the Northeast with system maturity. Some heavy downpours/runoff issues are possible. 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 25-35F 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 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