Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EDT Sun Oct 25 2020 Valid 12Z Wed Oct 28 2020 - 12Z Sun Nov 01 2020 ...Heavy precipitation threat from the Plains/Mid-Mississippi Valley eastward to the Mid-Atlantic later this week... ...Guidance Evaluation/Preferences... Deep closed low will exit out of New Mexico early Wed and travel ENE to the Mid-Atlantic by Friday as it weakens into an open wave. Just ahead of its path, newly formed Tropical Storm Zeta will likely make landfall on Wednesday per the National Hurricane Center just ahead of the upper low and its surface fronts. The two sources of moisture will merge into/along the frontal boundary over the Tennessee Valley Thursday and move to the East Coast Friday. The GFS runs remained quicker with the timing of both the upper low and Zeta while the UKMET was slowest. Preferred the more stable guidance near the 12Z ECMWF/Canadian and ECMWF ensemble mean along with the slightly quicker GEFS mean for the upper low. Zeta lied closer to the GFS but also noted an unusually high degree of uncertainty not uncommon in nascent storms. How the interaction evolves will dictate the axis/orientation of heavy rain but the event itself looks quite probable. Pattern becomes much quieter by next weekend CONUS-wide. ...Weather/Hazard Highlights... Very cold airmass in the Plains will moderate but many areas will see see temperatures 10-30 degrees colder than normal on Wednesday. To the east of the upper low, near to above normal temperatures will trend toward near to below normal as the system progresses eastward. In the Pacific Northwest, California and into the Great Basin region, temperatures will be 5-15 degrees above normal through most of the period. Over the Southeast, moisture from Zeta will stream northward toward the in situ frontal boundary on Wednesday into Thursday just ahead of the closed low. Heavy rain and flooding are possible from the mid-Mississippi Valley to the Tennessee Valley. As the system moves eastward en masse, rainfall will continue to focus along the boundary to the Mid-Atlantic. Guidance remains inconsistent where this may be and will depend on the interaction of the two features at the start of the period. On the northwest side of the upper low, snow is likely over New Mexico to the TX/OK panhandles and southwestern KS. With marginally cold air in parts of the Northeast, snow is possible in higher elevations as the precipitation streams eastward (primarily in a warmer air mass). The Pacific Northwest will see a few bouts of mainly light rain and higher elevation snow. Fracasso 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