Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 AM EST Fri Nov 27 2020 Valid 12Z Mon Nov 30 2020 - 12Z Fri Dec 04 2020 ...Dynamic and multifaceted storm to impact the northeast quarter of the CONUS early next week... ...Guidance Evaluation/Preferences... An active/blocky pattern continues to challenge the models/ensembles in consistency/speed/tracking of two main features over the lower 48 next week. First system of interest will be in the East Monday as an upper low closes off in the Midwest/eastern Great Lakes. Trend has been for a little quicker progression, but considerable spread still existed through the 12Z/18Z guidance between the slowest (UKMET) and fastest (ECMWF) solutions at the short-range/medium range interface ~12Z Mon. Split the difference between the ECMWF and GFS/Canadian clusters, nearest to the best ensemble overlap. Next system will enter the West as the eastern system wraps up, with timing differences between the GFS (quickest) and UKMET/Canadian (slowest), leaving the ECMWF in the middle. Trend was slower but then quicker most recently, so a GFS/ECMWF middle ground again seemed prudent. That system will push eastward, split into a northern/southern portion, and eventually dig through the Rockies or Front Range. This east-west spread in the guidance remains rather large, but with a shift eastward in recent runs. Quickening eastern system may allow for a farther east adjustment, along the lines of the GFS/ECMWF and not the much farther west UKMET/Canadian. Regardless, still expect this feature to travel slowly out of the Plains to the Mid/Lower Mississippi Valley, promoting cyclogenesis out of Texas through the Southeast and/or Gulf of Mexico. Incorporated an increasing weight of the GEFS/ECMWF ensemble means to account for spatiotemporal uncertainty. ...Sensible Weather and Hazards... Occluding low in the central Appalachians early Monday will lift into the eastern Great Lakes by early Tuesday. Ahead of the cold front, breezy southerly flow will bring in a surge of mild air and well above normal moisture levels. This will lead to a large area of modest to locally heavy rainfall Monday from the Mid-Atlantic northward into New England, especially east of I-81 and along I-91/95. Flooding may be a concern, especially in the urban areas with poor drainage. On the northwest/west/southwest side of the surface low, colder air will wrap around the system and support appreciable snowfall. As the low exits through the St. Lawrence Valley, flow over the Great Lakes will wring out additional snowfall or mixed rain/snow. Temperatures will be mild ahead of the front, falling to below normal readings once the cold front moves through from southwest to northeast. Largest areas of cool weather relative to normal will be across the Southeast and into Florida where highs may not reach 70 degrees into South Florida Tuesday. Incoming system to the Pac NW will be rather moisture-starved with only light amounts for the coastal ranges and Cascades Monday and across the Rockies Tuesday. Path of the upper trough/low will dictate how far west the cooler push of air will travel, favoring along/east of the Rockies per the forecast thinking. As the trough heads east of 100W, increased Gulf moisture will spread northward which favors an expanding area of rainfall along the northwest Gulf Coast into the lower Mississippi Valley late Wed into Thu. This should move eastward into Friday, perhaps through the Southeast into the southern Mid-Atlantic, depending on frontal wave development. 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