Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 AM EST Sat Nov 28 2020 Valid 12Z Tue Dec 01 2020 - 12Z Sat Dec 05 2020 ...Active pattern through the week... ...Guidance Evaluation/Preferences... Positive height anomaly in western Canada will favor troughing from the Southwest to the East Coast next week. Lead system will exit the Northeast Tuesday as a wrapped-up occluded system with heavy rain possible over eastern New England early in the day and wrap-around snow on the west side of the low. Concurrently, western system will dig through the Rockies to the Plains, but the models and ensembles continue to waver on how far west this may occur. Generally the GFS has been less aggressive and farther east while the UKMET has been farthest west. The ECMWF/Canadian have oscillated between the two extremes and the ensemble means have mostly followed their parent model but as a more muted depiction. Due to the ongoing uncertainty, opted to stay near the middle of the guidance via the 12Z Canadian. To the north, another system will likely dig southward out of Canada on the backside of the exiting Northeast system Tuesday. This system also has shown poor continuity, but may merge/interact with the southern system by next weekend in the East. Expect varied and at times boisterous model solutions until more certainty is shown. Course of least regret will be the ensemble means, and preferably the ECMWF ensemble mean which often performs well in amplified/blocky flow. GEFS mean may be too progressive overall but spread in the members overlaps with most, if not all, of the ECMWF members aloft. ...Sensible Weather and Hazards... With low pressure lifting across Lake Ontario early Tuesday, modest to locally heavy rain ahead of the cold front moving through New England will focus mostly on Maine. Wrap-around cold air to the west of the low will result in some additional accumulating snow over the Midwest and western NY/PA. As the low exits through the St. Lawrence Valley, flow over the Great Lakes will wring out additional snowfall or mixed rain/snow east of Lakes Erie/Ontario. Temperatures will be 5-20 deg F below normal behind the front, with the largest departures from normal over the Southeast. South Florida may not even break 70 degrees on Tuesday to start the month of December. Temperatures will rebound a bit for the rest of the week. System through the West will be rather moisture-starved, with generally light snow for the Rockies Tuesday. Path of the upper trough/low will dictate how far west the cooler push of air will travel due to the upper trough and cold front, but still favor 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 Wed into Thu. Some of this could be locally heavy. This should move eastward into the rest of the Southeast Friday and then perhaps up along the East Coast into next Saturday. 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