Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1202 PM EST Fri Dec 11 2020 Valid 12Z Mon Dec 14 2020 - 12Z Fri Dec 18 2020 ...A mid week Nor'easter Appears likely for the Northern Mid Atlantic to Eastern NY/New England... ...Model Guidance Evaluation and Preferences... The Days 3-7 product suite was manually derived from a composite blend of the cluster of guidance from the 12-00 UTC ECMWF/ECMWF ensemble mean, and 06 UTC GEFS Mean. Leaned this composite toward the ensembles and ECMWF Wed-Fri to maintain better continuity in a pattern with increasing embedded system forecast spread and uncertainty. This cluster develops a low off the Carolinas Tuesday and moves the storm north northeast to off the southern New England coast Wed., posing the potential for a snowstorm in the interior northern mid Atlantic and portions of Eastern New York and New England. ...Weather/Threats Summary... A southern stream upper trough moves across the Southeast Monday, spawning a frontal wave along a trailing East Coast cold front. Expect a swath of moderate precipitation from the southern and central Appalachians to the Carolinas and VA. Temperature profiles support some snow on the northwest periphery of the precipitation shield in the Appalachians. The models are in pretty good agreement here, so a consensus approach works well to resolve minor timing differences. The majority of solutions slowed the timing down a few hours from prior runs. The Pacific Northwest and adjacent northern California will be the focus of where Pacific moisture streams onshore through much of next week with a couple of systems moving onshore and then across the northern to central Rockies. Expect coastal rain and mountain snows with each wave. Periods of gusty winds will be possible as well, particularly near the coast and in channeling terrain. Greatest precipitation totals during this time should be across the Coastal Ranges, the Cascades, and northwestern California. Enhanced snows will also spread inland across the Intermountain West, with the ranges of Idaho/northwest MT, and northwest WY having the highest probabilities in the WPC Winter Weather Outlook for Days 4-7. The models show a northern stream wave crossing the Ohio Valley mid week that merges with a southern stream wave to produce a pair of low pressure systems, which eventually merge into a developing cyclone, with low pressure on the Carolina coastal front intensifying as it moves north northeast off the Mid Atlantic coast to a position off southern New England Wed morning, and then offshore Thu. The 06z GEFS Mean, 00z ECMWF, and the 12-00z ECMWF runs are closer to the coast than the 12z GFS on Wed. With the GEFS matching the ECMWF/ECMWF Mean forecasts, odds are the GFS will track further west in later runs. This offers a threat of heavy snow from the Appalachians through the interior Mid-Atlantic through the Northeast Wed as depicted on the WPC Winter Weather Outlook. The major uncertainties will be precipitation type in the cities along the corridor from Washington, DC to Philadelphia, New York, and Boston; and then how far inland the axis of heavy snow extends. Petersen Additional 3-7 Day Hazard 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