Extended Forecast Discussion NWS Weather Prediction Center College Park MD 140 PM EST Sat Dec 12 2020 Valid 12Z Tue Dec 15 2020 - 12Z Sat Dec 19 2020 ...Midweek nor'easter to focus significant winter threat over the Mid-Atlantic and Northeast... ...Overview... The medium range period will begin Tue with an upper trough/possibly closed low across the central CONUS. As this feature shifts eastward Wed/Thu, combining surface lows are expected to deepen near the Mid-Atlantic coast and form a nor'easter likely to spread significant wintry weather across portions of the Central Appalachians, Mid-Atlantic, and Northeast Wed and lingering in New England Thu. Meanwhile, precipitation persists in the Northwest with a progressive pattern and persistent Pacific moisture inflow. ...Model Guidance Evaluation and Preferences... 00Z/06Z model guidance is clustered reasonably well through the medium range period with the larger scale features described above, and the new 12Z guidance appears similar to the previous cycle. To start the period, a blend of deterministic models (00Z ECMWF, CMC, and UKMET and the 06Z GFS) was able to be used. Guidance has been persistent and agreeable with indicating a surface low/nor'easter tracking up the Atlantic coast, so confidence is higher than normal (for this far out in time) for a significant storm. However, what remains variable from model to model/run to run are the upper jet interactions and the mid-level shortwave energy moving through the broader trough, which lead to some differences in the exact surface low track and its timing. Even minor track differences can lead to big differences in the sensible weather. Preferred the 06Z GFS over the 00Z by Thu, as the 00Z run had a faster track away from the coast than the consensus. On the other hand, the 00Z ECMWF appeared to be on the slower side; the 00Z EC ensemble mean seemed to have a more reasonable position and timing. Upper troughing is forecast to come into the West Coast around Thu and move eastward for the end of the week. The depth and the timing of the upper energy as well as the associated surface low remains in question, but the 00Z UKMET created a closed low digging deeper in the eastern Pacific Wed-Thu than consensus would indicate, so leaned away from that solution for now. By the end of the week, shifted toward a 00Z EC mean/06Z GEFS mean heavy blend to minimize individual model differences, but kept some incorporation of the deterministic GFS and ECMWF. ...Weather/Threats Summary... At the beginning of the period Tue, there is potential for southern portions of the Plains toward the Ohio Valley to see modest but disruptive snow/ice in cool temperatures behind a low pressure system. This low is one that should combine and develop into the nor'easter impacting the Eastern Seaboard Wed into Thu. Significant heavy snow is likely to occur across parts of the Central Appalachians, Mid-Atlantic, and Northeast, with potential for ice and high winds in some areas as well. At this point, the highest probabilities for heavy snow appear to be just west of the major cities along I-95, but the large cities certainly could see impactful wintry weather as well. And as mentioned, even small shifts in the low track could cause sizable differences in the precipitation type at a particular place and with how far west over land the precipitation shield spreads. High pressure should settle into the East after this event to finish out the week. Across the northwestern U.S., rounds of Pacific moisture are expected to stream onshore as frontal systems approach. With this, periods of coastal rain and higher elevation snow are forecast through the end of the week in the Pacific Northwest and adjacent northern California, with higher totals in the favored higher elevations of the Coastal Range and Cascades. Snow will spread inland into the Intermountain West as well, and the Northern Rockies and Wind River Mountains/Tetons could see notable snow totals. Elsewhere, generally dry conditions should prevail toward the end of the workweek; then there is an increasing chance of Gulf of Mexico moisture return causing the possibility for rain around the Lower Mississippi Valley by Sat, but confidence with this remains low for now. Tate 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