Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 PM EST Thu Dec 31 2020 Valid 12Z Sun Jan 03 2021 - 12Z Thu Jan 07 2021 ...Weather Pattern Overview... A progressive flow pattern aloft is expected to remain in place for Sunday and going through the middle of next week, and a very active storm track across the northeast Pacific is likely to continue with deep surface lows across the Gulf of Alaska supporting multiple cold fronts moving inland across the western U.S. through the period. There will likely be an organized storm system across the Plains and then the Ohio Valley region by next Thursday. ...Guidance/Predictability Assessment... The timing and progression of multiple waves across the CONUS in the fast zonal flow aloft contributed to the main model differences. The deterministic model guidance has clustered reasonably well for the nor'easter early in the period such that a multi-model blend was sufficient there. Elsewhere, the biggest differences for the west was the timing/speed of the various shortwaves and frontal passages. By day 6/7 there is considerable spread with the evolution of a potential storm system across the Plains or MS River Valley. Most of the deterministic models used (00Z ECMWF/CMC/UKMET and 06Z GFS) showed the potential setup but some latitudinal differences. The ECENS/GEFS means offered a compromise solution and also trended well from continuity. ...Weather/Threats Highlights... Low pressure tracking from the Mid-Atlantic late this weekend and early next week continues to track closer to the coast compared to the previous cycle. A swath of precipitation on the northwest side is expected from PA to ME and interior portions of the Northeast and New England are likely to see moderate to locally heavy snowfall while closer to the coast rain or mixed precipitation is expected. The West Coast will remain active with several rounds of precipitation, some heavy, with the higher elevations experiencing heavy snowfall. Toward the end of the forecast period, return flow across the Gulf and MS River Valley ahead of another storm system will bring rounds of precipitation, some of which could be heavy along the Texas to central Gulf coasts. Temperatures across the continental U.S. during the period are expected to be near to above normal for most areas. The greatest departures from normal are forecast to be across the northern Plains for the Sunday to Tuesday time period (15-25F above normal) before moderating to 5-15F above normal for the remainder of the period. No significant intrusions of Arctic air are expected during this forecast period. Taylor Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Sun-Wed, Jan 3-Jan 6. - Heavy precipitation across portions of the Central Great Basin, California, and the Southwest, Mon, Jan 4 and Wed-Thu, Jan 6-Jan 7. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Wed-Thu, Jan 6-Jan 7. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sun-Thu, Jan 3-Jan 7. - Heavy snow across portions of the Central Rockies and the Northern Rockies, Mon-Tue, Jan 4-Jan 5. - Heavy snow across portions of the Mid-Atlantic and the Northeast, Sun-Mon, Jan 3-Jan 4. - Flooding possible across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Ohio Valley. - High winds across portions of the Central Plains, the Northern Plains, and the Northern Rockies, Sun, Jan 3. - High significant wave heights for coastal portions of California and the Pacific Northwest, Tue-Wed, Jan 5-Jan 6. - High significant wave heights for coastal portions of the Pacific Northwest, Sun, Jan 3 and Tue-Wed, Jan 5-Jan 6. 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