Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Dec 31 2018 Valid 12Z Thu Jan 03 2019 - 12Z Mon Jan 07 2019 ...Overview... The medium range period begins with a closed low over the Mississippi Valley slowly moving eastward--arriving in the Mid-Atlantic by between Day 4-5 (Fri-Sat) while additional waves of energy slide across the northern tier of the U.S. For the western U.S., an upper ridge will be the dominant feature starting out the medium range with a digging trough moving toward the West Coast by Day 4-5. This trough will likely move inland across the Intermountain West/Great Basin by the end of the medium range period. This pattern will bring active weather across the eastern U.S. on Days 3-5, shifting to the west by Days 5-7. ...Guidance Evaluation/Preferences... From the beginning, operational models were not in agreement with the trough/closed mid-level low over the Mississippi Valley. The 12Z ECMWF has a closed low over this area for Day 3 (Thu)--while the 12Z GFS has the trough stretching back into the lower MS Valley/southern Plains--with the 18Z GFS looking similar to this solution. The 12Z UKMet still remains the slowest (has the closed low in the southern Plains on Day 3). The 12Z CMC is the faster side (over the TN/OH Valley). By Day 4, GFS wastes no time in bringing the closed low over the Carolinas, while the ECMWF is over the TN/OH Valley. The ECMWF with this system has overall been consistent with itself while the GFS, to say the least, has not been consistent. The 12Z ECMWF ensemble mean and 12Z NAEFS were comparable to each other and to the ECMWF--this for the first 2 days the blend was based on mostly the 12Z ECMWF/ECMWF ensemble mean/NAEFS and a small portion of the 12Z GFS. By Day 5, as the trough in the East exits CONUS, an incoming trough in the Pacific will approach the West Coast and operational models still do not agree. The 12Z/18Z GFS brings the trough inland very quickly, while the 12Z ECMWF is much slower with this evolution. The ECMWF ensemble mean the last several runs has favored this slower solution--with the GEFS mean showing run-to-run inconsistencies. Thus, the blend from Days 5-7 was based mostly on an ECMWF/ECMWF ensemble mean blend with some 18Z GEFS mean. ...Weather Highlights/Hazards... On Days 3-4, as the surface low moves across the Southeast and into the Mid-Atlantic, precipitation will be expected across these areas. The heaviest axis of precipitation occurring in the northern portions of the Southeast eastward into the western Carolinas. The coastal region of the Northeast can also expect precipitation during this time--finally ending by Saturday night. On the northern end of this precipitation shield, there could be some wintry mix--with accumulating snow expected for New England on Friday and Saturday. Out west, rain and higher elevation snow will impact the Pacific Northwest--with accumulating snows particularly impacting the Washington/Oregon Cascades and the northern portions of the Northern Rockies. By Days 6-7 (Sun-Mon), as the trough approaches the West Coast and moves inland, precipitation will spread across California and into the Four Corners region. Expect higher elevation snow during these days especially for the Sierra Nevada mountain range in addition to the higher elevations across the Four Corners, and portions of the Intermountain West. The northern Plains and Upper Midwest will experience warmer than normal temperatures from Thu-Sat, with anomalies reaching 15-20 degrees above average. By Sunday and into Monday, the Ohio Valley will have the highest temperature anomalies--with 5-10 degrees above average. The western U.S. will stay near normal throughout the medium range period. Reinhart WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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