Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1102 AM EST Wed Jan 09 2019 Valid 12Z Sat Jan 12 2019 - 12Z Wed Jan 16 2019 ...Overview... Progressive and amplified flow across the North Pacific will continue to bring significant storm systems to the West Coast every couple of days during the medium range period. A strengthening upper ridge across the northern Rockies/Great Basin will result in split flow as these Pacific systems move eastward, with energy separating into northern and southern stream components. Northern stream energy will amplify as it enters broad cyclonic upper-level flow east of the closed upper high this weekend across the Midwest/Great Lakes/Northeast. This setup may result in some phasing of northern/southern stream waves along/near the east coast as low pressure exits off the southern Mid-Atlantic Sunday. While this pattern does not favor a widespread outbreak of cold air, persistent troughing across the East along with the amplified western U.S. ridge will support the transport of cold polar air southward into the eastern U.S., likely setting the stage for winter weather potential on the northern side of the low pressure system this weekend. By next week, another system in the Pacific may push into central/southern California and the Desert Southwest. ...Guidance Evaluation/Preferences... Model solutions seem to have settled on an amplified trough reaching California by the weekend, with an intensifying ridge across the northern Great Basin/Rockies causing the trough to shear and separate into the northern/southern stream components by Sun. Meanwhile, phasing of southern/northern stream shortwave energy will be underway across the Plains and Mississippi Valley. Models continue to struggle a bit with the phasing process, with some run-to-run variability still during the Sat-Sun time frame. A surface low is expected to track across the Gulf Coast states Sat-Sun with a second coastal low developing along the North/South Carolina coastline Sat night into Sun morning. By day 5 (Mon) there is some disagreement as to how quickly the low moves out to sea, although consensus has improved toward a track more east-northeasterly out to sea rather than closer to the Northeast U.S. coastline. Also by day 4-5 another amplified trough and associated frontal system will approach the West Coast. Models continue to show some variability on the exact structure of this feature (compact closed low or more elongated upper trough). Given the variability at smaller scales and general large scale agreement, opted to take a multi-model blend approach during days 3-5, including the 00Z ECMWF/00Z GFS deterministic models, with some ensemble means and continuity from WPC as well. By Tue-Wed (days 6-7), additional southern stream shortwave energy should cross the Southeast (the energy which began near CA on day 3) while the next Pacific trough moves into the Four Corners region and eventually the Plains (models have almost universally sped this feature up compared to previous runs which kept a closed low lingering offshore for longer). Model/ensemble solutions have shown a large degree of inconsistency/variability by this time frame so was leery of leaning too heavily toward deterministic guidance. Thus, weight was shifted toward the ECENS/GEFS ensemble means by days 6-7. ...Weather Highlights/Hazards... Expect the potential for rounds of precipitation (low elevation rain and mountain snows) across the West Coast, particularly California, as multiple Pacific systems impact the region. Light precipitation is expected for the Southwest as well. As shortwave energy reaches the Lower Mississippi Valley/Southeast by Sat-Sun, precipitation will overspread much of the region, as far north as the Ohio Valley, Appalachians, and Mid-Atlantic. Cold air on the northern side of the system will result in a threat of wintry precipitation from portions of the Ohio Valley to the southern Mid-Atlantic. Confidence has increased for the potential of accumulating snows in these areas, but the exact areas to receive any potentially heavy snow remain more uncertain at this time and will depend on the exact track of the surface low pressure system off the Southeast/southern Mid-Atlantic coastline, as well as the placement of the high in southeastern Canada. Generally above average temperatures will persist through much of the forecast period across the north central U.S. with max temperature anomalies in the +10 to +15 degrees F range (even warmer for some areas), due to the upper-level ridging there. The coldest temperatures during the forecast period will be across the Northeast during the weekend; highs in the single digits are possible in northern New England, 10 to 20 degrees below average. Some moderation is likely by next week as the trough moves off the coast in the wake of the weekend system. Ryan/Tate 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