Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Tue Jan 08 2019 Valid 12Z Fri Jan 11 2019 - 12Z Tue Jan 15 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., setting the stage for at least some 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... Models have generally trended toward a somewhat slower and more amplified solution with an upper trough expected to approach California Fri-Sat (days 3-4) and its surface front expected to move into southern California. Farther east, northern and southern stream shortwave energy will attempt to interact/phase across the central U.S. during the same time period and carry a surface low eastward through the lower Mississippi Valley/northern Gulf Coast. As is often the case in these situations, models show a bit more spread with the northern stream wave, with the 00Z GFS among the most amplified solutions while the 00Z Canadian was most amplified with the southern portion. Ensemble trends and surface setup (1040mb high to the north) seem to support a less phased (more sheared) and weaker solution (like the 06Z GFS and 00Z ECMWF) rather than a deeper surface low (like the 00Z GFS/Canadian). QPF was more uncertain given the blocking cold air to the north, but a stripe of appreciable snow is likely even given a sheared system with more "conversational snow" (less than a inch) on the northern fringe with more snow over higher elevation enhanced by upslope flow. Back to the west early next week, additional amplified energy will approach the west coast late Sun into Mon, with a trend a slower/deeper trough and likely a closed off upper low nearing southern California. Most recent deterministic runs show a closed low and the ensemble trend has slowed down quite noticeably in the ECMWF ensembles while the GEFS members have been slower for several runs. Elongation of the upper ridge into western Canada would support the continued split flow and rather repetitive pattern in the Southwest that is forecast for this weekend. ...Weather Highlights/Hazards... Expect the potential for multiple rounds of heavy precipitation (low elevation rain and mountain snows) across the West Coast during the forecast period as a rapid succession of Pacific systems impacts the region. As shortwave energy reaches the southern Plains/lower Mississippi Valley by Fri-Sat, areas of heavy rain will be possible, with precipitation spreading eastward into the Southeast/southern Mid-Atlantic by Sat night/Sun. Cold air to the north of the system would result in areas of wintry precipitation across portions of the mid-Mississippi Valley (Ozarks northward into southern Missouri) on Fri and across areas from the Ohio Valley to the southern/central Appalachians and Mid-Atlantic by Sat-Sun. The northern extent/amount of snow Sat-Sun across the Mid-Atlantic region remains highly uncertain given the degree of (or lack thereof) phasing/amplification. Generally above average temperatures will persist through the forecast period across the north central U.S. with max temp anomalies in the +10 to +15 deg F range (even warmer for some areas) through much of the period. The coldest temperatures will be across the Northeast given predominately northwest flow out of central Canada where temperatures as cold as 10 to 20 deg F below average will be possible from Fri through early next week. Some moderation is likely by next week as the trough moves off the coast in the wake of the weekend system. Fracasso/Ryan 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