Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 AM EST Sun Dec 02 2018 Valid 12Z Wed Dec 05 2018 - 12Z Sun Dec 09 2018 ...Cold pattern with increasing winter weather potential for portions of the central/eastern U.S. late this week... ...Overview... The flow across the CONUS during the medium range will lie in a transition zone from highly amplified/blocked flow extending from east Asia across the North Pacific to somewhat more progressive flow across the Atlantic Ocean. Anomalous ridging across Siberia will displace cold polar/arctic air toward Canada, and persistent ridging from western Canada north to Alaska will favor transport of cold air southward from Canada into the central/eastern U.S. A number of shortwaves will traverse an active northern stream from the Midwest to the Northeast during the period, with a strong cold front accompanying the most significant of these waves across the Plains, Ohio Valley, and Northeast Wed-Fri. Farther south, an active southern stream will spread Pacific energy from California east across the southern states. A wave of low pressure is expected to develop along the polar front near the western Gulf Coast by Sat morning in response as the southern stream wave approaches. Models continue to suggest the possibility for phasing interaction this southern stream wave with additional northern stream energy by Sat night/Sun, with potential development of a coastal low near the Southeast U.S. coast. ...Guidance Evaluation/Preferences... Models handled both northern and southern stream features sufficiently during days 3-4 (Wed-Thu) to justify use of a multi-model deterministic blend (ECMWF/GFS/FV3). Starting on day 5 (Fri), forecast confidence begins a gradual downward slide, especially surrounding a couple key areas. While most solutions agree that the southern stream wave should gradually deamplify as it moves east, differences emerge as to the timing of the deamplification as well as the timing of the eastward progression. Uncertainty is even a bit higher in the northern stream by late week, as models have shown a fairly large degree of variability and run-to-run inconsistency with a number of smaller scale impulses traversing the Midwest/Great Lakes. Both of these factors play a substantial role in any potential phasing of the two streams, with compounding uncertainty surrounding any such interaction. As a testament to the large array of possible solutions, even the usually under-dispersed GEFS becomes quite dispersed by next weekend, with similar behavior shown by the ECENS and CMCE ensembles. As a surface low develops along the western Gulf Coast Fri night, the precise evolution/track of this feature will hinge entirely on the eventual evolution of these upper-level features. Possibilities range from a consolidated low pressure area that moves north toward the Ohio Valley to a more easterly moving low with energy transferring to a new coastal system once phasing occurs. Given these considerations, a transition to majority weighting of ensemble means (ECENS/GEFS) was shown from day 5 onward, with the solution by day 7 (Sun) comprised almost entirely of ensemble means. ...Weather Highlights/Hazards... Systems traversing the northern stream across the Midwest/Great Lakes will be relatively starved for moisture, but lake effect enhanced snowfall will be possible. Heavy precipitation (rain and mountain snow) will be possible for portions of California and the Southwest Wed-Thu as the southern stream system moves onshore. As this system reaches the central U.S. on Fri it will begin to advect deeper Gulf of Mexico moisture northward, which will be enhanced ahead of a developing surface low in the northwestern Gulf. The result is expected to be a shield of widespread and potentially heavy precipitation Fri-Sat from portions of the southern plains to the Tennessee Valley. Wintry precipitation will be possible on the northern periphery of this system as a cold front and strong polar surface high provide an abundant source of cold air. Precipitation should spread into the Appalachians/Mid-Atlantic regions by Sat/Sat night as we see a new surface low potentially developing along the Southeast coast. See the latest Day 4-7 winter weather outlooks for details on areas expected to see the greatest winter weather threat in the medium range. Temperatures will be below normal across much of the CONUS through the medium range. The central U.S. will see the largest temperature anomalies through much of the period, where high temps are expected to range from 10 to 20 deg F below average. Surrounding areas from the Rockies to the Eastern Seaboard will not escape the cold conditions, with highs across a large area ranging from 5 to 15 deg below average. 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