Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sun Dec 02 2018 Valid 12Z Wed Dec 05 2018 - 12Z Sun Dec 09 2018 ...Cold pattern over the Lower 48 with a significant storm system likely to affect at least the southern half of the U.S. late week into the weekend... ...Overview... The contiguous U.S. will lie in a transition zone between highly amplified/blocked flow extending from eastern Asia across the North Pacific and 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 carry Pacific energy from near California eastward across the southern states. As this energy reaches the southern Rockies/Plains expect it to develop a wave of low pressure along the polar front near the western-central Gulf Coast by Sat. Models and ensembles continue to show varying degrees of phasing/interaction of this southern stream wave with additional northern stream energy by the weekend, along with likely development of a coastal low near the Southeast U.S. coast. ...Guidance Evaluation/Preferences... As has generally been the case in recent days the majority of operational guidance shows similar ideas during the first half of the medium range forecast, which today spans the Wed into early Fri time frame. Thus the latest runs of the GFS/FV3 GFS and ECMWF/UKMET/CMC all appear to provide useful information for depicting important features during this part of the forecast. Overall the model average (while downplaying the occasional more extreme solution) has been remarkably consistent in recent days for the system nearing California around midweek and likely to open up into an upper trough as it heads into the Southwest. Forecasts for the system tracking across the Upper Great Lakes/New England regions Thu-Fri have been a little more variable once they progress beyond the Great Lakes early Thu. The latest cycle of guidance has trended toward faster progression such as hinted at by some ECMWF/GFS runs at different times in recent days. Along the West Coast by day 6 Sat the ECMWF continues to be on the stronger side of guidance with shortwave energy that arrives from the Pacific. The 00Z ECMWF is on the extreme side of the full 00Z ECMWF/GEFS/CMC ensemble spread, plus the energy is heading into a region of mean ridging between a rapidly amplifying upstream trough and the west-central U.S./northern Mexico trough, so continue to have lower confidence in the operational ECMWF scenario. However there are some ensemble members that do still have a decent shortwave and the 12Z/01 CMC was fairly strong before adjusting to more deflection in the latest 00Z run--so this is a lower probability scenario that cannot be fully discounted yet. Solutions for the upstream amplifying trough off the West Coast are comparable through day 7 Sun. As the southern stream trough aloft progresses eastward and likely supports surface low pressure development from the western-central Gulf Coast late Fri-Sat with eastward/northeastward progression thereafter, so far there has been decent clustering/stability up to about early Sat. Then greater divergence arises owing to differences in stream interaction and the influence of West Coast/western U.S. flow details. At this time considerably more ensemble members side with the idea of greater emphasis on coastal low development by early day 7 Sun, versus a more inland track--indicating lower probability for significant phasing aloft and greater dominance of the low level cold air. Among latest operational runs the 00Z/06Z GFS runs are generally on the northern/southern sides of the spread respectively. Looking at the ensemble means, the ECMWF mean has been somewhat more stable than GEFS means over the past couple days. Through the 00Z cycle the GEFS had exhibited somewhat of a slower trend but the 06Z run reversed that trend. Given that most timing adjustments over recent days have tended to be slower rather than faster--plus the CMC mean is even slower--preferred to use the 00Z run for the GEFS mean component of the forecast late in the period. A model/ensemble mean blend yielded a forecast very close to continuity. ...Weather Highlights/Hazards... The system nearing the southern West Coast will initially bring locally moderate/heavy precipitation (rain and high elevation snow) to parts of California and the Southwest and somewhat less moisture to areas farther northward. Areas near coastal southern California and favored terrain over Arizona should see the highest totals with this event. Farther eastward expect areas of heavy rainfall to develop over the southern Plains around Thu night-Fri and then continue eastward into the weekend. The full array of model/ensemble guidance highlights an area from about the eastern half of Texas into the Lower Mississippi Valley as having potential to see the highest rainfall totals for the full event but locations farther eastward should see some heavy rainfall as well. Meanwhile the cold air in place will likely produce snow and possibly other wintry precipitation types in the northern part of the moisture shield, reaching the Appalachians and East Coast during the weekend. Currently the best potential for significant snowfall exists from the southern High Plains/south-central Plains into the southern/central Appalachians and parts of the Mid-Atlantic. Northern stream systems affecting the northeast quadrant of the nation will be relatively starved for moisture but lake effect enhanced snowfall will be possible. 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. Expect below normal temperatures over a majority of the Lower 48 through the period. The most extreme anomalies should be over the central U.S. behind the front that drops southward through the Plains mid-late week. Some locations may see highs at least 15-20F below average. The West should see near to below normal temperatures but with a gradual moderating trend from readings 5-15F below normal on Wed. The eastern U.S. will generally see highs 5-15F below normal with some day-to-day variability. Clouds and precipitation will bring one or more days of above average min temps to some locations across extreme southern tier locations. Rausch/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