Extended Forecast Discussion NWS Weather Prediction Center College Park MD 129 AM EST Sat Jan 05 2019 Valid 12Z Tue Jan 08 2019 - 12Z Sat Jan 12 2019 ...Overview and Guidance Evaluation/Preferences... A moderately progressive flow pattern will be in place across the CONUS during the extended forecast period, with fairly amplified upper waves traversing the country every few days. No signals for any significant incursions of arctic air are seen in the guidance. A look at hemispheric D+8 500 hPa anomalies reveals strongly positive anomalies centered between Greenland and western Europe, helping to displace the coldest air from the arctic regions into Eurasia and away from North America. Teleconnections associated with this and other hemispheric anomaly centers suggest a continued tendency for upper ridging to rebuild across western North America - but with continued progressive flow across the North Pacific the ridge will likely be a somewhat transient feature. Models/ensembles showed relatively good consensus early in the extended period with respect to one low pressure system crossing the Great Lakes/eastern states and another approaching the West Coast. A blend of the ECMWF/GFS/CMC was used as a basis for the forecast during days 3-5 (Tue-Thu). Models begin to show growing differences from Thu onward, both with respect how deep troughing will be along the Eastern Seaboard and how quickly it will lift out, as well as the timing and amplitude of deamplifying shortwave energy moving into the western U.S. and amplifying once again as it reaches the central U.S. from Fri onward. Finally, models show some consensus that another amplified upper trough should approach the West Coast by late Fri-Sat, with models showing the expected differences for the day 6-7 time period as to the amplitude and evolution of this feature as it nears the coast. Use of deterministic ECMWF and GFS solutions continued through days 6-7, albeit with weighting reduced substantially in favor of slight majority weighting of ensemble means (ECENS and GEFS). ...Weather Highlights/Hazards... The system crossing the eastern U.S. Tue-Wed may produce areas of heavy snow across northern New England, with the potential for lake effect snow to linger even after the low pressure system departs. Heavy precipitation, low elevation rain and mountain snow, are also likely from California to the Pacific Northwest Tue-Wed as a strong low pressure system approaches, pushing a cold front inland. This system may also produce areas of high winds near the coast. By Thu-Fri as shortwave energy crosses the Rockies and enters the central U.S., inflow of moisture from the Gulf of Mexico will increase in the vicinity of a warm front, with precipitation expected to increase in coverage for areas from the southern plains to the lower Mississippi Valley. Temperatures will initially be well above average for most of the central/eastern U.S. to begin the forecast period on Tue, with high temperature anomalies of +10 to +20 deg F for many areas. Cooler temperatures will prevail in the wake of a cold front by mid to late week, but temperatures will only decrease to near average levels. Meanwhile, temperatures across areas from the northern Rockies to the northern/central plains will remain above average through the middle of next week before the arrival of the next cold front late in the week. 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