Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 AM EST Sun Jan 06 2019 Valid 12Z Wed Jan 09 2019 - 12Z Sun Jan 13 2019 ...Overview... Moderately progressive flow pattern expected to persist across the CONUS during the medium range. Persistent positive height anomalies near the north pole as well as from James Bay east into the North Atlantic (per latest ensemble D+8 anomalies) will continue to prevent any significant or prolonged incursions of arctic air southward into the CONUS. However, teleconnections associated with a strong 500 hPa height anomaly dipole extending from south of Iceland (positive) to northeast Europe (negative) do support the tendency for periods of cold temperatures across the eastern U.S. These should be relatively short-lived, however, without a significant and prolonged means of southward transport for colder air from high latitudes. Meanwhile, an active polar stream across the North Pacific will bring energy to the U.S. West Coast and eastward across the country every 1-2 days, with some splitting of energy into northern/southern stream components as features move inland over the western states. ...Guidance Evaluation/Preferences... A multi-model deterministic blend (including the ECMWF/GFS/CMC) served as a basis for the forecast on days 3-5 (Wed-Fri). These solutions showed reasonably good consensus and clustering with ensemble means for the system initially moving away from the New England coast on Wed and with the strong low pressure system off the Pacific Northwest at the same time, with associated upper-level energy crossing the Rockies and north central U.S. Thu-Fri. By Fri-Sat (days 6-7) the southern stream component of the previous West Coast system is expected to interact with with a lingering frontal boundary across the northern Gulf of Mexico, resulting in a new frontal wave, with energy eventually perhaps transferring to a new low off the Southeast U.S. coast. As is typical of these systems involving some degree of interaction between northern/southern stream energy and resultant frontal waves, model/ensemble solutions show a significant degree of spread and variability, although the general evolution described has good consensus support. Nonetheless, it is certainly too early to speculate on specifics of any potential low pressure system along the Southeast/Mid-Atlantic coast next weekend. Meanwhile, the next highly amplified system should reach the West Coast during that time frame. Models show some consensus that as this Pacific system approaches another southern stream component should separate out and move more quickly into the Southwest states while a broad upper trough lingers off the Pacific Northwest coast. Models show expected differences as the specifics of this process, with a bit more consistency found in the ensemble means. Given these considerations during the latter portion of the extended period, a majority ensemble mean-based approach was used, with significant forecast weight placed on the ECENS and GEFS means. ...Weather Highlights/Hazards... Expect multiple rounds of potentially heavy precipitation (low elevations rains and mountain snows) along the West Coast through the forecast period. The focus for the greatest precip amounts on Wed-Thu will be from northern California north to the Pacific Northwest, shifting southward across much of California by Fri-Sat. Farther east, the system pulling away from New England on Wed could still produce areas of heavy snow for northern New England through Wed, with some potential for lake effect enhanced snows downwind of Lakes Erie and Ontario as well. Southern stream energy an aforementioned frontal boundary interactions will cause precipitation to gradually develop across the southern Plains by Thu, spreading east toward the Gulf Coast/lower Mississippi Valley and Southeast Fri-Sat. There is some potential for wintry weather on the northern periphery of this system across the southern/central Appalachians by next weekend, but confidence in this aspect of the forecast is low at this time. The largest temperature anomalies will be from the northern/central Rockies east into the Plains through much of the forecast period. High temperatures ranging from 5-15 deg F above average are expected, with some areas approaching +20 deg anomalies on Thu. Meanwhile, temperatures from the Great Lakes to the Eastern Seaboard will be near to several degrees below average. The best potential for significantly below average temperatures appears to be across New England on Fri-Sat, where highs of 10-15 deg F below average are possible - the result of strong cold air advection in the wake of the mid-week low pressure system. 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