Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1103 AM EST Sun Jan 13 2019 Valid 12Z Wed Jan 16 2019 - 12Z Sun Jan 20 2019 ...California Heavy Precipitation Wed-Thu... ...Cold air surges through the North-Central U.S. later this week... ...Central to Eastern U.S. Heavy Rain/Snow Threat Next Weekend... ...Overview and Guidance Evaluation/Preferences... While the overall larger scale flow pattern evolution remains reasonably well-supported by the guidance, an uncertain split of smaller scale features over the eastern Pacific and subsequent progression inland across North America continues to offer significant run to run differences in deterministic solutions right from the start of the medium range time period. A blend of the deterministic models to start (Wed-Thu) offered a reasonable consensus-based forecast that has served well the past few days as a weaker lead system approaches northern California Wednesday and a stronger system pushes inland on Thursday. For Fri-Sun, trended toward an ensemble mean consensus that was slower than the 00Z/06Z GFS but not as slow as the 00Z ECMWF/Canadian with the upper trough through the Plains. This will support surface cyclogenesis out of Texas and through the lower Mississippi Valley late Friday into Saturday as arctic high pressure sinks through the High Plains/Upper Midwest. By next Sunday, models and ensemble members show considerable spread in track/speed/strength/evolution in the eastern third of the CONUS. Trend has been for a slower/deeper system like the 06Z FV3-GFS that may exit off the southern Mid-Atlantic coast per the preferred cluster of solutions. Back to the west, another system may approach/enter the coast of WA/OR by next Sunday. ...Weather Highlights/Hazards... In the west, an initial and subsequent system will bring increasingly more precipitation to mostly California and to a lesser extent the Pacific Northwest Wed/Thu. 1-2" of valley rain and several feet of mountain snows are likely through central/northern California and the Sierras, respectively, with less across southern California. With a healthy influx of moisture, precipitation will work inland through the Intermountain West/Great Basin/Rockies late Thursday and Friday with windy conditions and valley rain/mountain snow. Snow levels will fall as heights lower which will turn some lower elevations over to snow as precipitation ends by Saturday. Downstream, glancing blows of cold Canadian air will become more significant and longer-lasting as it surges southward Thursday into next weekend as the northern stream amplifies around the wobbling Hudson Bay upper low. Concurrently, the robust western system will organize over the southern Plains with increased Gulf of Mexico moisture and lift through Arkansas on Saturday as the arctic front to the north pushes through the Northeast. As precipitation overruns the cold airmass to the north, snow will spread eastward out of the central Plains through the Midwest/Ohio Valley as potentially heavy warm-sector rains/convection move through the Southeast. Though confidence decreases in specifics in the east next weekend, the ensemble signal for a significant and fairly widespread heavy snow/ice event exists to the north of the low track. Please consult the WPC Day 4-7 Winter Weather Outlook for a probabilistic assessment of the winter weather threat from coast to coast. Fracasso/Schichtel 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