Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Fri Jan 11 2019 Valid 12Z Mon Jan 14 2019 - 12Z Fri Jan 18 2019 ...California Heavy Precipitation Pattern Next Week... ...Overview... Steadily progressive and amplified Pacific flow will bring a series of significant storms to the West Coast next week. As these systems move eastward, split flow will emerge around an amplified downstream Great Basin/Rockies upper ridge. Northern stream energy will amplify as it enters broad cyclonic upper-level flow east of the ridge, with frequent shortwaves and mean upper troughing from the Upper Midwest to the Northeast. Lead Mid-Atlantic coastal low development this weekend will track northeastward over the western Atlantic and Canadian Maritimes early next week. Meanwhile, an active West Coast will feature amplified trough/upper lows to especially threaten California all next week. ...Guidance Evaluation/Preferences... Model agreement during the forecast period was fairly good in the large scale, with smaller features such as shortwaves moving through the northern tier of the U.S. and the West not handled as well (per usual). The deterministic GFS and EC and the GEFS and ECENS were all used to derive the WPC forecast throughout the period, with increased emphasis applied to ensemble mean guidance in the days 5-7 timeframe. Despite model uncertainties in the details, forecast confidence is high for the threat of heavy precipitation focused in California next week. ...Weather Highlights/Hazards... A lead coastal low pressure system lifting off the Eastern Seaboard Mon/Tue will be a maritime threat as the system deepens offshore. The West Coast will remain quite active given the rapid frequency of shortwave energies and frontal systems. The heaviest precipitation (low elevation rain and mountain snow) will be focused across California, with multiple rounds of precipitation possible through the forecast period. Total precipitation amounts over the period could be heavy, with upslope flow enhancing QPF to 4 to 8 inches in the Sierra and Coastal Ranges. Later in the week, precipitation is expected to spread to the Pacific Northwest as well. A temperature pattern change is expected for the north central tier of the CONUS--early in the week, above normal temperatures are expected due to the influence of an upper ridge across the Great Basin/Northern Rockies, which will erode by midweek. Then, due to incoming shortwaves related to the large upper low in central Canada, the Northern Plains and Upper Midwest are likely to see below average temperatures later next week. The Northeast may experience periods of below normal temperatures for the same reason. Tate/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