Extended Forecast Discussion NWS Weather Prediction Center College Park MD 135 AM EST Sat Dec 21 2019 Valid 12Z Tue Dec 24 2019 - 12Z Sat Dec 28 2019 ...Overview and Guidance/Predictability Assessment... Models/ensembles show a persistent upper-level mean trough generally across the West Coast and Great Basin through the medium range. Meanwhile, moderately progressive flow will persist downstream across the central and eastern U.S. as shortwave energy ejects from the western trough and encounters broad anticyclonic flow across the south central and southeastern states. A persistent upper-level low across the Arctic Ocean well north of Alaska along with weakly positive height anomalies across Canada will keep the risk of any significant outbreaks of cold air across the CONUS to a minimum during the forecast period. A relatively deep upper-level low and associated low pressure system are forecast to begin be pulling away from the Southeast on Tue (day 3). Over the past 24 hours, models have converged a bit toward the slower side of the spread. With the ECMWF remaining slower than the GFS, leaned somewhat toward the slower ECMWF solution. Farther west, model timing/amplitude differences continued to be evident in the guidance with a shortwave reaching California early Tue, and then deamplifying across the central/eastern U.S. Wed-Thu (days 4-5) as a relatively weak frontal wave moves from the central U.S. to the Ohio Valley. Models show general agreement that another trough or potentially a closed upper low should approach the West Coast either late Wed or early Thu. Fairly significant differences remain, however. The ECMWF has trended toward a much deeper/closed and slowly progressing system, while the GFS has remained a more open wave, albeit trending slower/deeper with each consecutive run, and the 00Z GFS at least hinting at the potential for a closed upper low around the time the feature nears the coast. Given the trends, preferred to lean more toward the ECMWF since it seemed to be leading the way on the current trends. During the day 6-7 (Fri-Sat) time period, additional northern stream shortwave energy appears likely to move across the CONUS northern tier, with deterministic solutions showing a range of timing/amplitude for these features, favoring heavier use of ensembles means. Based on the described considerations, the WPC forecast was initially based on a blend of the 12Z ECMWF/18Z GFS during days 3-4, with quite a bit more emphasis placed on the ECMWF. For days 5-7, a blend of the ECMWF along with the the ECENS/GEFS ensemble means was used. ...Weather Highlights/Threats... Scattered rain/mountain snow showers are expected across the Four Corners region Tue as a shortwave passes overhead. Increasing moisture return across the Southern Plains starting Thu night and continuing through late in the week may produce areas of rain in an overrunning-type setup north of a frontal boundary. Some wintry weather may be possible on the northern fringe of this area of precipitation, from portions of the Central Plains to the Midwest, although precipitation amounts in those areas look rather light at this point in time. Rain and mountain snow are expected to become much more widespread along the West Coast by Wed-Thu, continuing through late in the week for the Northwest, as the last of the described troughs and additional smaller scale northern stream energy move onshore. Temperatures are forecast to be above average across much of the central and eastern U.S. The largest temperature anomalies are forecast from the Central Plains to the Midwest/Ohio Valley Tue-Wed, where high temperatures of 15 to 20 deg F above average will be common. A broad area of the central/eastern U.S. will see highs 5 to 15 deg above average persisting through the end of next week. Meanwhile, with persistent upper-level troughing, much of the West will see temperatures near or slightly below average. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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