Extended Forecast Discussion NWS Weather Prediction Center College Park MD 133 AM EST Sun Dec 22 2019 Valid 12Z Wed Dec 25 2019 - 12Z Sun Dec 29 2019 ...Overview and Guidance/Predictability Assessment... An amplified and moderately progressive flow pattern is expected across the CONUS during the medium range forecast period. A consolidated vortex across the Arctic Ocean and an active Pacific wave pattern will keep any significant arctic air outbreaks away from the CONUS, with many locations, especially across the central and eastern U.S. experiencing unusually mild conditions for late December. Models show good consensus early in the forecast period that an upper-level trough should amplify along or just off the U.S. West Coast, with a closed upper low developing in the vicinity of southern California by Wed night/Thu morning. Models continue to differ on the timing of this feature as it moves eastward across the Southwest through Fri, with the ECMWF on the slow side of the spread and the GFS on the fast side, and the ensemble means largely in the middle. Farther north, an active northern stream will transport numerous smaller scale impulses across the CONUS northern tier, with several waves of low pressure traversing a frontal boundary across the central U.S. Wed-Fri. Overall, model consensus was sufficient during days 3-5 (Wed-Fri) to base the forecast on a multi-model deterministic blend, including the 12Z ECMWF/UKMET/CMC/18Z GFS, with average forecast confidence. By days 6-7 (Sat-Sun), confidence decreases as model spread increases on the specifics of how quickly the southwestern system moves east into the central U.S., and any potential interactions with northern stream energy/resultant surface cyclones across the Plains or Midwest. One area where consensus does exist is on the idea that the flow across the CONUS should become more amplified through time, with the setup on day 7 likely featuring an amplified upper trough, the product of some degree of some northern/southern stream phasing, somewhere across the central or eastern U.S. The specifics of such a system, however, are elusive. Despite rather large deterministic spread by next weekend, ensemble solutions were somewhat agreeable. Thus, the day 6-7 forecast was based heavily on a blend of the 12Z ECENS/NAEFS ensemble means, with only very modest amounts of deterministic guidance used. ...Weather Highlights/Threats... Rain and mountain snow are expected along the West Coast on Wed, and continuing into Thu across southern California, as the upper trough moves onshore. The Pacific Northwest will remain wet through much of the forecast period with persistent rain and mountain snow as a number of shortwaves and weakening Pacific frontal systems affect the region. Moisture return ahead of the southern stream trough will create an overrunning setup north of a frontal boundary across the Southern/Central Plains by Thu, with showers expected to develop. Precipitation should expand in coverage and intensity across the Southern/Central Plains by Fri as the upper low moves east across the Southwest, intensifying moisture return into the central U.S. By next weekend, with potential development of a low pressure system across the central U.S., models indicate the potential for a band of moderate to possibly heavy precipitation from the Southern Plains to the Ohio Valley/Great Lakes. The details are quite murky by that time, for reasons described above. Nonetheless, there appears at least some potential for an area of winter weather on the north side of this system (despite the relative lack of significant cold air). Once models get a better handle on the specifics of next weekend's system, a clearer picture will emerge as to what degree of heavy rain and/or winter weather threat exists. Temperatures are forecast to be well 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 Wed-Thu, where high temperatures may approach or exceed 20 deg F above average in some areas. A broad area of the central/eastern U.S. will see highs 5 to 15 deg above average persisting into next weekend. Meanwhile beneath the relatively slow moving upper-level trough, much of the Great Basin and Southwest will see high temperatures 5 to 10 deg 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