Extended Forecast Discussion NWS Weather Prediction Center College Park MD 149 PM EST Sun Dec 22 2019 Valid 12Z Wed Dec 25 2019 - 12Z Sun Dec 29 2019 ...Overview and Guidance/Predictability Assessment... The large scale pattern across the CONUS is expected to feature a relatively amplified and progressive flow during the medium range period (Dec 25-29). 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. Troughing across the West and ridging for the central/eastern U.S. at the start of the forecast period has above average agreement in the latest forecast deterministic guidance. A closed mid/upper level low will drop into southern California Thursday where the latest GFS remains a bit faster compared to the ECMWF solution. For the northern stream, a fast moving shortwave trough will skirt the Great Lakes region where model agreement is fairly high at this point. As a result, for the day 3-5 forecast (Dec 25-27), the forecast was primarily a blend of the 06Z GFS, 00Z ECMWF/UKMET/CMC with some components of the ECENS/GEFS means. For Day 6-7 (Dec 28-29), model differences increase substantially, particularly with the ejection of the next shortwave trough and associated low pressure in the southern/central Plains. The GFS continues to advertise a faster solution and more phasing with the northern stream energy that drops toward the Great Lakes region by the weekend. Meanwhile, the ECMWF offers a less phased, slower scenario with the southern stream energy becoming cut off from the overall flow. The CMC offers a solution in the middle, though trends toward the ECMWF idea. The ECENS/GEFS means remain a bit different as well (GEFS fast, ECENS slower) and with low overall confidence the general approach for the forecast blend was based largely on the ensemble means. ...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/Taylor 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