Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 AM EST Fri Dec 20 2019 Valid 12Z Mon Dec 23 2019 - 12Z Fri Dec 27 2019 ...Heavy rain and gusty winds possible along parts of the Southeast U.S. coast on Monday... ...Overview and Guidance/Predictability Assessment... Model/ensemble 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 medium range. A relatively deep upper low and associated low pressure system are forecast to begin pulling away from the Southeast Mon-Tue (days 3-4), and models/ensemble members have shown a convergence toward the middle of the spread in terms of timing over the past couple cycles, with the ECMWF/GFS safely in the middle of the spread. Farther west, modest timing/amplitude differences were evident in the guidance with a shortwave reaching California Mon night/Tue, and then deamplifying across the central/eastern U.S. Wed-Thu (days 5-6) as a relatively weak/multi-centered surface frontal wave moves from the central U.S. to the Ohio Valley. Models show general agreement that another shortwave should approach the West coast late Wed, although some differences and run-to-run variability remain as to the precise timing as well as the structure (closed-off upper low or a more open wave). The 12Z ECMWF and the past couple runs of the GFS have trended toward a more open wave. Finally, a similar thing can be said about another trough forecast by the ECMWF/GFS to reach the West Coast on Fri (day 7) as the previous shortwave reaches the Plains. A blend of the 12Z ECMWF/18Z GFS was heavily used in the WPC forecast for days 3-5, with a shift to gradually heavier weight placed on ensemble means during days 6-7. Overall, forecast confidence through the period was average to slightly above average. ...Weather Highlights/Threats... Models show some degree of consensus that a band of moderate to heavy rains is possible on Mon on the north side of the low pressure system slowly pulling away from the Southeast U.S. coastline. The model consensus suggests 1-3 inches of rain are possible on Mon along portions of the coast before the system pulls out to sea by Tue. This system may also produce areas of strong winds due to a rather intense pressure gradient north of the surface low. Farther west, scattered rain/mountain snow showers will be possible across the Four Corners region Mon-Tue as a shortwave passes overhead, with rain (and potentially wintry precipitation on the northern fringe) from the Central Plains late Tue into the Midwest on Wed. Increasing moisture return across the Southern Plains by late Thu ahead of the next shortwave could produce areas of rain in an overrunning-type setup north of a frontal boundary. Rain and mountain snow is expected to become much more widespread along the West Coast, California in particular, by the mid to latter portion of next week as the last of the described upper troughs reaches the coast. Temperatures are forecast to be above average across much of the central and eastern U.S. during the medium range. The largest temperature anomalies are forecast from the Central Plains to the Midwest Mon-Tue, where high temperatures may approach or exceed 20 deg F above average. A broad area of the central/eastern U.S. will see highs 5 to 15 deg above average into at least the latter portion 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