Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 PM EST Wed Dec 18 2019 Valid 12Z Sat Dec 21 2019 - 12Z Wed Dec 25 2019 ...Heavy rain possible over parts of the Southeast this weekend... ...Overview... Upper troughing off the West Coast will be reinforced from the northwest and slowly move onshore next week, maintaining an unsettled pattern. Concurrently, a somewhat unusual southern stream system will track through the Gulf of Mexico and across Florida late this weekend. Locally heavy rain will be possible around its circulation. ...Guidance/Predictability Assessment... Across the West, the general troughing pattern seems well resolved, but there are some differences in the wavelength of the trough and the amount of energy that makes its way into the West Coast. A model blend of the 06Z GEFS and 00Z EC means was used along with some incorporation of the operational models. In the Southeast, a potent upper low likely cut off from the northern stream is expected to move eastward from the Southern Plains on day 3/Sat across the Gulf Coast through day 4/Sun, while a surface low tracks eastward with it. Model spread is mainly related to timing--GFS and GEFS mean runs have been consistently faster than the ECMWF and ECENS runs on Sun/Mon. The 00Z UKMET was an outlier even Sat, because it separated the 500 mb vorticity into pieces instead of wrapping up one upper low, but it appears the new 12Z UKMET is in better alignment with other guidance. The 00Z CMC seemed to be a good compromise between the GFS and EC solutions with the low position, so incorporated that into the blend, as remaining in between the faster and slower solutions seems prudent. Overall this forecast issuance did not change significantly from continuity. ...Weather Highlights/Threats... The moisture plume associated with the atmospheric river into the Pacific Northwest during the short range period should continue to sink southward through coastal OR/CA as the cold front moves onshore Saturday. An additional few inches of precipitation are possible over northern Califonia before the focused plume diminishes and precipitation becomes lighter through central and then southern coastal California. As the upper trough and surface front move inland next week, precipitation will spread through the southern Great Basin and Four Corners region. Then there is uncertainty as to how much precipitation moves into the Plains around Christmas Day, with considerable run-to-run variability. Temperatures will trend cooler behind the front. The Gulf system will provide at least modest rainfall for much of the Southeast with possible locally heavy amounts. Models have been showing over five inches over several days with sufficient moisture in the lower atmosphere. Some light snow is possible on its northern fringe in the marginally colder air of the southern Appalachians, but the event is expected to be mainly rain. Warmer than average temperatures are forecast to spread from the Northern Rockies and Plains eastward across much of the U.S., with high temperatures up to 20 degrees above average. Dry conditions should persist across much of the Mississippi Valley to Ohio Valley and Mid-Atlantic/Northeast. Tate/Fracasso 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