Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 AM 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. Much of the rest of the CONUS will be dry from the southern Rockies/Plains northeastward to the Great Lakes/Northeast under upper ridging. ...Guidance/Predictability Assessment... In the West, consensus of the GFS/ECMWF/Canadian served as a good starting point as the models will continue to shift in the interaction/timing of the lead and secondary shortwaves in the overall longwave trough. These fit within the current and multi-cycle ensemble envelope with expected differences that grew larger with time. In the Southeast, 12Z ECMWF/Canadian lied in between the quicker GFS runs and the slower UKMET. The 18Z GEFS mean and 12Z ECMWF ensemble mean clustered well with the ECMWF/Canadian, so a consensus near there was preferred. The GFS, though quicker, was still included as it was well within the middle 50% of ensembles with respect to forward speed. ...Weather Highlights/Threats... Moisture plume associated with the short term atmospheric river into the Pac NW should continue to sink southward through coastal OR/CA as the cold front moves onshore Saturday. An additional several inches of precipitation are expected over 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. 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 snow will be possible on its northern fringe in the marginally colder air of the southern Appalachians. Temperatures will be above average east of the Rockies (and outside the Southeast) with overall dry conditions from eastern Montana/Colorado to the Plains where some locations will see temperatures 15 to 20 deg F above average. The Northeast will see a moderation to above average temperatures by Sunday and especially Monday. 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