Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1249 AM EST Wed Dec 26 2018 Valid 12Z Sat Dec 29 2018 - 12Z Wed Jan 02 2019 ...Overview and Guidance Evaluation/Preferences... Positively-tilted troughing will reload in the west as subtropical upper ridging lingers over the Bahamas. This allows northern stream flow to progress along the US/Canadian border to the south of an upper low over Hudson Bay. The recent 12Z/25 FV3-GFS/ECMWF represent a good deterministic weight to the forecast along with parts of the UKMET and Canadian (early in the period). A front will linger over the Gulf of Mexico with surface waves moving northeastward through the Southeast as a Pacific system pushes east Sun-Tue from northwestern Montana to the Great Lakes. The operational 12Z/18Z GFSs were not included due to its handling of a Pacific trough that should close off near 38N/150W early Saturday. Its 26/00Z run appeared much more in line. Trended toward the ECMWF ensemble mean by next Wednesday as the GEFS mean was likely too far east with the trough due to upstream mishandling of the Pacific. ...Weather Highlights/Hazards... Cold front in the east will linger in the Gulf through the period, spreading rain over the Southeast in batches as a sfc wave or two ride along the boundary. This may even fall as snow on the northern fringe given sufficiently cold air. Next Pacific system enters late Saturday into Sunday with widespread but mostly light to locally modest rain/snow through much of the interior west continuing on Monday but winding down. That surface low will move along the US/Canadian border and drag its cold front to the south and east. This will allow Gulf moisture to again flow northward and expand/enhance the rain shield through the Southeast next Monday/Tuesday depending on how the front in the Gulf responds. Temperatures will be cooler than average in the West and Plains with coldest anomalies (10-20F below average) likely focused over the High Plains and southern Rockies/west Texas. Much above average warmth (plus 10-30F anomalies) will be drawn northward ahead of the front Saturday along/east of the Appalachians. Cooler air will filter into the East in the wake of the cold frontal passage Saturday but with near average temperatures for late December considering the above average mid-level heights. Central and southern Florida will stay under the influence of subtropical ridging and continue with above average temperatures. Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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