Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1039 AM EST Wed Dec 26 2018 Valid 12Z Sat Dec 29 2018 - 12Z Wed Jan 02 2019 ...Overview... 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. 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. ...Guidance Evaluation/Preferences... A consensus of the latest deterministic guidance (00Z ECMWF/UKMET/CMC and 06Z GFS) served as a good starting point for the forecast on day 3 (Sat). Differences surrounding northern stream shortwave energy and the associated low pressure system crossing the northern Plains/Midwest Sun-Mon resulted in a trend away from the GFS and toward heavier ECMWF/CMC weighting during days 4-5 (Sun-Mon). The GFS and most GEFS members were on the northern edge of the ensemble spread with the track of this surface low, and a solution closer to the more southern ECMWF/CMC track was preferred. Further differences with additional shortwave energy diving south across the Rockies and reinforcing the broader upper trough Mon-Tue resulted in a continued trend away from the GFS and toward ECMWF/CMC and ECENS/NAEFS mean solutions. The GFS has continued to be much more progressive than the overall consensus with the large scale upper trough, while the aforementioned preferred solutions keep higher heights along the Eastern Seaboard west of the subtropical ridge, and keep the lower heights through day 7 (Wed) confined to from the Rockies to the Ohio Valley. By very late in the forecast period, run-to-run continuity among deterministic solutions declines to the point that ECENS/NAEFS ensemble means comprised over 2/3 of the forecast blend. ...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. Ryan/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