Extended Forecast Discussion NWS Weather Prediction Center College Park MD 134 AM EST Thu Dec 27 2018 Valid 12Z Sun Dec 30 2018 - 12Z Thu Jan 03 2019 ...Overview... Positively-tilted troughing will reload in the west as subtropical upper ridging slowly retreats eastward past the Bahamas. This will allow 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 perhaps several surface waves moving northeastward through the Southeast or southern Mid-Atlantic as a Pacific system pushes eastward Sun-Tue from Montana to the Upper Midwest and Great Lakes into the Northeast. ...Guidance Evaluation/Preferences... A consensus of the latest deterministic guidance (18Z GFS and 12Z ECMWF/UKMET/CMC/FV3-GFS) served as a good starting point for the forecast Sun-Mon. Differences surrounding northern stream shortwave energy and the associated low pressure system crossing the northern Plains/Midwest Sun-Mon favored an ensemble consensus near this cluster (away from the 12Z GFS/GEFS). Wavy front in the Gulf will release several waves of low pressure along its boundary as upper ridging over the Bahamas prevents any eastward movement of the front until late in the forecast. Models/ensembles have had poor continuity with these smaller features (unexpectedly) so a consensus approach seemed prudent. By next Wed/Thu, models/ensembles differ on how quickly to progress the reinforcing height falls in the west eastward -- GFS/Canadian generally quicker than the ECMWF. The slower ECMWF ensembles have been trending just a bit quicker while the quicker GEFS ensembles have been trending just a bit slower but their means were still were ~10 deg longitude apart by next Thursday. Preferred the slower side of the ensembles (near the 12Z ECMWF ensemble mean) given the best overlap of ensemble clusters near the 12Z ECMWF/FV3-GFS and its stability. ...Weather Highlights/Hazards... Cold front in the east will linger in the Gulf through the period, spreading rain over the Southeast and southern Mid-Atlantic in batches as a few surface waves ride along the boundary. This may even fall as snow on the northern fringe given sufficiently cold air and/or at higher elevations. A Pacific system will produce widespread but mostly light to locally modest rain/snow through much of the interior west continuing on Monday but diminishing with time. Bulk of Gulf moisture will be focused farther east along the wavy front that will flow northward and expand/enhance the rain shield through the Southeast especially Monday as low pressure lifts from Louisiana to off the Northeast coast. Once the fronts clear the coast late Tuesday high pressure should dry things out for much of the central/eastern states except in the Southeast. Tail-end of the front will finally make some eastward advance later next week as upper ridging to its east relents. Temperatures will be quite cold in parts of the southern Rockies/west Texas and Upper Midwest early in the week with anomalies about 10-25F below average. This will mean low temperatures in the negative teens over parts of ND/MN Tue/Wed and highs near zero. Above average temperatures (plus 10-25F anomalies) will be drawn northward ahead of the Pacific front Mon/Tue east of the Mississippi. Central and southern Florida will stay under the influence of subtropical ridging until about next Thursday and continue to see 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