Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Dec 22 2018 Valid 12Z Tue Dec 25 2018 - 12Z Sat Dec 29 2018 ...Significant central U.S. storm after Christmas... ...Overview... A storm system moving through the Southwest states next Tuesday (Christmas Day) will lift through the central Plains Wed-Thu and intensify, setting the stage for a major rain and snowstorm for much of the center of the country. The system will move into the Great Lakes on Friday and then off the Northeast/Mid-Atlantic coast next Saturday as another storm moves through the West/Southwest and Texas. The large scale remains quite predictable but the important details will be refined over the next several days. ...Guidance Evaluation/Preferences... A majority deterministic model blend served as a good starting point given the good ensemble agreement through the end of the forecast period. The 12Z/18Z GFS runs were different enough out of the Pacific to start the period that they were not included, but as the system moves into the central Plains their forecasts were closer to the consensus but still just a bit quicker. The 12Z parallel GFS (FV3) was closer to the ensemble consensus and was used in place of the operational GFS through the forecast period. The 00Z/21 ensembles slowed the system progression through the central Plains but the 12Z/21 ensembles were a tough quicker, but only minor changes to continuity were made as the overall deterministic/consensus blend has been fairly steady. ...Weather Highlights/Hazards... Early in the week the precipitation focus will be on the Southwest as the Pacific system pushes through northern Arizona. As the system starts to lift northeastward through Kansas, precipitation will expand around and ahead of the deepening low with the flow out of the Gulf. To the northwest of the surface low, snow will come down heavy at times with increasing wind and possible blizzard conditions over parts of Kansas and Nebraska Thursday. Several inches of snow are likely over a large area with over a foot quite possible from near the CO/NE/KS borders through central Nebraska toward southeastern South Dakota (roughly Goodland, KS to Sioux Falls, SD). Please consult the latest winter weather probabilities for a daily graphical depiction. In the warm sector, rainfall could become locally heavy near the Gulf Coast as the system infuses Gulf moisture northward up and over the warm front. Temperatures will trend cooler than average in the West behind the front Tuesday but will be above average over the Plains and much of the East as high pressure moves off the Mid-Atlantic coast. As the surface low deepens over Kansas, milder air will be drawn northward from the Mississippi Valley eastward as the synoptic warm front and leading warm front surge northward. This will yield temperatures 10-20 deg F above average, especially overnight, progressing eastward ahead of the cold front. 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