Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Fri Dec 21 2018 Valid 12Z Mon Dec 24 2018 - 12Z Fri Dec 28 2018 ...Significant central U.S. storm potential after Christmas... 16 UTC Update... No significant change in forecast thinking from overnight. Model consensus during Days 3-5 (Mon-Wed) was sufficient to use a blend of the latest deterministic guidance as a forecast starting point (06Z GFS and 00Z ECMWF/UKMET/FV3). Models/ensembles have shown a bit of a slowing trend with the significant low pressure system affecting the central U.S. by the middle of next week. The 00Z FV3 was the significant outlier after day 5, continuing to race the system quickly northeastward across the Great Lakes more quickly than the consensus, while the 00Z ECMWF was on the slower/western side of the spread. Ensemble members continue to show a range of solutions with respect to timing, intensity, and track for this low pressure system, and weighting of ensemble means (ECENS and NAEFS) was increased during days 6-7 to account for this spread. Ryan Previous Discussion (issued at 07 UTC) ...Overview... The upper flow next week will turn increasingly amplified as a Pacific system pushes through the Great Basin late Monday and out of the Four Corners Wednesday. It will track northeastward toward the Great Lakes next Thu/Fri as downstream ridging builds into the East Coast. The large scale remains quite predictable but the important details have yet to come into focus, including the exact location and type of precipitation. ...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 just a bit quicker than the ensemble consensus (including the 12Z Canadian) while the 12Z ECMWF was a bit slower -- all nearly fitting their long-term biases. The 18Z parallel GFS (FV3) was closer to the ensemble consensus and was used in place of the operational GFS by next Thu/Fri. ECMWF ensemble mean has been consistent in the track of the system out of the Southwest toward ~Lake Michigan but the ensemble spread encompassed about a 300 mile radius next Thu and about a 375 mile radius next Friday, roughly the extent of the cold sector precipitation shield. ...Weather Highlights/Hazards... Early in the week the precipitation focus will be on the West -- mostly the northern California and southern Oregon line but also into the Sierras -- as the Pacific system moves ashore. NW to SE motion would generally keep lower elevation rainfall amounts down but higher elevation snow could be well over a foot above ~5000ft or so. Lesser rain/snow is expected from Nevada into the Rockies with many areas seeing at least measurable precipitation. Best chance of snowfall over several inches will lie around the upper low track including the Mogollon Rim. As the system starts to lift northeastward, precipitation will expand around the deepening low as well as through the warm sector nearer to the Gulf Coast. Locations over the northern half or third of the Plains would see snow with this system based on the current most likely scenario. Strong winds may add to the hazardous conditions. Rainfall could become locally heavy near the Gulf Coast as the system wraps up through the central Mississippi Valley Thursday. Temperatures will start near of a bit above average to start the week but trend cooler than average in the West behind the front. The central/eastern states will trend above average as the low center lifts into the Great Lakes. Anomalies will be in the +5 to +15 deg F range but +10 to +20 deg F for overnight mins. 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