Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1217 AM EDT Thu Apr 05 2018 Valid Apr 05/0000 UTC thru Apr 08/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z GFS/NAM evaluation with preliminary model preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ NAM/GFS initialization errors do not seem to degrade their short range forecasts. Weak trough/quasi-zonal flow moving into the West Saturday Lows emerging into the Plains Saturday night/Sunday morning ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-Canadian compromise; above ave confidence The 00z Canadian was much farther south with its upper low/main bundle of upper level energy moving into the West on Saturday than the other guidance. With the amplitude of the flow minimizing, believe the more northerly guidance (00z GFS, 00z NAM, 12z UKMET, and 12z ECMWF) have a better grasp on the synoptic situation. A compromise of the consensus is preferred with above average confidence. Mid-level vortex slowly dropping through Central Canada Amplitude of the Eastern trough Saturday onward Surface lows expected offshore the East Saturday/Sunday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: NAM/UKMET compromise; confidence no better than ave The 00z GFS is quicker to move energy through central Canada which leads to a broader/quicker moving trough in the East. This is at odds with the other deterministic runs and available ensemble means, so the GFS front in the western Atlantic is too fast. Also supporting this idea is the slowing trend seen between its 18z and 00z runs. The belief is that two waves of low pressure will move/exist off the East coast with this trough, which is well represented by the 12z global ensemble guidance -- one streaking offshore the Mid-Atlantic coast late Saturday with a second main low held up by the amplifying trough offshore the Mid-Atlantic coast Saturday night into Sunday. While no model presents the best idea independently, a compromise of the 00z NAM and 12z UKMET gets reasonably close to this idea. Confidence is no better than average in this solution due to the larger than average deterministic and ensemble spread at this time range. Disturbance moving by Michigan late Thursday/Friday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET compromise; above average confidence The 12z UKMET appears too narrow with this mid-level disturbance as the general flow pattern in the northern stream is one off lesser amplitude. Believe a broader shortwave makes more sense, which also befits the 18z GEFS/12z ECMWF ensemble mean solutions. A compromise of the 00z NAM, 00z GFS, 12z Canadian, and 12z ECMWF is preferred with above average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Roth