Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Sun Apr 15 2018 Valid Apr 15/0000 UTC thru Apr 18/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation and preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ ...Midwest Cyclone Becoming Centered Over New England by Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: Blend of 00z NAM/ECMWF/UKMET Confidence: Slightly Above Average A cyclone over the Midwest states early in the period will not encounter blocking, per se, but will move only slowly as it skirts beneath northern stream shortwave ridging, the stacked low reaching New England on Tue/Wed. The ECMWF has been very consistent from run to run, but did trend as we had hoped, back in the direction of the consensus Day 2/3 position with a less suppressed solution over New England. The GFS and Canadian solutions are more questionable, gaining latitude more quickly and progressing faster than the ensemble consensus. Given this is a broad scale closed low in April, we prefer the slower idea which matches better to consensus. While the GFS and Canadian may be useful to some extent, we prefer a blend of the NAM/UKMET/ECMWF. ...Trough Progressing Through the West, Reaching the Plains late Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~ Preference: Blend of 00Z ECMWF/UKMET/Canadian Confidence: Average A parade of energetic, medium wavelength systems is set up from the Pacific Ocean toward western and central North America. The next system will enter the West on Sunday, reaching the continental divide early Tuesday, and swinging negatively tilted onto the Plains by Wednesday morning. Late in this process the southern portion of the trough becomes more distinct, with the possibility of a small closed mid level circulation forming in the Central Plains / lee of the Rockies. Throughout the 3-day period the operational GFS appears too fast, and does not induce its deepening up through 500 mb early Wednesday despite a favorably energetic flow emerging from the mountains. The NAM, on the other hand, may be showing its typical bias by producing a solution that represents the strong, cold end of the spectrum, particularly when looking at the cold air at 850-700 mb versus the other operational models and ensemble means. The ECMWF and UKMET were good middle ground solutions with greater ensemble support, and as of the 00z cycle, also shared support from the Canadian. ...Next Pacific Trough Reaching the West Coast Tuesday Night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00z Operational Models, Excluding the UKMET Confidence: Average As the above system pulls into the Plains, the next Pacific trough will approach the West Coast Tuesday night. As with the other systems during this period, the ECMWF seems to be handling this pseudo-progressive, medium wavelength flow very consistently. Spaghetti plots containing all GEFS, EC, and CMC ensemble members and with multiple model cycles plotted side by side, show the other modeling centers' solutions trending in aggregate toward the ECMWF solution. As of the 00z cycle, the NAM, GFS, and Canadian are similar enough to the ECMWF to blend these all into a consensus solution, excluding only the UKMET which fails to achieve as much depth and is therefore a fast outlier. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Burke