Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 100 PM EDT Fri Mar 13 2020 Valid Mar 13/1200 UTC thru Mar 17/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed Low developing over Pacific NW through Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 00Z/16 non-12Z GFS blend 00Z/16 - 00Z/17 Confidence: Slightly above average Model differences remain small until Sunday evening when the 12Z GFS ends up farther south with the 700-500 mb low off of the West Coast. The 12Z GFS is not an outlier when considering the available ensemble guidance but given greater support for a non-GFS solution by Sunday, the preliminary preference will exclude the 12Z GFS after 00Z/16. ...Weakening mid-level shortwave tracking through the Southwest today, weakening across the Midwest late Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-12Z NAM blend Confidence: Slightly above average Only minor timing/amplitude differences remain with the mid-level shortwave as it tracks east through the OH Valley. The 12Z NAM, as with previous cycles, continues to look too fast, however not by a lot. A non-12Z NAM blend of guidance looks reasonable with this system. There are some latitude differences with the trailing front across the southern states by Saturday into Sunday with a weaker surface low noted in the 12Z GFS tracking across the Mississippi River Valley allowing for a more southern-placed warm front. The GFS is not an outlier, but there is less support for it among the remaining deterministic guidance. Ensemble means support a middle ground which can be represented by a general blend, removing the faster timing of the 12Z NAM across the eastern U.S. ...Base of longwave trough over the north-central U.S. by Monday evening... ...Related cold front across the Upper Midwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-12Z GFS blend Confidence: Average The 12Z GFS stands out as the faster model with this system, a non 12Z GFS blend will be preferred given the remaining deterministic and ensemble guidance (including the 06Z GEFS) is slower than the 12Z GFS. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto