Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1257 PM EDT Tue Mar 19 2019 Valid Mar 19/1200 UTC thru Mar 23/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS Model Evaluation Including Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Trough and Developing Low in the East... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, 00Z ECMWF Confidence: Average Considerable model spread persists through Day 3 with guidance still shifting the track of the coastal low expected to develop off the Mid-Atlantic Wednesday night and move over the Northeast through Friday. The 12Z GFS has trended slow and farther inland which brings it into better agreement with the 00Z ECMWF operational. However, notable spread in both the 06Z GEFS and 00Z ECENS suggest uncertainty persists and further shifting of the track can be expected. The model preference listed above was based predominately around that system, but the preferred blend is applicable elsewhere. The 12Z ECMWF and 00Z GFS are in reasonably good agreement across most of the CONUS, and a blend of the two should represent a consensus forecast close to ensemble means (with an average amount of model spread). The track of this low is sensitive to northern stream shortwaves that will be interacting, including a phase on Thursday and of particular concern is the reinforcing shortwave that reaches the Great Lakes Thursday night which will influence the track of the primary trough/low through Friday and where the phasing takes place over the weekend (beyond the current Day 3 outlook). The reinforcing northern stream shortwave is still faster in the 00Z ECMWF than the 12Z GFS, though the difference has shrunk and hence a much more similar track of middle and lower level components. Furthermore, the intensity of the components are similar among the GFS and ECMWF resulting in similar QPF magnitude. The 00Z UKMET and CMC are considerably slower with this reinforcing trough and allow the low to track well off the Carolina coast and remain considerable outliers. The 12Z NAM is more intense with both mid-level waves which results in similar tracks to the GFS and ECMWF, but with much greater magnitude to the QPF (despite higher central surface pressure). The preference remains with the GFS/ECMWF with the caveat that much is still to be determined with the timing and intensity of the interacting components of this low. ...Low and approaching trough in the West... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Above Average An amplified/full-latitude trough currently off the West Coast comes ashore tonight before closing into a low over the Desert SW Thursday and swinging into the northern Rockies through Friday. Good agreement is seen among the global guidance so far today with this feature, so a general model blend is recommended. The next trough approaches the West Coast late Friday with good timing agreement among guidance with the arrival of QPF into CA. A general model blend also holds for this second feature. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Jackson