Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1216 PM EST Thu Mar 08 2018 Valid Mar 08/1200 UTC thru Mar 12/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS evaluation with preliminary preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Coastal low currently near coastal Maine, weakening westward into southern Quebec through Friday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The models show similarly with this system. ...Secondary surface low near Nova Scotia Friday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-00Z CMC blend Confidence: Above average By 00Z/10, the 00Z CMC still remains a little bit farther east than the remaining deterministic consensus with the surface low. The latest ensemble scatter low plots continue to show a modest amount of spread between 00Z-12Z/10, with the 00Z CMC on the eastern end of the ensembles. The non-00Z CMC models show minor differences with location and strength, but a blend of these is considered a reasonable compromise with this system. ...Shortwave dropping south across British Columbia Thursday night with the southern portion reaching into the Plains Saturday... ...Surface low developing over the southern Plains late Saturday and moving toward the Southeast on Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/06Z GEFS/00Z ECMWF/00Z CMC blend Confidence: Below average The shortwave coming through southwestern Canada shows some differences in the models early on with the 12Z NAM and 00Z ECMWF less amplified across the southern portion of the trough compared to the 12Z GFS/00Z UKMET/00Z CMC. These differences grow downstream into the Plains with little agreement seen among the models. Most model run to run continuity is poor, with perhaps the GFS showing the most consistency. However, the 12Z GFS appears to be a stronger outlier with its surface low as it develops and tracks across the South. At 500 mb, ensembles and the majority of deterministic guidance shows a trend toward with the mid-level trough east of the Mississippi River on Sunday into Monday. While ensemble scatter low plots show a moderate to large degree of spread on Sunday, a center point of the lows, when considering trends, is nearest the position of the 12Z GFS and 00Z CMC. The 00Z ECMWF appears too weak with the system and the 12Z NAM significant faster, while the 00Z UKMET appears too far to the south. No single model or two-way blend is preferred here given the large differences and poor run to run consistency. Therefore, a multi-model blend of the 12Z GFS/06Z GEFS/00Z ECMWF/00Z CMC is preferred with hopefully the strengths of each model being maximized while minimizing apparent weaknesses. ...Weak shortwave approaching California from the west Saturday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF/00Z UKMET blend Confidence: Average The 00Z ECMWF is much faster with the shortwave coming into California than many of the latest ensemble guidance, while the 12Z GFS is slower but weaker. The 00Z UKMET is also on the slow side but appears to be a favorable amplitude. Given the southern stream jet stream is rather zonal across the Pacific with a meridional northern stream, the preference is not be toward the slower side of the models. With a weaker amplitude system like the 12Z GFS, a slower/weaker solution does not fit the conceptual model, so a middle ground between the 12Z GFS/00Z ECMWF/00Z UKMET is recommended at this time. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto