Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1216 PM EDT Wed Apr 17 2019 Valid Apr 17/1200 UTC thru Apr 21/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Preliminary Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Longwave trough amplifying across the Plains before closing over the Southeast Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECENS mean/12Z GFS/06Z GEFS mean Confidence: Average Amplified full latitude trough moving across the Plains will with neutral tilt will gradually become negative and close off across the Southeast. Although the GFS is a bit slow on day 1, it catches up to the consensus Friday, and is consistent with itself closing off Friday night. Otherwise, there are still considerable discrepancies in the guidance suite, especially by day 3. The NAM is fast to negatively tilt, and by the end of the forecast period is strong and NW both with the upper low and surface reflection. The 00Z/17 ECMWF is also well NW of the consensus, likely due to a stronger jet max rotating around the base of the trough which drives stronger left exit diffluence and a further NW surface low. However, the gradual trend has been for a slightly further NW solution among all the guidance, and this was noted as well with recent significant systems. The UKMET and CMC are not preferred at this time for their evolution seemingly too suppressed and struggling to determine a center leading to elongated upper features. One thing to note is that while there is truly one primary upper trough, there will likely be no less than 3 surface waves, each of which may have impact on placement of the subsequent baroclinic gradient and next surface low. This lowers the confidence slightly from the previous blend as determining exactly where these lows move is difficult even at short ranges. ...Shortwave digging into the Pacific Northwest Friday and Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General blend through day 2, 12Z NCEP suite/00Z UKMET day 3 Confidence: Above average through day 2, average on day 3 Leading shortwave moving into Washington State Friday features little spread and a general model blend is reasonable. The only caveat would be less weight on the GFS which, while consistent with itself and its mean, it on the weaker and more positively-tilted edge of the consensus compared to nearly all other global guidance. Spread increases dramatically into Saturday with a secondary impulse dropping into Northern California as the above shortwave dissipates. The 12Z NCEP guidance has gotten much stronger with this system as it moves onshore Saturday, trending towards the previously strongest UKMET. The 00z/17 ECMWF has slowed from its prior two runs, suggesting it may be trying to strengthen as well, and hope to see this with today's 12Z run this afternoon, but until then has limited viability in the blend. The CMC remains a very weak outlier, and is almost non-existent with its secondary shortwave, trending away from the NCEP suite. For this reason a general model blend can be used through day 2, with a 12Z NCEP suite and 00z/17 UKMET preferred on day 3. ...Closed low ejecting through Baja Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Above average Guidance remains in a relatively small cluster both with the amplitude and speed of a shortwave ejecting east into the Baja Peninsula Friday night and moving across into Southern AZ Saturday. The most notable difference exists in the 00z/17 CMC which is significantly weaker with the amplitude of this shortwave, which also causes it to race out ahead of the consensus, even ahead of its ensemble mean. Otherwise, the clustering is tight, and although the UKMET is a bit stronger, it is not outside of a reasonable D3 envelope. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Weiss