Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 253 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 Final Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Longwave trough amplifying across the Plains before closing over the Southeast Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/00Z ECENS mean/12Z GFS/00Z GEFS mean Confidence: Average 19Z Update: The Canadian and UKMET have improved with their mid-level center location but still are struggling with the potent vorticity impulses rotating through the trough. The ECMWF is more in line with the previous blend, and while it is still on the west side of the envelope, cannot rule it out into day 3 as it is more inline with the ensemble means than it was at 00Z. Previous: 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: 12Z ECMWF/00Z ECENS mean/00z GEFS mean Confidence: Average to slightly below average 19Z Update: Spread has increased with the leading shortwave with the full 12Z suite. The NAM/GFS are fast and on the weak edge of the envelope, while the UKMET lags considerably despite having modest amplitude. The CMC timing is inline with the ECMWF and the global ensemble means, but it strong and south of the consensus. This leaves the ECMWF and the GEFS/ECENS mean as the preferred blend, but with lower confidence than earlier. The previous spread issues for the second impulse persist with the updated guidance, likely due to increased uncertainty with the leading shortwave. No significant changes are noted so the previous blend is unchanged. Previous: 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 19Z update: 12Z CMC remains a shallow and fast solution and will remain excluded from the preferred blend. The concerns about the strength of the UKMET have resolved with a subtly weaker solution and the ECMWF remains in line as well. No changes needed to the blend. Previous: 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