Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Fri May 29 2020 Valid May 29/1200 UTC thru Jun 02/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper level trough and cold front across the Eastern U.S... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average ...19Z update... The 12Z CMC sped up with a lead shortwave trough passing through southern Quebec Saturday morning which puts it in better agreement with the remaining models. Otherwise, a small trend to be faster Sunday into Monday with the longwave trough axis toward the east was noted in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...previous discussion follows... The mean axis of a longwave trough will slowly edge east from the Great Lakes region today, to the Northeast on Monday. The orientation of the trough axis will likely become negatively tilted by Monday morning with troughing extending from the Northeast to the eastern Mid-Atlantic region. While the 00Z CMC is a bit slower than preferred with a lead shortwave embedded within the flow over southern Quebec Saturday morning, the models paint an overall similar picture for this area of the country over the next 3 days. Therefore, a general model blend is recommended. ...Closed upper level low that reaches northern Mexico on Friday night and becomes slow moving through the weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z CMC blend (least weight on 12Z GFS) Confidence: Average ...19Z update... The 12Z ECMWF/UKMET/CMC joined the 12Z NAM/GFS with the strength of the 500-300 mb low over northern Mexico/southern TX for the weekend. There is now good agreement with strength and position of this feature but the 12Z GFS remains strongest and the 12Z CMC is a bit east of the consensus by late Monday. ...previous discussion follows... A mid-upper level closed low will develop over northern Mexico late today, followed by southward translation through the day on Saturday and a northward/northwestward drift for Sunday and Monday. The 00Z UKMET/CMC are weakest with this feature, and the 12Z GFS is strongest. Ensemble means support 700-500 mb ridge strength over the Central Plains to Mid-MS Valley and 500-300 mb low strength over northern Mexico/southern TX...similar to a blend of the 12Z NAM, 12Z GFS and 00Z ECMWF. The 00Z ECMWF is weakest of the three regarding the upper low while the 12Z GFS is strongest. Subtle trends toward a stronger system were noted over the past 24-36 hours of model cycles, but prefer to have the least weight on the more aggressive GFS at this time. ...Trough evolution along the West Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z NAM blend (first shortwave) General model blend (offshore western trough) Confidence: Average ...19Z update... The 12Z UKMET/CMC adjusted toward the 12Z GFS/00Z ECMWF with the lead shortwave tracking across the Northwest into southern Canada. The 12Z ECMWF remained similar to its 00Z cycle. Now, only the 12Z NAM shows significant differences with its evolution into Canada. Regarding the offshore trough axis just off of the West Coast for Sunday and Monday, the UKMET and CMC nudged slower compared to their previous 00Z cycles, enough such that a general model blend can be used. ...previous discussion follows... A closed low that is currently offshore of CA will reach the mainland tonight and weaken into an open wave as it tracks through interior portions of the Northwest Saturday night. The shortwave will then track south-central Canada, over a strong ridge axis extending through the central and northern Rockies. On the heels of the exiting shortwave from the West Coast, an elongated mid-upper level trough will setup just offshore of WA/OR/CA by Monday morning, with northern and southern stream components. At least through Monday evening, the ensemble means favor the slower 12Z NAM, 12Z GFS and 00Z ECMWF. Given the 00Z UKMET/CMC are faster to bring this feature inland, they will not be included in the preference. Ensemble spaghetti height trends over the past four 12/00Z cycles do not show any change with respect to faster or slower timing, and if anything indicate a tightening of the models from the west and the east. However, despite the good agreement of the SREF/GEFS/EC ensemble means, there is still a fair amount of spread in the individual ensemble members so confidence is only average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto