Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1214 PM EST Tue Jan 28 2020 Valid Jan 28/1200 UTC thru Feb 01/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Split flow trough crossing the Great Plains today and reaching the Southeast Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF blend Confidence: Slightly above average The latest numerical guidance remains in decent agreement with the evolution of a shortwave which was over the Southern Plains at 12Z today/Tuesday. As the wave tracks east, the 12Z NAM slows compared to the remaining model consensus and the 00Z UKMET also slows likely due to differences in the handling of the northern stream component discussed below. A 12Z GFS/00Z ECMWF blend looks good for this system with the 00Z CMC serving as a secondary preference. ...Split flow trough in northern stream from the Upper Midwest to the Mid-Atlantic Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 00Z UKMET blend Confidence: Average the northern component of an elongated mid-level trough tracking across the Upper Midwest on Wednesday now shows fairly good agreement outside of the 00Z UKMET. The 00Z UKMET is more amplified than the remaining consensus regarding the 500 mb vorticity max moving across the Ohio Valley Wednesday night. There are minor latitude differences outside of the 00Z UKMET with the shortwave as it reaches the East Coast late Thursday and an accompanying surface low in the western Atlantic. However, a non 00Z UKMET blend appears reasonable given the differences are relatively small. ...Mid-level trough over the Pacific Northwest today and amplifying into the Desert Southwest Wednesday, reaching the Southern Plains Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF blend Confidence: Average Ensemble guidance shows the 12Z NAM and 00Z CMC as slower/more amplified than the consensus as the trough axis reaches the Southern Plains. Meanwhile, the 00Z UKMET becomes much faster with the trough by 12Z/31. The ensemble means are in the middle along with the agreeable 12Z GFS/00Z ECMWF. ...Mid-level trough entering British Columbia on Wednesday and amplifying southward into the Great Plains by Friday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF or 06Z GEFS/00Z ECENS means Confidence: Below average A high degree of uncertainty exists with how the southern end of a shortwave trough evolves once it crosses through a ridge centered over southwestern Canada and the northwestern U.S. Run to run consistency has been poor and ensemble spread is large. However, 540 dam ensemble spaghetti plots at 500 mb seem to suggest the 00Z UKMET/CMC are on the far southern end, if not outliers, with their troughing over the Upper Midwest by 00Z/01. Given the 12Z NAM appears too slow with the system referenced above into the Southern Plains by 12Z/31, it will not be included as part of the preference. This leaves the 12Z GFS and 00Z ECMWF which are different but a blend between the two ideas is recommended at this time, or near the GEFS/ECENS means. ...Leading edge of longwave trough, offshore of the west coast of Canada Friday evening and downstream atmospheric river into the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM/00Z ECMWF blend Confidence: Average After the passage of a shortwave and accompanying cold front into British Columbia on the 30th, initially zonal flow into western Canada will begin to amplify as ridging increases ahead of an upstream shortwave near 40N 145W by 00Z/01. Ensemble means and the remaining deterministic consensus suggest that the 12Z GFS is too quick with the shortwave, located at the leading edge of a longwave trough between 12Z/31 and 00Z/01. A blend of the 12Z NAM and 00Z ECMWF is closest to the relatively agreeable ensemble mean's position with the shortwave and downstream atmospheric river into the Pacific Northwest. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto