Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Wed Apr 04 2018 Valid Apr 04/1200 UTC thru Apr 08/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation...including final preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper Level Trough over the Great Lakes this morning wrapping into east side of Hudson Bay Low by tonight... ...Occluding surface low moving into Quebec today and tonight... ...Trailing cold front sweeping to the East Coast on Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similarly with this system. ...Peripheral shortwave around Hudson Bay low rotating south into the Northern Plains by late Thursday and digging into the Great Lakes Friday morning... ...Leading low amplitude shortwave dropping southeastward from western Canada, reaching the Great Lakes late Thursday... ...Associated surface cyclogenesis across the Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM, 12Z GFS and 12Z ECMWF Confidence: Average The 12Z UKMET/CMC remain different from the remaining model consensus with the upper flow evolution across the Great Lakes region. They both are stronger at 500 mb due to a slower leading low amplitude shortwave moving across the Midwest Thursday night, while the 12Z NAM trended a bit faster with this feature while the 12Z GFS/ECMWF remain in the middle. Run to run changes are still occurring in the guidance but ensemble means support a broad blend of the 12Z NAM, 12Z GFS and 00Z ECMWF with this system...both aloft and at the surface. ...Low-amplitude shortwaves pushing into the Western U.S. in split flow through Friday... ...Surface low development over the Southern Plains Thursday associated with one of the waves, developing east into the Southeast for Saturday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/CMC blend or 00Z/12Z ECMWF blend Confidence: Average No discernible differences or preferences with the lower amplitude waves are noted until early Saturday as one of the shortwaves begins to amplify east of the Plains. The 12Z GFS is much faster and flatter with this wave, resulting in a quicker surface low track across the Southeast and Mid-Atlantic coast. The 12Z NAM is also a bit faster and more amplified across the Southeast with the 500 mb trough axis. The 00Z ECMWF appears to be the closest match to the preferred solution, but a nudge toward the NAM/GFS cannot be ruled out in future cycles. 19Z update: The 12Z UKMET trended slower aloft with the shortwave over the Southeast, while the 12Z CMC/ECMWF trended flatter and faster, in the direction of...but not to the extent of...the 12Z NAM/GFS. Given these trends, a nudge in the direction of the faster trends will be preferred, with shifts in ensemble guidance possible as run to run changes continue. This nudge is best represented by a 12Z ECMWF/CMC blend or a 00Z/12Z ECMWF blend. ...Large upper level low and trough approaching British Columbia by Thursday with occluded marine surface low offshore... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The models show similarly with this system. ...Strong shortwave pushing toward the Pacific Northwest by Friday Night with new surface low development just south of the prior occluded cyclone... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 12Z CMC blend Confidence: Average Only the 12Z CMC stands out with a slower and less amplified wave approaching the West Coast. While the 12Z NAM/GFS remain quicker, the ensemble trends and latest means support a blend between the faster 12Z NAM/GFS and slightly slower 12Z ECMWF/UKMET. These relatively minor timing differences outside of the 12Z CMC can be worked out with a non 12Z CMC blend given better agreement on timing and placement from the 12Z model cycles compared to the 00Z cycles. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto