Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1240 AM EDT Wed Apr 29 2020 Valid Apr 29/0000 UTC thru May 02/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation Including Initial Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Plains to East Coast storm system ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z NAM/12Z UKMET/12Z ECMWF Confidence: Moderate The overall upper level pattern over the central and eastern U.S. will become more amplified over the next couple of days as an upper level ridge builds across the Rockies and western High Plains. Height falls on the eastern side of the ridge is supporting a rapidly developing closed low over the Midwest and deepens further as it reaches the Ohio Valley region by Thursday. As the main surface low over Michigan occludes by Thursday night, a secondary surface low forms over the Mid-Atlantic and lifts northward as the upper level trough acquires some negative tilt. Beyond 00Z Thursday, the 00Z GFS becomes a little faster with shortwave energy pivoting around the upper low, and farther north with the upper low by 00Z Friday. The CMC remains a little weaker with the low. The UKMET, NAM, and ECMWF are relatively well aligned and close to the ensemble means, so a blend of these three solutions should work well as a starting point. Multiple shortwaves affecting the Pacific Northwest to the Northern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/12Z EC MEAN/00Z NAM Confidence: Moderate There will be three individual shortwaves that will affect the West Coast through the end of the week and eventually flatten the western U.S. ridge some in the process. The lead shortwave reaches southern British Columbia Thursday morning, followed by a flatter wave reaching northern California and Oregon Thursday afternoon. These eventually phase into a stronger trough over south-central Canada and the northern Plains by Friday evening. The third shortwave approaches the Pacific Northwest coast by Friday night associated with an upper low over the northeast Pacific. The 18Z GEFS mean shows less definition to the lead shortwave across British Columbia, and then the NAM become slightly more amplified. There are amplitude difference with the trough crossing the Pacific Northwest, with the ECMWF and CMC not as strong as the UKMET/GFS/NAM, and the GFS is weaker with the downstream ridge axis. But Saturday morning, the GFS is slightly stronger and farther south with the low over Manitoba, and the CMC slightly faster with the surface front across the northern Plains. Model agreement is good overall with the system approaching the Pacific Northwest later in the week. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick