Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 117 AM EDT Sun Mar 25 2018 Valid Mar 25/0000 UTC thru Mar 28/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS evaluation...including preliminary preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Amplification of the western U.S. trough with eventual evolution into northern and southern stream components by Tuesday morning... ...Lee cyclogenesis over the Plains today and Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 75% 12Z ECMWF/25% 12Z UKMET blend Confidence: Average A complex evolution aloft across the western U.S. continues to make a challenging forecast given multiple shortwave facets to consider. By Tuesday morning, a northern and southern stream trough component are evident, with the northern stream trough axis over the north-central U.S. while a mid-level trough/closed low sets up over the Southwest. The 00Z NAM/GFS remain a bit faster with the northern stream component compared to the ensemble means and remaining deterministic model consensus. Meanwhile down south, the 12Z UKMET is a little slower with a shortwave moving across northern Utah which causes the southern stream wave to close off and be slightly slower than the ensemble consensus, evident in the 500 mb 558 dam spaghetti height plots Monday evening. The 00Z GFS slowed down some with its southern stream component, but remains faster, like the GEFS mean which is significantly ahead of the ECMWF and CMC members. While some convergence has been noted over the past 24 hours of model cycles between the slower and faster camps, the large amplified flow pattern would tend to favor a slower progression to the east than seen by the GEFS and even the 00Z GFS to some degree. The 12Z ECMWF is considered the best of the deterministic models for both northern and southern locations, with the 12Z UKMET considered next best, although some component of the 00Z GFS could be considered usable across the South. Farther north, a weak surface low will move eastward through Kansas into the Midwest late Monday into, with ensemble scatter low plots quite varied in placement. However, the faster 00Z NAM/GFS are considered less likely than the 12Z ECMWF/UKMET given the ECMWF/UKMET have a more favorable upper evolution matching closer to the relatively consistent ECMWF mean. ...Shortwave weakening toward the Southeast U.S. today... ...Weakening associated surface low transferring to the Carolinas coastline by Sunday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 12Z CMC blend Confidence: Above average The 12Z CMC is weaker with a shortwave over the Midwest compared to upper air plots from 00Z tonight, and when compared to the remaining model consensus. This leads to a flatter wave in the 12Z CMC and a slightly weaker surface low that ends up slower and further south than model consensus. Otherwise, models are in very good agreement. The preference is for a broad consensus with this system with the exception of the 12Z CMC. ...Closed low developing off the Northeast today and Monday... ...More accelerated development of the Carolinas coastal low well offshore on Monday and Tuesday... ...Associated strong surface high settling into New England... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Only minor differences remain with the closed low moving into the western Atlantic today/tonight with the 12Z CMC a little weaker than the consensus, the 12Z ECMWF is on the western side of the deterministic/ensemble spread and the 00Z GFS is slightly faster/east than the consensus. All in all, the differences are minor and have little to no impact on the strength and position of the strong ridge settling into New England on Monday. A general model compromise can be used for this system. ...Shortwave rounding a Pacific ridge through the Gulf of Alaska on Monday and digging into the Northern Rockies Tuesday... ...Surface low developing over Alberta and Saskatchewan Tuesday with trailing cold front reaching the north-central U.S. Tuesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z NAM, 00Z GFS, 12Z ECMWF blend Confidence: Slightly above average The 12Z UKMET is the most amplified with a mid-level shortwave as it reaches southwestern Canada and eventually Montana Tuesday night. The resulting surface low in the 12Z UKMET is much deeper over Canada than the remaining model consensus. The 12Z CMC surface low by Wednesday morning is toward the northern side of the model spread, while the 00Z NAM, 00Z GFS and 12Z ECMWF show similarly and between the latest GEFS and ECMWF means. Therefore, a blend of the 00Z NAM, 00Z GFS and 12Z ECMWF is considered best for this system. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto