Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 242 PM EDT Mon Apr 02 2018 Valid Apr 02/1200 UTC thru Apr 06/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS evaluation...including preliminary model preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper level trough over the Northwest today continuing to dig to the Upper Midwest on Tuesday... ...Surface cyclogenesis over the Plains today with a deepening low moving into the Great Lakes Tuesday... ...Low lifting into Quebec with trailing cold front clearing the East Coast through Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 12Z NAM blend Confidence: Slightly above average The 12Z NAM has a surface/925 mb low early Tuesday morning over the upper Midwest Tuesday morning and is subsequently a little farther north with a warm front into the Ohio valley. The NAM's lower level temperatures are also colder than the remaining deterministic models in the vicinity of the surface low. Outside of these possible issues in the 12Z NAM, the upper level timing of the mid-level wave lies on the slow side of the deterministic guidance with the 12Z ECMWF/UKMET trending faster compared to their 00Z cycles. The 12Z GFS and 12Z GEFS are faster and lie toward the faster end of the latest ensemble spread with the previously slower ECMWF members trending faster over the past few cycles. The latest guidance continues to show the 12Z GFS slightly faster along with the 12Z CMC, with the 12Z NAM slowest. Given the trends to converge toward the middle, a blend of the 12Z GFS and 12Z ECMWF appears the best route at this time, with greater convergence of model cycles compared to yesterday. ...Low-amplitude shortwaves approaching Northwest U.S. Tuesday morning into Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model compromise (corrected from prelim disc.) Confidence: Above average After a period of brief shortwave mid-level ridging over the northwestern U.S. this afternoon, low amplitude shortwaves are expected to enter the Northwest tonight, downstream of a large scale trough over the coast of British Columbia. There are no significant differences with these smaller scale features, but large differences tied to the next system to impact the Northeast are described in the below section. ...Upper trough and surface cyclone approaching the Pacific Northwest late Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z ECMWF and 12Z UKMET blend Confidence: Average A mid-upper level trough approaching the West Coast will allow a surface front to near the coast late Thursday. There are timing differences in the model guidance, with the 12Z CMC toward the faster side of the latest deterministic and ensemble models. There is decent support in the ensemble means to be near the 12Z NAM/12Z ECMWF/12Z UKMET regarding a negatively tilted mid-level trough offshore of the Northwest near 00Z/06, but the 12Z GFS is lacking this feature. Therefore a 12Z NAM, 12Z ECMWF and 12Z UKMET blend is recommended. Only relatively minor changes were noted with the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...Mid-level closed low/shortwave trough near the central U.S./Canadian border late Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 12Z GFS blend Confidence: Slightly below average The ensemble means show the 12Z GFS is too fast and displaced westward with a shortwave/closed low forecast to near the north-central U.S. by 00Z/06. The ensemble means also support a surface low north of Lake Superior at this time, but the low is absent in the 12Z GFS. A non 12Z GFS blend (only the remaining 12Z NAM/ECMWF/CMC/UKMET) shows enough agreement to be included in the preference given alignment with the ensemble means. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto