Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Sat Apr 14 2018 Valid Apr 14/1200 UTC thru Apr 18/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation with final preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Longwave trough over the Central Plains early today, becoming negatively tilted tonight and reaching the eastern CONUS Mon... ...Deep surface low over the Plains early today, occluding and tracking into the Ohio Valley and Great Lakes into Mon... ...Secondary surface low impacting the Northeast on Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/ECMWF/UKMET blend through 00Z/17 12Z ECMWF/UKMET blend 00Z/17-00Z/18 (12Z NAM secondary option) Confidence: Slightly above average The 12Z NAM is slightly slower with the frontal progression into the East, and given some of the slower biases seen with the ongoing system across the South and the fact that the upper trough axis will be swinging negatively tilted through the South and East, prefer not to be slowest. However, the 12Z GFS seems to go astray with the development of a secondary, triple point low near Washington D.C. Monday morning, tracking into upstate New York Monday night. The 12Z GFS also differs from the consensus regarding its 500 mb low progression into the Northeast on Monday, positioned more to the west. Between 00Z/17 and 00Z/18, there is decent ensemble consensus for a non 12Z GFS blend with the surface low and 500 mb low center, led by the 12Z ECMWF/UKMET with the 12Z NAM a secondary option for New England. The 12Z ECMWF adjusted a bit south of its 00Z run with the whole mid-level low across the East, which could be a temporary adjustment toward the south side of the latest model spread. Blending either the 00Z ECMWF or the 12Z ECMWF with the 12Z UKMET is considered the best route in the 00Z 17-18 time frame. ...Longwave trough slowly dropping into the Pacific Northwest this weekend... ...Surface cyclone emerging into the Plains during the day on Tue... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/UKMET blend Confidence: Average Timing differences appear with the timing of the upper trough into the West Coast beginning on Monday with the 12Z GFS a bit faster than the consensus. This is reflected in the ensembles as well with the GEFS on the leading edge of the guidance while the 00Z ECMWF is the slowest of the deterministic guidance. The amplified nature of the flow pattern across the Pacific supports the idea for future slowing, but given the 00Z ECMWF mean is ahead of the 00Z ECMWF, prefer to stay somewhat conservative on the timing of the upper trough. A blend of the 00Z ECMWF with the 12Z NAM is preferred at this time. This blend also proves true for the surface low across the Plains on Tuesday, with the middle of the ensemble clustering supportive of a blend between the slower 00Z ECMWF and faster 12Z NAM. 19Z update: The 12Z ECMWF/UKMET adjusted to the favored middle ground, replacing the preference with the latest available model guidance. The 12Z CMC is a bit faster than the favored middle ground, so it is left out of the preference along with the somewhat faster 12Z NAM which was included in the preliminary guidance. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto