Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 203 PM EST Tue Feb 20 2018 Valid Feb 20/1200 UTC thru Feb 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... ...12Z model evaluation with final preferences... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ***Note that due to data flow issues at WPC/NCWCP, the 12Z ECMWF/UKMET/CMC were not available for comparison and the preliminary discussion below will serve as the last discussion for this data cycle*** ...Positively tilted mid-upper level trough over the western U.S. with strong cold front passing through the Northeast on Wednesday...extending southwestward into the southern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS / 00Z ECMWF Blend Confidence: Slightly Above Average The 12Z GFS was noted to be slightly quicker with the progression of the leading edge of the mid-upper level trough into a potent ridge situated in the western Atlantic, when compared to the remaining model guidance. This is noted early on Thursday from the northern Mid-Atlantic into the lower MS valley. Ensemble means support their respective deterministic runs with a blend of the faster GFS and slower ECMWF appearing reasonable at this time. At the surface, the 12Z NAM looks to take a weak surface low along the MS valley too far north and west when compared to the remaining consensus. ...Shortwave amplifying southward along the West Coast on Thursday...possibly closing off in the mid-levels before reaching the Four Corners region on Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Average There are relatively minor differences with this wave...the 12Z NAM shows stronger/more closed off with the shortwave, the 12Z GFS is slightly faster and the 00Z ECMWF less amplified and slower. The 00Z UKMET/CMC seem to split these differences with varying degrees of spread depending on the forecast hour noted through 00Z/24. Overall, a general model blend is preferred as no single model appears to be best at all forecast hours, with a blend appearing to smooth out the differences and result in a solution nearest to the ensemble means. ...Lower amplitude shortwave nearing the Pacific Northwest on Friday with associated cold front... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-00Z ECMWF Blend Confidence: Below Average The 00Z ECMWF is flatter and faster with this shortwave as it approaches the Pacific Northwest, while a non-00Z ECMWF consensus supports closer to the more amplified 12Z NAM/GFS. Given the amplified nature to the upper flow over the central and eastern Pacific Ocean, more amplification downstream of a building ridge over Alaska by Friday makes sense. However, confidence is below average given larger than average ensemble spread with no clear trend and a wide range of possibilities seen in the ensemble spaghetti data. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto