Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1145 AM EST Fri Feb 21 2020 Valid Feb 21/1200 UTC thru Feb 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed mid level low reaching the Rockies Sun...weakening over the Upper MS Valley Mon... ...Surface low tracking from northeast NM into Mid MS Valley by Mon ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z ECMWF/00z ECMWF ensemble mean/00z UKMET blend Confidence: Average The 12z NAM/GFS are close to the consensus with the closed mid level low off the southwest CA coast early Sat, but by 23/00z, both models become faster than the 00z non-NCEP model solutions,and the difference maximizes near 23/12z. After that time, the timing differences lessen, though the 00z ECMWF/ECWMF ensemble mean lags a general consensus with the mid level system as it open up over the Mid and Upper MS Valleys before 25/00z. The NCEP camp has slowed with this feature over the past three runs, and the non-NCEP guidance has increased its forward speed with the mid level feature, bridging the gap. At this point, a blend of the two camps would eliminate the differences with the mid level system. However, differences between the NCEP and non-NCEP camps regarding the surface low forming over northeast NM become larger with time, maximizing just before the end of the period (25/00z). By this time, the 00z ECMWF/00z ECMWF ensemble mean and the 00z UKMET seem to form the best cluster with the surface low position over southeast MO (which is supported by the preponderance of the multi model ensemble surface low positions). Given the above, a blend of the 00z ECMWF/00z ECMWF ensemble mean and the 00z UKMET are preferred, though the differences between the NCEP and non-NCEP solutions appears to the narrowing. ...Weak mid level shortwave and attendant cold front reaching the Great Lakes Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The 12z NAM/GFS are close to the consensus as the short wave energy comes ashore across west central British Columbia around 22/12z. After that time, the 12z GFS becomes faster with the mid level short wave (and its surface cold front), a trend seen in the last three runs of the operational GFS.The 12z NAM remains closer to the relatively tightly clustered model consensus with the short wave and cold front. From this vantage point, the 12z GFS is not significantly faster with the mid level and surface systems, so a general model blend is preferred here. Due to the speed of the mid level flow, it is possible model solutions may not yet have a good handle on how fast the mid level short wave crosses Saskatchewan into Ontario, so forecast confidence is only average. ...Mid level shortwave crossing the Pacific Northwest Sun...closing off over the Northern/Central Plains Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12z GFS blend Confidence: Slightly below average Both the 12z NAM/GFS are close to the consensus bringing short wave energy from 48N 150W at 22/12z to the Pacific Northwest before 24/00z. After that time, the 12z GFS becomes faster than the consensus (and the 06z GEFS mean) with the short wave as it closes off over the Northern and Central Plains by 25/00z. Much of the rest of the guidance also closes off the mid level system in the wake of the mid level trough over the Upper Great Lakes, but does so further northwest over the Northern Plains. Based on the timing of the upstream mid level system, a slower solution seems to fits the pattern better. For that reason, the 12z GFS was not included in the preference. However, given the spread in the guidance toward the end of the period, forecast confidence is slightly below normal. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hayes