Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 319 PM EDT Tue Apr 07 2020 Valid Apr 07/1200 UTC thru Apr 11/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Surface wave moving through the Great Lakes tonight, then sliding offshore along 40N Wed/Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non-12Z NAM blend Confidence: Slightly above average ...19Z update... Adjustments from the 12Z ECMWF/UKMET/CMC were small but in the direction of the previous preference noted below. This leaves the 12Z NAM as a faster/stronger outlier with this system. The final preference is for a non-12Z NAM blend ...previous discussion follows... Large east/west spread in the ensemble scatter low plots, but a subtle trend toward the faster camp rules out the slower 00Z CMC. The 12Z NAM is fastest and strongest with the low level wave, but the preference is to be closer to the middle to faster side of the latest guidance. There is some latitudinal spread as well, but a blend of the 12Z GFS, 00Z ECMWF and 00Z UKMET appears reasonable at this time given their placement away from the extremes. ...Upper Trough Digging Across Great Lakes; Surface Low Developing Off New England Coast Thursday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average ...19Z update... Adjustments from the 12Z UKMET/CMC were in the direction of the 12Z GFS and 00Z ECMWF, shifting the surface low a little farther north. The 12Z ECMWF is similar with position, but just a little weaker with the surface low. This leaves the 12Z NAM as farther south than the latest 12Z consensus but the differences are small enough, even considering the NAM, to allow for a general model blend. Confidence remains only average given the time frame of the forecast and potential for future shifts. ...previous discussion follows... Model spread is relatively small, but there is strong agreement for a rapidly deepening surface low in the Gulf of Maine late Thursday into Friday. Prior to Thursday, only minor timing differences exist with the shortwave trough as it swings east through the upper Ohio Valley into the Northeast with the 12Z GFS/00Z ECMWF a little ahead of the remaining 12Z NAM/00Z UKMET/00Z CMC. Valid 00Z/10, the ensemble scatter low plots would suggest the 00Z CMC is too slow/south with low development so it will not be preferred. The 12Z NAM/00Z UKMET are located a bit farther south as well while the 12Z GFS and 00Z ECMWF are farther north. There has been a subtle trend in the guidance since yesterday to be slightly quicker/more negatively tilted with the mid-level trough axis which would place the surface low a little farther north and perhaps inland. The latest ensemble means also support a solution closer to the northern 12Z GFS/00Z ECMWF but cannot rule out the 12Z NAM/00Z UKMET. Ensemble spread is relatively tight for a Day 3 system and all of the models agree with bomb-cyclogenesis, but the preference will favor the middle to northern side of the non-00Z CMC deterministic guidance. ...Slow moving southern stream (S CA/Desert Southwest) closed low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF mean or (12Z ECMWF/UKMET/CMC blend) Confidence: Average ...19Z update... The 12Z ECMWF/UKMET remain similar to their previous cycles and the 12Z CMC shifted westward a bit. The 00Z ECMWF mean is still preferred with the 12Z ECMWF/UKMET west of the ECMWF mean, and 12Z CMC east of the 12Z ECMWF mean (near the 12Z GEFS mean). ...previous discussion follows... Trends since yesterday support a westward shift with the 500 mb closed low over the Southwest valid Friday. The 12Z NAM and 12Z GFS are farthest east with the closed low but their trend has been toward the west. By Thursday and Friday, the ECMWF and GEFS mean split the difference between the farther east 12Z NAM/GFS and farther west 00Z ECMWF/UKMET. The 00Z CMC is also a bit east, but not to the degree of the 12Z NAM/GFS. Given westward trends, prefer to lean toward the slower/westward side of the middle of the ensemble spread, or near the 00Z ECMWF mean. No single deterministic model is in great agreement with the 00Z ECMWF mean, but perhaps the 00Z ECMWF is closest. ...Upper trough and related surface cold front reaching MT/ND on Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z CMC blend Confidence: Average ...19Z update... No significant changes were noted with the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...previous discussion follows... Model agreement is fair with this system, but the 00Z CMC becomes a deterministic outlier for late Friday with a more aggressive low, displaced south of the majority of guidance. Outside of the 00Z CMC, a blend of guidance looks just fine. The 12Z NAM timing looks reasonable as cold air filters south into MT/ND, but it is stronger with the low level flow and therefore colder by 00Z/11 with the surface to 700 mb layer into MT and ND. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto