Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 341 PM EDT Tue Apr 03 2018 Valid Apr 03/1200 UTC thru Apr 07/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation...including final preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper Level Trough over the Plains this morning, pushing east into the Great Lakes tonight and then wrapping into east side of Hudson Bay Low on Wednesday... ...Midwest surface low developing northeast into the Great Lakes tonight and occluding over Quebec by Wednesday... ...Trailing cold front sweeping to the East Coast on Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Continued convergence has been seen in the model guidance since yesterday and sufficient agreement remains to prefer a general model blend for this system. There remain minor detail differences and future minor wobbles in low placement/strength can be expected. No significant changes were noted between the 00Z and 12Z runs of the ECMWF/UKMET/CMC. ...Peripheral shortwave around Hudson Bay low rotating south into the Northern Plains by late Thursday and digging into the Great Lakes Thursday Night... ...Leading low amplitude shortwave dropping southeastward from British Columbia/Alberta...reaching the Great Lakes late Thursday... ...Associated surface cyclogenesis across the Great Lakes Thursday night and impacting the Northeast on Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS; 12Z ECMWF blend Confidence: Slightly below average Differences exist with a lead shortwave (weaker) which drops into the U.S. ahead of a more potent shortwave over south-central Canada on Thursday. A rough interaction between these two features results in a complex forecast. The 12Z NAM is notably slower with the lead wave Thursday night near the upper Mississippi valley. The 12Z UKMET/CMC are displaced eastward with the south-central Canadian shortwave/closed low which affects the surface evolution across the northern U.S. An initially weak surface low over the upper Mississippi valley is forecast to strengthen as it crosses the Great Lakes Thursday night and into New England on Friday. The ensemble means are in relatively good agreement with the upper level pattern and surface low track, and split the difference between the slightly faster 12Z GFS and slower 12Z ECMWF. While ensemble spread remains large and made large changes from yesterday's output, agreement appears to be converging toward a blend of the 12Z GFS and 12Z ECMWF at the moment, hence the preference. ...Low-amplitude shortwaves pushing into the Western U.S. in split flow from Wednesday to Friday... ...Lowering Pressures from the Great Basin into the Southern Plains with surface low development by Friday morning in the Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly below average While detail differences are large in the mid-level flow, there does not appear to be any one model to lean toward or discount given their broad agreement. Therefore, a general model blend is preferred with the large scale evolution, with blow average confidence in the details. ...Larger upper level low and trough approaching British Columbia by Thursday with occluded marine surface low offshore... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Convergence continues to occur with each model cycle and currently, the models show similarly with this system. ...Warm front and approaching surface low from the eastern Pacific, ahead of a mid-level shortwave tracking eastward to the south of a closed low west of British Columbia through Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 12Z UKMET, 12Z ECMWF blend Confidence: Slightly below average A shortwave is forecast to amplify as it tracks toward the West Coast at the end of the short range period...00Z/07. Tracking this feature back on the hemispheric charts puts it just west of the International Dateline at 12Z this morning. Flow across this region of the Pacific is fairly zonal and fast...150+ kt at 250 mb...to the south of a closed low in the North Pacific. As this feature tracks eastward, flow aloft remains fairly quick and the result is timing differences with the shortwave approaching the West Coast late Friday. The 12Z NAM is fastest, followed by the 12Z GFS and the 12Z CMC is slowest. While some trends to be faster have been noted in the GFS, the 12Z ECMWF/UKMET/CMC did not deviate faster or slower compared to their 00Z cycles. Ensemble spread remains large with the GEFS on the faster side with the CMC and ECMWF members slower, so do not want to hedge too quickly with this system like the 12Z NAM/GFS. A blend of the 12Z GFS, 12Z ECMWF and 12Z UKMET seems to be a reasonable blend in the middle of the spread. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto