Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 144 AM EST Sun Dec 15 2019 Valid Dec 15/0000 UTC thru Dec 18/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS Evaluation with Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Mid-level trough tracking from the western U.S. today to the eastern U.S. Wednesday morning... ...Surface low organizing east of the Mississippi River Monday into Tuesday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS/ECMWF/UKMET blend Confidence: Average ...07Z update... Regarding the timing of the 500 mb trough moving east, the CMC remains faster with its 00Z cycle, while the 00Z ECMWF slowed down relative to its previous 12Z cycle. The 00Z UKMET remained relatively similar with timing but was slightly slower at 500 mb. There was a southward shift from the 00Z ECMWF/UKMET/CMC with the warm front from the Mississippi River to the East Coast, compared to their previous 12Z cycles. The shift places the new 00Z non-NCEP guidance in better agreement with the 00Z GFS. This agreement should not be seen as a lowering of uncertainty though, as run to run changes will still be possible with a subtle shift back to the north or even farther south a possibility. ...previous discussion follows... Timing differences appear with the 700-500 mb trough axis as it reaches the Great Plains with the 00Z NAM slower and 12Z CMC faster. Ensemble agreement with this trough looks better than yesterday to support a 12Z ECMWF, 00Z GFS and 12Z UKMET blend. There are larger differences at the surface however, regarding the latitude of a warm front extending east of a surface low that develops over the Lower Mississippi Valley on Monday. The 00Z GFS is farthest south, while the 12Z UKMET/ECMWF are north with the boundary. Ensemble means support a middle ground which will be preferred, essentially blending the 00Z GFS, 12Z ECMWF and 12Z UKMET. ...Mid-level shortwave clipping the Great Lakes Tuesday into Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 00Z CMC blend Confidence: Slightly above average ...07Z update... The 00Z UKMET trended toward the preferred NAM/GFS/ECMWF but the 00Z CMC continued to stray from the better model clustering with a 500 mb low track farther south and west by Wednesday morning. ...previous discussion follows... A deep closed but somewhat compact low is expected to reach the Great Lakes region Tuesday night. There is good agreement in the latest deterministic guidance through Tuesday morning but then some differences begin to develop. Ensemble means support a 00Z NAM, 00Z GFS and 12Z ECMWF blend, with the 12Z CMC faster and 12Z UKMET slower with the closed low by Wednesday morning. ...Trough approaching the western U.S. Tuesday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 00Z CMC blend, led by the 00Z GFS Confidence: Below average ...07Z update... The 00Z CMC trended faster with the 500 mb trough nearing the West Coast Tuesday night, and it is now faster than the remaining deterministic guidance. The 00Z ECMWF trended toward the previous preference however, while the 00Z UKMET showed only minor changes from its prior 12Z cycle. The new preference includes the new 00Z ECMWF but excludes the 00Z CMC with forecast thinking unchanged from the previous discussion below. ...previous discussion follows... Large differences exist in the ensemble and deterministic guidance with this system. Differences are with weather more of the energy within the approaching trough focuses farther south into a closed low or stretches latitudinally into an elongated north/south axis by Tuesday evening. The 12Z ECMWF appears to stand out the greatest with three main and separate vorticity maxima within the trough axis whereas the remaining deterministic guidance shows two. The result is that the 12Z ECMWF takes a compact closed low farther south/more offshore of the California Coast by Wednesday morning and the remaining guidance takes the feature inland across California and Nevada. While differences remain among the non 12Z ECMWF guidance, a non-12Z ECMWF blend is preferred at this time, led by the 00Z GFS. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto