Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 132 PM EST Wed Feb 27 2019 Valid Feb 27/1200 UTC thru Mar 03/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low/trough across the Pacific Northwest/Northern Rockies... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 01/12z 12Z GFS/00Z ECWMF ensemble mean blend after 01/12z Confidence: Average The 12Z NAM/GFS are close to the consensus with the evolution of the closed low off the OR coast as it transitions into a strong short wave after 01/12z. After that time, the 12Z NAM become faster and flatter with the short wave as it crosses the Northern Rockies. By contrast, the 12Z GFS remained fairly consistent with the timing of the short wave, especially after 02/00z over the Northern Rockies. The 12Z ECMWF/UKMET remain slower than the consensus after 01/12z, though there are members of the 00Z ECMWF ensemble output that are faster than the consensus. Given the amount of spread with this system, the 12Z GFS/12Z ECMWF ensemble mean comprise the preferred blend, as they have exhibited the best consistency, especially after 01/12z. However, given the spread, confidence is average at best. ...Short wave energy crossing the Midwest/OH Valley... ...Weak surface wave crossing the OH Valley/Mid Atlantic... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-12Z NAM blend Confidence: Slightly above average The 12Z NAM is close to the consensus with the short wave shearing out of the fast mid level flow south of the strong closed low over central Canada through about 01/12Z. After that time, however, the 12Z NAM stands alone as it deepens and slows the short wave as it crosses Upper MS Valley and Upper Great Lakes. When compared to the tightly clustered consensus, it stands out as an outlier, and was not included in the preferred blend. ...Deep closed low/trough over southern Canada... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/12Z ECMWF blend Confidence: Average The 12Z NAM is close to the consensus as it brings a dumb belling strong closed mid level system across Manitoba and Saskatchewan through 02/00Z. After that time, the 12Z NAM becomes slower than the consensus (as well as slower than its 06Z solution), The 12Z GFS remains closer to the consensus as it crosses Ontario by 03/00Z. This position is supported by the 12Z ECMWF/00Z ECMWF ensemble mean, so the blend of operational 12Z GFS/12Z ECMWF is the preferred, as the 12Z CMC/UKMET bookend the consensus position late in the period. ...Gulf Coast/Southeast/Mid Atlantic short waves... ...Surface waves over the Southeast/Mid Atlantic Thu into Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend...first shortwave/surface low 12Z UKMET/CMC/ECMWF blend...second shortwave/surface low Confidence: Average...first shortwave/surface low Slightly below average...second shortwave/surface low There are subtle differences in the timing of short wave energy in the fast mid level flow across the Southern US (which would be expected in such as fast flow), with the 12Z NAM/GFS close to the consensus with the short wave energy that spins up a surface low off the Southeast coast through 02/00Z. However, at the surface, the 12Z NAM becomes much further north with the first surface wave on the frontal boundary extending off the Southeast and Mid Atlantic coasts (as it has been for at least three model solutions). Because of this, the 12Z NAM was not included in the blend with the first surface low. Both the 12Z NAM/GFS remain faster and flatter with the second short wave in the fast mid level flow, resulting in a weaker and more progressive second surface low that passes well southeast of southeast New England by 03/00Z. In contrast, The 12Z non-NCEP guidance is a bit deeper with the short wave, producing a slightly stronger surface low that passes closer to the southeast New England coast. The 12Z CMC came back to the non-NCEP pack with its surface low placement and strength, resulting in a very tight cluster among the 12Z UKMET/CMC/ECMWF. Bringing the 12Z CMC back into the non-NCEP fold increases the confidence here, ...Southern stream trough/surface low approaching CA Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 02/00z 12Z UKMET/CMC/ECMWF blend after 02/00z Confidence: Slightly above average The 12Z NAM/GFS are close to the model consensus as a closed mid level low near 34N 133W at 02/00Z. After the time, both solutions become slower and deeper (even slower than the 00Z/06Z versions of these models). The non-NCEP camp is faster and flatter with the short wave weakening as it reaches the CA coast by 03/00Z. At this point, given the strong mid level flow upstream of the short wave, a faster and flatter solutions appears more likely, so a non-NCEP solutions is favored later in the period. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hayes