Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1258 AM EDT Tue Apr 03 2018 Valid Apr 03/0000 UTC thru Apr 06/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper Level Trough over the Rockies pushing east into the Great Lakes today and then wrapping into east side of Hudson Bay Low on Wednesday... ...Plains surface low developing northeast into the Great Lakes today and occluding over Quebec by Wednesday... ...Trailing cold front sweeping to the East Coast by Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend; weighted to 00Z GFS/NAM Confidence: Slightly above average Models have converged to a reasonably good consensus with this system, showing a tight clustering of surface low positions through 05.00Z (Wednesday evening) when the low should be into eastern Canada and the cold front should be clear of most of the East Coast. Therefore, a general model blend is the preference. There are some subtle differences noted in the 00Z NAM and 00Z GFS as compared to their previous runs at the initial hour of the forecast, and as compared to the 12Z model consensus. Ahead of the western trough, the shortwave ridge was initialized a little stronger than previous runs had forecast, particularly on the NAM. The NAM and GFS had slightly different initializations in the vicinity of Wyoming, and these mesoscale details could have an impact on the downstream forecast. Overall, though, the big picture is similar enough that the general model blend is still the preference. Will recommend a slight weight to the GFS and NAM, though, with the assumption that the 00Z initialization has captured a more amplified shortwave ridge than initially forecast -- which may lead to a slightly slower surface low. However, a close comparison with previous model forecasts does not reveal substantial differences. ...Peripheral shortwave around Hudson Bay low rotating south into the Northern Plains by late Thursday... ...Digging into the Great Lakes Thursday Night with surface cyclogenesis in the region... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS; 12Z ECMWF; 12Z UKMET Confidence: Slightly below average There is fairly good similarity through 05.06Z (Wednesday Night) in the model mass fields across much of the region, but differences grow fairly quick as the shortwave digs into the northern tier of states. The 12Z CMC was showing a high height bias aloft across the Northern Plains and also kicked the rotating shortwave out furthest to the west. The eventual surface and mid-level low positions are much further north than the other global models, and thus it is not preferred. The 00Z NAM holds on to a strong closed low over central Canada for another 12 hours beyond the global models, and this leads to a later arrival of the trough into the northern tier of states. There isn't much support for this among the global models, so it is also not preferred. The remaining global models (GFS, ECMWF, and UKMET) are close enough and similar to the ECMWF Ensemble that a blend of the three could be used. ...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: 18Z GEFS Mean; 12Z ECMWF Ensemble Mean Confidence: Below average Recent equatorward shift and extension of the North Pacific Jet is typically associated with reduced predictability, and this is the case by late in the week with quite a bit of model spread across the Western US. Small shortwaves will be pushing into the region in an increasingly split flow regime as the nose of an upper level jet approaches the West Coast. As a result, there is lower than normal confidence in the specifics. Given reasonably close agreement between the GEFS Mean and ECMWF Ensemble Mean in the large scale pattern, the preference is to weight the forecast increasingly toward a blend of the ensemble means by the end of the week. ...Larger upper level low and trough approaching British Columbia by Thursday with occluded marine surface low offshore... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 18Z GEFS Mean; 12Z ECMWF Ensemble Mean Confidence: Slightly below average As discussed in the previous section, the pattern shifts over the Pacific favor reduced model predictability, and thus will prefer a greater weight of the forecast to the ensemble means. The GEFS and ECMWF Ensemble means do have some differences over the eastern Pacific by Friday morning, but largely agree on an upper level low near 48N/143W and broad southwesterly to west-southwesterly flow to the south that should be impacting the West Coast. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers