Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1245 AM EDT Wed Apr 04 2018 Valid Apr 04/0000 UTC thru Apr 07/1200 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 Great Lakes this morning wrapping into east side of Hudson Bay Low by tonight... ...Occluding surface low moving into Quebec today and tonight... ...Trailing cold front sweeping to the East Coast on Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Models continue to be in excellent agreement with this system, and a general model blend is preferred to account for subtle, minor differences that may still lead to a small degree of uncertainty. ...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 western Canada reaching the Great Lakes late Thursday... ...Associated surface cyclogenesis across the Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z GFS and 12Z ECMWF; weighted to GFS Confidence: Slightly below average The best agreement with the timing of the shortwave rotating around the Hudson Bay low and pushing into the Great Lakes is between the GFS and ECMWF, which are also relatively close to the ECMWF Ensemble Mean. The primary difference appears to be as the primary shortwave (dropping south) begins to phase with the the low-amplitude shortwave pushing SE/ESE from western Canada. The GFS shows the low aloft opening up more quickly, with a positive tilt wave resulting as it begins to get sheared from the strong jet streak just to the south. Meanwhile, the ECMWF holds on to a closed low aloft longer, with a more neutral tilt to the wave. This results in a surface low that is a bit stronger and further northwest as compared to the GFS. A blend of these two models is the preference. The GFS has greater support from the GEFS-ECMWF ensemble clusters, and so the preference will be weighted slightly more to the GFS. The 12Z UKMET appears to progress the wave faster than the GFS-ECMWF consensus, while the 00Z NAM is slower and much flatter overall. The 12Z CMC has a surface low well to the northwest of even the ECMWF, which has less support from ensembles; thus the CMC is also not favored for this system. ...Low-amplitude shortwaves pushing into the Western U.S. in split flow from Wednesday to Friday... ...Surface low development over the Southern Plains Thursday associated with one of the waves, developing east into the Southeast by Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS; 12Z ECMWF; 12Z CMC Confidence: Average With the low amplitude nature of this wave, subtle differences may affect the sensible weather impacts across the Southern US. However, most of the global models show no obvious systematic bias that would make them a less likely scenario at this point. The two exceptions are the 12Z UKMET, which has considerably higher heights across much of the Southern US, and the 00Z NAM, which has a stronger wave and thus a stronger surface low that is further north as compared to the other models. A blend of the remaining models is the preference. ...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 Models continue to show relatively good agreement with this system, and a blend of the operational models is fairly close to a blend of the GEFS Mean and ECMWF Ensemble Mean. The preference is for a general model blend to account for subtle detail differences that have yet to be worked out. ...Stronger shortwave pushing toward the Pacific Northwest by Friday Night with new surface low development just south of the prior occluded cyclone... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z GFS and 12Z ECMWF; weighted to ECMWF Confidence: Average All the available models show a strong shortwave rapidly pushing northeast toward the Pacific Northwest, and closing off a low aloft by Friday Night as it crosses 140W. This should lead to relatively fast surface cyclogenesis with some fronts affecting the West Coast by Saturday morning. There is a spread in timing among the models, with the 00Z NAM the fastest, then the 00Z GFS, with the 00Z ECMWF and 12Z UKMET after that, and the 12Z CMC the slowest. A blend of the ECMWF and GFS would offer an approximate average timing of the models, and they are close enough that less of the detail in the mass fields would be washed out by a blend. However, given the potential for rapid deepening of the surface low, the preference is to lean more on the slower end of model spread, so the ECMWF is given more weight in the preference. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers