Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 129 PM EST Wed Dec 11 2019 Valid Dec 11/1200 UTC thru Dec 15/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Southern Stream/Southeast to Mid Atlantic and Northeast surface low Fri/Sat... Preference: 12z GFS/NAM and 00z ECMWF Confidence: Slightly below average All the models have the same general idea of an area of low pressure developing over the Southeast and moving up the coast Friday night into Saturday...however differences with the details of the evolution remain. Quite a few pieces of shortwave energy interacting within a quick zonal flow, thus makes sense that models would struggle a bit with the details of how everything comes together. The initial wave that triggers the low development seems pretty well forecast, however it is the trailing, stronger, energy that eventually catches this low that is a bit less certain. The 12z GFS/NAM and 00z ECMWF seem pretty well clustered with the system, and while differences still exist within these solutions, a compromise of the three seems like a pretty good starting point. The 06z GFS was an outlier in how it handled the evolution, but the 12z run appears much more aligned with the better consensus. The 00z CMC appears too strong with the lead wave, which allows for a stronger low initially moving up the coast. While this is not impossible, it appears like a lower probability event. The 00z UKMET is much stronger with the trailing shortwave energy causing an earlier formation of a negatively tilted trough and a further west low track...and this too seems less likely at this point. Thus both the 00z CMC and UKMET are not in our preferred blend at this time. ...Northern Stream shortwaves across the Northern Plains to the Upper MS Valley and Upper Great Lakes... Preference: 12z GFS/NAM and 12z ECMWF Confidence: Slightly above average The northern stream 500 mb shortwave departs the northern Rockies across the northern Plains tonight and into the upper MS Valley Thu. The wave deamplifies and turns northeast across the upper Great Lakes Thu night and across the Canadian border. Minor differences in wave amplitude and phasing and general good clustering of solutions lead to a recommendation to use a multi-model blend. The next shortwave drops out of southwest Canada into the northern Plains Thu afternoon and night. This wave gradually amplifies as it crosses the northern Plains into the Upper MS Valley Fri with the southern stream portion noted above developing in the mid to lower MS Valley. The 12z ECMWF has slightly amplified this wave to reduce the differences. The northern stream portion of the trough is slow to move out of the upper MS Valley with a slow drift towards the upper Great Lakes on Sat. There is good agreement on the location and timing among the NAM, GFS, and ECMWF as a result of the slow movement, supported by the 12z GEFS mean and 00z ECMWF Ensemble Mean. ...Pacific Northwest to the Northern Great Basin, Northern to Central Rockies/Central Plains... Preference: 06z GEFS Mean in the Pacific Northwest, in between the 12z NAM/GFS over the Great Basin to the Rockies and central Plains Confidence: Average For Days 1-2 across the Pacific Northwest, CA, and the Great Basin, a parade of small amplitude shortwaves in the strong zonal upper jet graphic is best handled by a general model blend . Still, toward the end of the train, a stronger/greater amplitude shortwave tracks through with some moderate neutral to negative tilt orientation late Friday into early morning Sat with an associated surface cyclone. Remarkably, the global guidance is fairly agreeable with the timing and mass fields as a whole, providing some confidence. The large differences are the width of the upper level system, with the GFS a bit more slower and broader supported more by the CMC while the NAM/ECMWF are well placed/shaped but narrower and the UKMET is most narrow and negatively tilted. Will hedge more of a compromise between the 00z NAM/GFS and 12z ECWMF. The latter half of day 2 through day 3, differences develop in the 700 mb wave structure from the central Great Basin across the central Rockies and high central Plains. The 00z-12z ECMWF is faster than the NAM/GFS/UKMET, with the slower NAM and UKMET also more amplified. Confidence is lower here as the GFS trended faster on the 12z run. I recommend intermediate timing and amplitude between the faster/less amplified ECMWF/GFS and the slower/more amplified NAM/UKMET. The 12z ECMWF has increased the amplitude of the central Plains 700 mb trough Sat to narrow the gap in wave amplitude, so the 12z run looks good. By Day 3 in the Pacific Northwest, the upstream closed low over the northeast Pacific drifts slowly east, with most solutions opening the closed low at 500 mb into an open wave during the day Saturday, with the 500 mb wave possibly approaching the CA coast late Sat. The GFS is slower opening up the closed low into a wave, resulting in stronger height falls from western OR into northwest CA Sat afternoon. The 12z GEFS Mean clusters well with the 12z NAM 500 mb wave timing and a bit less amplified than the operational GFS. A blend of the 12z ECMWF and UKMET meshes well with the 12z GEFS Mean. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Petersen/Chenard