Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 235 AM EDT Thu Mar 29 2018 Valid Mar 29/0000 UTC thru Apr 01/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As the 00Z model run is being completed, the preliminary preferences are generally maintained as model spread looks quite similar. The most notable differences were with the Great Lakes surface low this weekend, and the model distribution there may have narrowed slightly. However, the 00Z ECMWF continues to be northwest of the ECMWF Ensemble Mean from 12Z, so will maintain the preliminary preference. ...Potent shortwave trough over Texas ejecting quickly northeast in convergent flow to New England by Friday... ...Secondary shortwave moving from the Northern Rockies to the Mid Atlantic in a similar time frame... ...Surface low tracking from the Lower Mississippi Valley to the Northeast by Friday and associated cold front... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET Blend; weighted more to 00Z GFS, 00Z ECMWF Confidence: Slightly Above Average A combination of the new 00Z GFS and 12Z ECMWF appear to offer a reasonable intermediate timing of the surface low as it develops and pushes off to the northeast, and seem to be closest to the 12Z ECMWF Ensemble mean. The 12Z UKMET seemed too slow, particularly given the flow pattern aloft, and also advanced the front slower to the east as compared to the other global models. The 12Z CMC and 00Z NAM are also fairly close to the ECMWF and GFS in their mass fields. There are slight differences, but they are reasonable possibilities. Therefore, the preference is for a non-UKMET blend, but weighted more toward the GFS and ECMWF. ...Shortwave in the Gulf of Alaska today, digging to the Northern Rockies by Friday with surface cyclogenesis over Montana... ...Continued trough amplification and surface low development into the Great Lakes on Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF Ensemble Mean Confidence: Slightly Below Average Despite a similar pattern aloft on all the models, subtle detail differences are still having an impact on the forecast -- perhaps related to how the digging shortwave interacts with a lobe of vorticity rotating around the Hudson Bay low. Model surface low positions by 31/12Z (Saturday morning) still have quite a bit of spread, and have trended substantial over the past couple model cycles. The GFS run 24 hours ago had only a weak surface low/trough into the Lower Great Lakes, while the 00Z run tonight now has a strong surface low over Wisconsin. The models will likely continue to shift over the next couple model cycles, but should continue to gradually converge. Although the 00Z NAM and GFS sit in the middle of the operational models for surface low position, there are quite a few GEFS and ECMWF ensemble members to the north and west, and the 00Z GFS has trended slightly slower and further north as compared to its 12Z run. Given the recent trends and ensemble members, the preference is to lean toward the ECMWF Ensemble Mean. That's not quite as far NW as the operational ECMWF, but more representative of ensemble spread than the GFS or NAM. The 12Z UKMET was considerably slower and the 12Z CMC was on the faster end of model spread. ...Low-amplitude shortwave reaching the Oregon coast on Saturday, and reaching the central High Plains by Sunday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET Blend Confidence: Slightly Below Average Models generally agree that a low-amplitude shortwave will be pushing through portions of the West this weekend, but offer differences in the details of their forecasts. The subtleties of these differences may be difficult to tease out when the shortwave in question is still well out over the Pacific, and its specific trajectory will be largely influenced by downstream developments. The most notable difference is with the 12Z UKMET which seems to bring the wave in slightly stronger and at a lower latitude, which makes differences in the sensible weather forecast. ...Shortwave beginning to dig into the Pacific Northwest late in the weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend, weighted to 00Z GEFS Mean; 12Z ECMWF Ensemble Mean Confidence: Slightly Below Average Yet another shortwave will be digging into the northern tier of states late in the weekend, this one into the Pacific Northwest. There are considerable differences in how far south the models bring the wave by 01.12Z (Sunday morning), and the general structure of the wave, with the 12Z ECMWF and UKMET offering more elongated waves and a more expansive upper level anticyclone over the Bering Strait, while the 00Z GFS shows a narrower ridge upstream and a more compact wave digging faster into the Northwest. A general model blend is preferred at this time without a clear systematic bias to exclude any models, although a heavier weighting toward the ensemble means is suggested. ...Lingering trough and stalled surface front in the vicinity of Florida this weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS, 00Z ECMWF Confidence: Slightly Above Average ---06Z UPDATE--- The UKMET now shows the weakest trough over the Gulf, with the 00Z CMC closest to the GFS and ECMWF. Given the lack of consistency from the CMC/UKMET, the preference is to go with a blend of the more consistent GFS and ECMWF forecasts. ---PREVIOUS DISCUSSION--- The new 00Z GFS and the 12Z ECMWF/UKMET are fairly close in their mass field forecasts in this region, and are also very close to the ensemble means. Therefore, the preference is to lean toward a blend of those models. The 12Z CMC shows the weakest trough aloft of the global models, and also builds the surface ridge in further than the other models. The 00Z NAM, meanwhile, shows a more amplified trough this weekend over the Gulf. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers