Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 212 PM EDT Sat Jun 27 2020 Valid Jun 27/1200 UTC thru Jul 1/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 18Z Update: The 12Z CMC trended eastward with the trough axis across the Western U.S. and the upper high over the Great Lakes and Ontario by the end of the forecast period, and closer to the other guidance compared to its 00Z run. It is the most amplified with the degree of amplification of the upper trough across the Intermountain West, and inline with the model consensus across the East. The 12Z UKMET also trended stronger across the western U.S. for the Day 3 period. The 12Z ECMWF remains stronger with the second shortwave crossing the central and northern Plains compared to the other guidance. Closed low over Quebec and accompanying shortwaves Trough impacting the Northeast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average An upper level low initially over Quebec will sustain a broad upper level trough centered over the East Coast through the weekend. The models are in decent agreement on the structure of this low over Quebec and the southward extension of the trough across the Northeast U.S. this weekend. On Sunday, a reinforcing shortwave trough will cross the Great Lakes and help to amplify the synoptic scale trough over the Northeast, and the models support the closed dropping south across New England for the beginning of the week. The main difference noted in the models is a southward displacement with the 12Z NAM by Tuesday regarding the upper low placement, and similar to earlier NAM forecast runs, so a non-NAM blend will be preferred for this region. Deep upper trough/closed low across the Rockies and Intermountain West ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS/UKMET/ECMWF Confidence: Slightly above average A deep upper trough and associated closed low is forecast to amplify across the northern Rockies and Intermountain West this weekend as an upper-level ridge builds across the Gulf of Alaska and also across the Midwest and Great Lakes region, suggesting a potential omega-block pattern. The models remain in good agreement on the evolution through Sunday evening, after which the 00Z CMC is displaced to the southwest with the trough axis and also with the downstream ridge across the central/northern Plains. Although the GFS trended a little slower in lifting the trough towards the northeast by the end of the forecast period compared to its 6Z and 00Z runs, it still remains a bit more progressive than the model consensus for the Day 3 period. Taking these factors into account, a ECMWF/UKMET/NAM blend should serve well as a starting point in the forecast process. Shortwaves lifting northeast over the Plains/Midwest ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z UKMET/GEFS mean/00Z ECENS mean Confidence: Average Ahead of the deep western U.S. trough/closed low will be a pair of shortwave troughs that eject northeast out of the Southwest and across the Plains and Midwest. Similar to earlier runs, the 12Z NAM continues to suggest a stronger solution with the first shortwave as it tracks towards the northeast from the central Plains to the upper Midwest through Sunday, in contrast to the global models being less amplified. The second shortwave has some model spread noted as the 00Z ECMWF/ECENS mean support a somewhat stronger solution versus the remaining guidance, which is a little less amplified. For now, a blend of the ensemble means and the 12Z UKMET will be preferred with both shortwave impulses. Remnant energy across the lower MS Valley Weakness lingering over Texas ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The model support a gradual building of the subtropical ridge this weekend across the FL Peninsula and adjacent areas of the Gulf Coast states. Some of the weak shortwave energy lingering over the lower Mississippi Valley should shear out over top of the ridge through Saturday. However, there will still be a broad mid-level weakness/trough lingering over Texas which should tend to retrograde westward through Saturday, before then lifting north across the southern and central Plains ahead of the deep Western U.S. trough/closed low. A general model blend will again be preferred with the weak trough over Texas. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick