Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1226 AM EDT Fri May 29 2020 Valid May 29/0000 UTC thru Jun 01/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Model Evaluation Including Initial Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Upper level trough and cold front across the Eastern U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Moderate-High The upper level low that had been tracking across the south-central U.S. is now in the process of becoming absorbed and phasing with an amplifying northern stream trough across the Great Lakes region. This will sustain the passage of a rather strong cold front by late May standards across the eastern U.S. and exiting the East Coast on Saturday, with a high quality airmass settling in behind it with cooler temperatures and much lower humidity. A reinforcing shortwave will pivot around this trough and further amplify the pattern across the Northeast states, and an upper level ridge building in behind across the Plains. The models are all in agreement on the phasing of the two disturbances through Friday night, with only minor differences noted thereafter. The GFS is only slightly faster with the progression of the cold front across New England, and the NAM slightly more amplified with the upper trough across the Mid-Atlantic and Northeast by Sunday. The differences are generally small enough to support a multi-deterministic model blend. Closed upper level low that reaches northern Mexico on Friday and becomes slow moving through the weekend ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Moderate-High An upper level trough initially over the southern Plains, mainly evident at the 250mb level, will continue to drop southward to northern Mexico by Friday night. It will likely become trapped south of the building upper ridge over the Great Plains, with little in the way of movement. There should not be much of a surface low reflection with this trough, however. The models are in relatively good agreement on the handling of this feature, and thus a general model blend can be incorporated. Trough evolution along the West Coast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through Saturday afternoon, then 00Z NAM/GFS/12Z ECMWF blend Confidence: Moderate A well developed upper level low situated near 30N/125W with subtropical origins is forecast to reach northern California by midday Saturday, and then evolve into an open trough as it lifts northward across the Pacific Northwest around the western side of the building ridge across the Plains. A second trough builds in west of California by Sunday evening as the first trough lifts northward across western Canada. In terms of the model evolution, the 12Z UKMET is portraying a faster track northward across the Pacific Northwest by Saturday night, and the 00Z NAM and GFS hold onto the closed low the longest before becoming an open wave. Confidence is a little lower regarding the second disturbance off the California coast, with the GFS/NAM/ECMWF showing a stronger 500mb trough compared to the flatter CMC and UKMET. There is good overall ensemble support for the stronger set of solutions here, and a NAM/GFS/ECMWF blend works well and also maintains earlier forecast continuity. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick