Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1227 PM EDT Sat Jul 21 2018 Valid Jul 21/1200 UTC thru Jul 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including preference and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Eastern CONUS closed low & coastal low ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z UKMET/ECMWF blend Confidence: Average The closed low over the S Great Lakes into the Ohio valley is fairly agreeable as it descends southward into the upper TN Valley and GA by late Mon/Tues. The 00z CMC is a bit weaker initially and more concentric by the end of the forecast period but this is well within the ensemble suite/noise to keep it in a general model blend for this system. The upper low's interaction with the shortwave along the East Coast shows some variation to the strength, track to the latter system. The 12z NAM is showing some strength (too deep) bias within the suite that maintains through the NW and W retrograde over the top of the closed low into the Great Lakes by late Sun into Mon. The 12z GFS followed it well deepening about 30dm in the 7H to 5H compared to the 06z run, leading to greater uncertainty. Combine this with uncertain development residing on the magnitude of latent heat release to support strengthened upper level features...and while there is some modest instability and moisture flux across the Mid-Atlantic, the expected values are not as strong as forecast, casting some doubt. The GFS/NAM may be on a trend or may be over performing on the latent heat release. At this point, would favor shifting away from the GFS/NAM as it falls in line with known bias but reserve the right to adjust if we seen the UKMET/CMC/ECMWF and 12z GEFS solutions shifting harder in this direction. Canadian System and frontal zones into northern tier ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM through 24/12z 00z UKMET/ECMWF after 24/12z Confidence: Slightly above average shifting to average As for the closed low developed across Western Canada, and the trailing compact shortwave currently entering the Olympic peninsula, appears to be best analyzed in placement/strength by the 12z NAM, but not significantly so. Still, some of the typical bias to retain a very strong, compact wave remains with the NAM even passing through the rugged disruptive terrain of the Northern Rockies. There is some support from the 06z GEFS and 12z GFS which depict a stronger negative tilting trof with this feature across the S Prairie Providences but not to the magnitude of the NAM, and is more sensible within the overall suite. So with a further north surface low, the frontal timing/strength of low level flow into the frontal zone is more sensible across the northern tier with the NAM exclusion. Greater uncertainty/model spread exists with interaction of the closed low and a S/W in the Arctic stream into Mon, the 12z GFS shows reduced spacing and an less W-E troughing associated with this wave compared to the ECMWF/UKMET and a slower CMC. This makes the GFS stand out with timing of a secondary frontal boundary associated with this feature that resides along/just north of the border by late Tues. As such would shift preference here away from the GFS toward the ECMWF/UKMET (as the CMC is also a bit too slow). Gallina