Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Wed May 29 2019 Valid May 29/1200 UTC thru Jun 02/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Preliminary Model Evaluation Including Preferences and Forecast Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...CONUS from Rockies eastward... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: GFS and 00Z.29 ECMWF and their means Confidence: Slightly below average Closed mid-level low which had been stalled over the Southwest will eject and finally weaken as it becomes absorbed into a deep closed low over Southeast Canada, finally breaking down the heat ridge across the Southeast CONUS. There remain significant differences in the global suite as to how this will evolve, with the 00Z.29 UKMET the most unrealistic with its rapid ejection of the upper trough into the shear axis near the Great Lakes, breaking down the Southeast ridge far too quickly. This same model also handles the, admittedly, challenging interaction of multiple shortwaves across Canada day 2 and 3. The UKMET continues its fast trend diving a secondary impulse south of the main closed low, and amplifying the trough as far south as the northern Mid-Atlantic. This again seems unlikely based off the strength of the ridge to its south, and the UKMET has been removed from the preferred blend. The CMC is also driving this trough too strongly into the Southeast ridge, and should be used cautiously. Also on day 2, the NAM becomes weak and progressive with the shortwave moving through the Mid-Atlantic, a trend repeated often in recent NAM runs, most of which have not occurred due to the strength of the ridge. By day 3 /Saturday/ model differences are even more significant in how they handle the true breakdown of the ridge. A lagging piece of energy from the Southwest low will eventually traverse eastward driving a cold front through the Mid-Atlantic and causing temperatures to return to normal. Initially the slower solution are more reasonable, but as flow flattens and becomes more zonal, this final shortwave should race eastward to be off the coast Saturday, but confidence is lower than normal due to uncertainties into how small scale features will affect the synoptic pattern by day 3. ...Texas... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~ Preference: Non-Canadian Confidence: Above average A shortwave rotating through the base of the amplifying western longwave trough will eject eastward into Texas in relatively benign flow Thursday into Friday. While all the guidance shows this shortwave interacting with a thermal trough and dry line to produce significant rainfall across West Texas, the CMC is a fast outlier and brings the best convection into central and northern Texas. With mid-level flow weak, expect the CMC is too fast and will support the rest of the guidance which is in very good agreement and show a small ensemble envelope by day 2. ...West Coast/Great Basin... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average A trough diving southward through California will re-amplify the longwave trough pattern across the west as it closes off in the vicinity of the Great Basin D2/D3. Guidance still exhibits considerable intensity differences in this feature, but the trend in the operational runs has been for something subtly weaker, which is agreed upon by the ensemble means. This would jive with the downstream pattern of the ridge finally breaking down in response to shortwaves aloft, whereas otherwise a stronger trough would likely support stronger downstream ridging. For this reason a general model blend is appropriate, with slightly more weight on the 00Z.29 ECMWF/00Z.29 CMC and 12Z.29 NAM along with the ECENS and GEFS means which are the weaker and more positively tilted solutions. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Weiss