Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 311 PM EDT Tue Apr 09 2019 Valid Apr 09/1200 UTC thru Apr 13/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Final Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Amplifying through Intermountain West by Wed spurring deep Plains cyclone, Wed into Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend but minimal weight on NAM Confidence: Slightly above average 19Z Update: Previous concerns about the UKMET/CMC being too slow to strengthen the mid-level low have eased with a more rapid deepening noted on the 12Z suite. The ECMWF appears to be very consistent with itself and its ensemble means, and the overall guidance cluster is tight outside of the NAM which may still be usable D1 into D2 for its lead on the subtly warmer solution. However, by D2 and into Friday, the NAM outpaces the consensus considerably and should be used with caution into the Great Lakes. The previous preferred blend remains the same however. Previous: The NAM continues its trend of being on the north side of the generally small model envelope as the mid-level low center closes off across the Western High Plains. The GFS remains in remarkably good agreement with its run-to-run consistency, and by Wednesday night there is very good agreement in a closed mid-level low across Western KS, while the NAM remains north into NE. This is echoed in the surface low positions as well. The NAM has the strongest vorticity maxima rotating beneath the closed low, suggesting it may not even be in agreement with itself as this energy could pull the low center southward towards the general blend consensus. Despite this, a subtle NW and warm shift has been noted in thermal fields which suggests at least a small percentage of the NAM could be used thermally as the other guidance is trending towards that warmer solution across the Midwest. On day 1 the 00z/9 CMC/UKMET are a bit slow and broad with the upper feature, and should be used cautiously, but later on Tuesday the above preference is suggested. ...Great Lakes/New England (northern stream shortwaves)... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 19Z Update: 12Z UKMET has slowed from its earlier runs to better match the remaining 12Z Suite and can be used with more weight in the blend. The 12Z ECMWF remains a bit slow which appears less likely due to the fast flow developing across the Northeast. Previous: A series of shortwaves will interact across the Northeast. Although a general model blend can be used, the GFS/NAM/CMC appear to handle the shearing of the first two vorticity centers better as the flow becomes pinched ahead of a third closed impulse dropping around the upper low near 50N/50W. The 00z/9 ECMWF does not respond well to this increasing geostrophic flow and is likely too slow pushing the energy across Maine, while the 00z/9 UKMET is on the fast side of the model suite. ...Southeast (southern stream closed low)... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average 19Z Update: 12Z UKMET remains just a tad weak and fast with the upper feature moving off the Southeast coast. This allows the earlier preference to remain unchanged. Previous: Closed low clearly evident on WV imagery this morning across MS will drift eastward to be off the Southeast coast Wednesday morning. The UKMET is a bit weaker and then fast ejecting this feature, but otherwise the remaining guidance is in generally good agreement. ...Wed/Thurs shortwave entering Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non NAM/UKMET Confidence: Average 19Z Update: Interaction between multiple vorticity centers dropping out of the Gulf of Alaska on the UKMET causes a flattening of the shortwave ridging across the Pacific. This allows secondary energy to become dominant and pull the trough westward. While this exact solution is not supported by other guidance, the ECMWF and CMC have also shifted west due to stronger vorticity energy and a more robust jet. While the idea of a more westward pull of this energy into the Pacific NW is reasonable, this is likely to occur with a more phased and stronger primary center as shown by the CMC/ECMWF/GFS versus the separate impulses noted by the NAM/UKMET. For these reasons the UKMET has been removed from the preferred blend while the CMC is in line with the global ensemble mean. Previous: A shortwave diving from the Gulf of Alaska will approach the Pacific Northwest Thursday. As this occurs it will likely shear out to the south in response to ridging developing across S Central Canada north of the significant cyclone in the middle United States. The guidance is in reasonable agreement diving this feature inland from the Pacific Coast beyond this forecast period, but the general trend has been for a slower evolution of this occurrence. The NAM seems to drive the trough too strongly into the ridge while the 00Z/9 CMC is an outlier with the amplification of the ridge across the Pacific producing a shorter wavelength and leaving the main vorticity energy too far to the west. For these reasons a 12Z GFS/00Z ECMWF/00Z UKMET is preferred with average confidence. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Weiss