Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Sat Apr 25 2020 Valid Apr 25/1200 UTC thru Apr 29/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Shortwave and associated developing low pressure moving from the Midwest through the Northeast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend until 28.00z 12z GFS/UKMET/ECMWF blend thereafter Confidence: Slightly above average until 28.00z Average to slightly below average thereafter 19z update: The 12z CMC shifted north, being a bit stronger...but remains on the southern periphery of the surface low cluster; so it builds further confidence toward the remaining cluster. However, after 60hrs, the southern stream, like the 00z UKMET becomes very strong/compact with the digging replacement shortwave. The 12z UKMET backed off this evolution a bit and is similar (if equally quick) as the 12z GFS. The 12z GEFS would suggest the operational is a tad to aggressive; especially as the 12z ECMWF is slower/weaker and therefore north with the axis of development. Even though there remains moderate spread due to multiple wave interactions after 60hrs; a 12z GFS/UKMET/ECMWF blend remains preferred at average to slightly below average confidence. ---Prior Discussion--- Stacked cyclone moving along the Ohio River today and weakens in favor of a coastal low that moves through the Hudson Canyon from the DE coast south of Long Island before parking near Cape Cod Tues. The transition is fairly solid in the guidance, with the exception of the CMC which is a tad weaker and therefore tracks well south of the tight cluster. So a non-CMC blend is supported until 60hrs (28.00z). While the 12z GFS continues to depict greater latent heat release due to convection along the deformation zone across NE PA/S NY as it sharpens Monday morning; the temps are not as warm and more in tolerance to keep in the blend, but would continue to favor the ECMWF with profiles over the GFS when digging down to p-type influences, etc. Afterward, as the deeper cyclone is reinforced by upstream shortwave energy, the model spread increases due to differences in timing and magnitudes of interaction. The 12z NAM moves out of preference first given a stronger/more concentric organization that shifts the surface cyclone track north of the ensemble suite. By the end of the forecast period, there are some typical biases starting to manifest with the faster 12z GFS (even relative to the 06z GEFS suite), slower to weaken the occlusion with the ECMWF and the UKMET having a stronger, negative tilt as it weakens the larger scale closed low over New York/SE Canada relative to the other guidance. These are small but important details that will need to be determined in future runs; but for now a 12z GFS and 00z UKMET/ECMWF blend should be solid after 60hrs at average to slightly below average confidence. Trough axis affecting the Northwest into the Upper Midwest/Great Lakes through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend through 27.12z 12z NAM/GFS/ECMWF blend thereafter Confidence: Above average 19z update: With more preferable evolution downstream, the 12z UKMET and CMC both trended better with the base shortwave under-cutting the block ridge and sliding through the Great Lakes by the end of day 3. Still, both are generally much weaker with the UKMET fast and the CMC slow. Overall, a general model blend would likely suffice, but for a stronger blend...will keep with initial preference of 12z NAM/GFS/ECMWF blend after 27.12z; but increase confidence to above average ---Prior Discussion--- Shortwave currently entering southwest BC will translate across Southern Canada under solid model agreement through 48 hours as it clips the northern tier of the US northern Plains. Downstream blocking ridge across Lake Superior will lead to a bifurcation of the upper level trof, while the bulk of the energy will lift north, the trailing tail-end of the trof will move under the ridge and shear into the confluent region of the larger scale broad cyclonic weakness over SE Ontario/Lower Lakes. Much of the model difference/spread with this evolution is really related to the breakdown of the downstream cyclone. The UKMET (as stated in the section above) is quick to weaken and press the cyclone out, allowing for a faster eastward translation and helps to kick/enhance the shortwave in the Maritime. The CMC is on the other side is much stronger and dominates/shears out the energy. The 12z NAM/GFS and ECMWF are all more central and while there are some differences, they remain small. As such a general model blend is supported through 48 hours (27.12z) moving toward a 12z NAM/GFS and 00z ECMWF blend at slightly above to above average confidence. Shortwave energy affecting the Pacific Northwest on Monday into the Plains by Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 1/3rd GFS; 2/3rd ECMWF blend Confidence: Slightly below average 19z update: The 12z UKMET started to remove the strong convective feedback across the Northern Plains into the Upper Midwest, but it still remains stronger and faster with this feature. The 12z ECMWF trended a bit stronger with this southern cluster but more importantly moved it faster, leading to greater spacing between it and the main wave entering the Northern Plains, this allows for a broader (less sharp) broadening of the larger trof by 84 hour, weakening the surface wave. This increases spacing with the 12z GFS, which dug faster further south (though not as dramatic as the 12z NAM). The 12z CMC, appears to be a middle-ground solution between the UKMET (convective upscale) and the ECMWF, slightly slower with the main wave. As such it amplifies very deeply by 84hrs over S MN and looks dramatically out of place in depth, though shows a similar placement to the ECMWF. Overall, this speaks more to the uncertainty of timing and interaction of ingredients (instability and moisture) with this wave. The 12z GEFS is very close to the ECENS mean, and both split the difference between the GFS/ECMWF. So will keep with initial preference of 1/3rd GFS and 2/3rd ECMWF but reduce confidence to slightly below average (mainly after 72hrs) ---Prior Discussion--- The generally zonal pattern over-topping a strong ridge that dominates the eastern Pacific into the desert Southwest. The next strong shortwave comes along with the bulk of energy in the core of the system lifting north to build the larger Gulf of AK vortex. The tail end of the trof and associated cold front will track through SW Canada clipping the Pacific northwest. This wave will be a bit weaker/flatter having lost most of the energy north, but with a strong associated jet is progress quickly and finding weakness in the wake of the prior system, begins to amplify/dig into the Northern Plains by Tuesday. This system, like many in this pattern before it, because of the strong ridge over the West, will dig sharply and loaded on the upstream side of the trof. The CMC and UKMET have downstream blocking issues that are already less favored and this continues for this system after translating across the Northern US Rockies, to the point the 00z UKMET has a significant feed-back issue across the northern Plains in the late day 3. While this convective complex is likely to be there, given the GFS/ECMWF and their ensemble suites, the reflection in the UKMET is too much to consider as it messes up the entire mass fields. The 12z NAM is a bit broader crossing the Canadian Rockies, and atypical as it is, does not have a strong amplification...which seems a bit less likely given the longer term pattern. The 12z GFS is a tad faster than the ECMWF to the point the GEFS is closer in line to the ECMWF than the GFS. Still, both are similar in evolution, and will prefer a 1/3 12z GFS and 2/3rd ECMWF blend by the end of day 3. Confidence is average. Cold front reaching Pacific Northwest late Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GEFS/00z ECENS mean Confidence: Below average 19z update: The effects of model difference in the Gulf of AK and the timing of the moisture arrival to the Pacific Northwest may be minor; but sizable model shifts increased the overall spread/uncertainty for the closed low/surface center. The UKMET/CMC didn't change too much remaining slow, more elongated between two shortwave centers...but the 12z ECMWF trended slower nearly matching the track/timing of the UKMET/CMC. This is away from the consensus of the 00z CMCE/ECENS and the 06/12z GEFS. The changes also highlight a glaring fast/strong bias of the NAM as well as it approaches the West coast late on Tuesday. To keep with continuity, will support a 12z GEFS/00z ECENS mean solution for this setup and reduce confidence to below average ---Prior Discussion--- After a slight delay, the Pacific parade of shortwaves continue with a pair of closely packed shortwave features; the first of which approaches Tuesday. The lead wave, is delayed slightly due to the spacing of the later...but the GFS is clearly very fast and well north of the cluster, pressing the cold front and precursory moisture feed into the Pacific northwest at the end of the forecast period. The 12z NAM and 00z ECMWF are central to the ensemble suite while the UKMET and CMC are slower, showing no real distinction between wave packets (ie no short-wave ridging between) by the end of the forecast period, which delays the frontal zone. Will favor a 12z NAM/00z ECMWF blend for this system at average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina