Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1242 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 and 00z UKMET/ECMWF blend thereafter Confidence: Slightly above average until 28.00z Average to slightly below average thereafter 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 and 00z ECMWF blend thereafter Confidence: Slightly above to above average 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: 12z GFS(1/3rd)/00z ECMWF (2/3rd) Confidence: Average 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 NAM/00z ECMWF blend Confidence: Average 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