Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 238 PM EDT Thu Apr 16 2020 Valid Apr 16/1200 UTC thru Apr 20/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 19Z Update: There was a slightly faster trend with both the UKMET and the ECMWF on all three systems discussed below compared to their 00z runs. The GFS remains the strongest solution with the southern stream system across the central/southern Plains by this weekend. The UKMET is a little more amplified with the Pacific Northwest system across Oregon with respect to its 500mb heights. Overall, the large-scale model agreement has improved with the Midwest to Northeast storm system to merit a multi-deterministic model blend. Western High Plains trough crossing the Midwest and then reaching the East Coast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Deterministic multi-model blend Confidence: Moderate-High A positively tilted trough across the Rockies and northern Plains is forecast to cross the Midwest and reach the East Coast by Saturday afternoon, while supporting a surface low that develops along a frontal zone and crosses the Ohio Valley and then exiting the coast just south of New England. The 00z ECMWF/UKMET have consistently been slower with the mid level feature and the surface wave, and the 12z GFS has trended a little bit closer to these models, so this model trio would work well as a blend here. The NAM is a little slower with the progression of the upper trough across the Midwest and Ohio Valley, but similar with surface low features. It should be noted that as the surface low deepens off the New England coast Sat, the 00z UKMET becomes faster than the 00z ECMWF/00z ECMWF ensemble mean. By that time, most of the sensible weather is expected to be off the coast. Closed low near California evolving into open wave over southern Plains ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z ECMWF/UKMET/CMC blend Confidence: Moderate As the closed mid level low initially off the California coast drops southeastward across the Desert Southwest, it opens up into a long wave trough. Trailing shortwave energy from northern California follows a path similar to the first mid level wave later in the period. These two features eventually consolidate over the southern Rockies and become a well-developed southern stream trough over the central/southern Plains by Sunday. The interaction between the two short waves is causing some issues for the guidance, with the 00z CMC consolidating these shortwaves the quickest and the GFS remaining the most amplified with the second trough across Arizona Saturday evening. The GFS becomes more progressive by the end of the forecast period across the Plains and also ahead of its ensemble mean. Short wave over the Pacific Northwest this weekend ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/12z ECWMF/00z EC mean blend Confidence: Moderate Short wave energy dropping across western Canada after Friday morning starts eroding the mid level ridging across the Pacific Northwest, eventually forming a closed mid level low over Washington and Oregon by Sunday morning. There is good overall model agreement through Saturday afternoon, after which the 12z NAM becomes stronger with the upper low across Oregon and a bit more progressive. The UKMET is slightly weaker and the ECMWF is a little slower. Given that there is still a fair amount of spread with the speed and magnitude of the mid level system, forecast confidence is only moderate at this time. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick