Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1243 PM EDT Mon Apr 20 2020 Valid Apr 20/1200 UTC thru Apr 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Initial Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Surface low tracking off the Mid Atlantic coast today ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Moderate-High Shortwave trough now exiting the coast with a dual-low surface structure will continue to intensify offshore as it consolidates into one main surface low well to the southeast of New England. The main difference noted in the models is a slower progression with the 00Z CMC, but otherwise a multi-deterministic model blend should suffice. Developing long wave trough over New England with surface low ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF/00Z UKMET blend Confidence: Moderate-High Short wave energy diving out of central Canada in the fast mid level flow drops to the base of a developing long wave trough over the Upper Great Lakes by Tuesday morning, and then the long wave trough takes on a negative tilt. This will support a primary surface low over Quebec and a triple point surface low forming near the coast of Maine by Tuesday night. Most of the guidance is close with the evolution of the long wave trough (although the 00Z CMC is slightly more amplified aloft), and the surface low is displaced south of the model consensus with the CMC as well. Otherwise, the model differences with the surface lows has improved compared to earlier model runs. Mid level trough tracking from Desert Southwest to the Deep South ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General blend through 18Z Wednesday, then 00Z ECMWF/00Z EC mean/12Z NAM Confidence: Moderate The models are in generally good agreement with this southern stream storm system through Wednesday afternoon, after which the GFS becomes more progressive compared to the other models. The 00Z UK is stronger with that low and the CMC is stronger with the upstream ridge over the Rockies. Surface low tracking from western Canada to the Northern Plains ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF/00Z CMC Confidence: Moderate A clipper type surface low tracks southeastward out of Alberta and reaches the Northern Plains by Wednesday morning in progressive northwest flow aloft. The 00Z UKMET is a bit faster with the arrival of this low, and the 12Z NAM is a little stronger with the upper level trough that accompanies it. A blend of the well clustered GFS/ECMWF/CMC should work as an initial blend for this region. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick