Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Sun Apr 19 2020 Valid Apr 19/1200 UTC thru Apr 23/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 19Z Update: The main trend noted in the arrival of the 12Z non-NCEP guidance is a faster progression of the southern stream trough reaching the southern Plains by Day 3, and represents a rather noteworthy change with respect to timing. This now leaves the 12Z NAM as the slowest solution, and the GFS remains a little faster than the other guidance, albeit not to the same degree earlier today. Given the run-to-run model variability, the 00Z EC mean will be substituted for the 12Z NAM as part of the model preference for this storm system. The 12Z UKMET remains the most amplified with the trough exiting the East Coast on Monday. Storm tracking across the Gulf Coast region ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM/12Z ECMWF blend Confidence: Moderate The southern stream shortwave currently over the southern Plains will continue tracking eastward across the Deep South through tonight and then exit the East Coast by Monday afternoon, sustaining a well developed surface low that will result in widespread heavy rain and severe weather. A dual-low surface low structure is appearing likely by Monday morning as a second low develops off the Mid-Atlantic coast, and passes well south of New England based on current guidance. The 12Z GFS becomes faster as early as Monday morning, and this trend continues as the low exits the coast. This is similar to what the 6Z GFS was portraying. The surface low tracks are similar in position among the non-GFS guidance through Monday afternoon, so the main differences between the models remains the placement of the axis of highest rainfall, which has more to do with mesoscale interactions and the overlay of the instability and mid level lift. The 00z UKMET remains slower and left of the consensus with the shortwave and the surface low once it exits the coast, likely due to the slower short wave energy in the developing trough. Because of this, this model was also excluded from the preferred blend. Because of the differences with the QPF placement and the track of the surface low off the coast, forecast confidence remains average. Building trough over the Great Lakes and the Northeast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/12Z ECMWF blend Confidence: Moderate The large synoptic scale trough over central/eastern Canada and the northern tier of the U.S. gets reinforced with a strong shortwave tracking southward across Manitoba, and then evolving into a cold upper low over southern Ontario and Quebec. This will sustain a cold front crossing the Great Lakes and Ohio Valley, and then reaching the East Coast by Tuesday evening. The 12Z GFS/NAM are close to the consensus tracking short wave energy in the progressive mid level flow from Manitoba across the Upper Great Lakes through Tuesday afternoon. Beyond that time, the 12Z NAM is slightly slower and a little more amplified with the developing long wave trough approaching northern New England. The 00Z CMC becomes even more amplified across New England by Wednesday morning as the trough acquires negative tilt. Since the GFS/ECMWF have the support of their ensemble means, these solutions comprise the preferred blend. Trough across the Southwest and then the southern Plains ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z EC mean/12Z UKMET/12Z ECMWF blend Confidence: Limited-Moderate The next short wave in the flow tracks toward the central California coast after Monday morning. There is still some model spread concerning the timing of this short wave, as it undercuts mid level ridging across the Pacific Northwest. Though there is a loose consensus taking the mid level system over Nevada and Arizona on Tuesday, more noteworthy differences become apparent by Tuesday night and into Wednesday as the system crosses the southern Rockies and eventually the southern Plains. Similar to yesterday, the GFS remains more progressive and also ahead of its ensemble mean, and the 00Z CMC is on the other end of the spectrum with a slower solution and weaker with the surface low that emerges over the Plains. The ECMWF is slightly weaker with the trough but is close to the consensus regarding timing, whilst the UKMET is slightly stronger. The 12Z NAM is also close to to ECMWF/UKMET regarding timing, so a blend of those three solutions should serve well as a starting point in the forecast. Clipper system reaching northern Plains by Wednesday morning ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/12Z UKMET/12Z ECMWF blend Confidence: Moderate A subtle mid-level shortwave tracking around the upper ridge across southwestern Canada will progress quickly towards the northern Plains in strong northwest flow, along with a clipper-type surface low reaching North Dakota and then Minnesota by early Wednesday along an existing quasi-stationary front. The 12Z NAM is displaced to the northeast with the track of this low and the 00Z CMC has a slightly weaker low. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick