Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 238 PM EDT Sun Mar 29 2020 Valid Mar 29/1200 UTC thru Apr 2/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Closed low over Great Lakes and New England with surface cyclogenesis near the coast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average 18Z Update: The 12Z UKMET made a very minor southward adjustment with the surface low near the New England coast and is solidly within the model consensus, and a general model blend of the 12Z guidance remains best. There is very good synoptic scale agreement on the evolution of the parent upper low over the Upper Great Lakes that progresses eastward to New England by Monday night. This will support the development of a coastal low near southeast New England that is forecast to track slowly eastward roughly near 40 degrees north. In regard to the developing surface low, there are minor placement differences that develop, with the 00z UKMET slightly closer to the coast and the CMC a tad farther to the south, but still close enough to merit a deterministic multi-model blend. Southwest shortwave and southern tier surface low reaching the East Coast by Tuesday evening ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM/GFS/ECMWF Confidence: Average 18Z Update: There was a slightly faster trend with the 12Z non-NCEP guidance with the low track across the southern tier of the nation. The 12Z CMC has trended slightly less amplified with the shortwave trough, although still slightly stronger and slower than the other models. By Wednesday morning, the 12Z UKMET becomes considerably faster and loses ensemble support. The previous model preference holds, except to substitute the 12Z ECMWF. Shortwave currently along the CA coast will progress eastward across the southern tier states through early week. The 00z CMC is already out of phase, a bit south, and more compact than other guidance and is quickly dismissed from preference. The remaining difference initially through Tuesday across the Southern Plains and into the southern MS Valley is in depth and timing, with the UKMET and GFS a tad weaker than the ECMWF/NAM; which leads to slightly different placement of the deeper moisture/unstable environment ahead of it. The weaker/faster solutions suggest a slower transport north and stronger convective response farther south near the Gulf. This seems logical at this time of year, but continuity and overall forcing/mass fields suggest northward shift. The 00z UKMET is much flatter than the ensemble/continuity and is more dramatically so on Day 3 across the southeast. While this is plausible, it is more dramatic than the GFS/GEFS which will represent the uncertainty with its incorporation over the UKMET in a preferred blend. Still, the uncertainty here is going to be mesoscale convective placement and its upscale influence. So the preference will be near a 12z GFS/NAM and 00z ECMWF blend at average confidence. Closed upper low/trough approaching Pacific Northwest through Monday then reaching western High Plains by midweek ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/12z UKMET with some 12z ECMWF/CMC at lower weight Confidence: Average 18Z Update: The 12Z NAM remains quicker to bring mid-level height falls to the region compared to the model consensus. The ECMWF trended deeper with the upper low over southwest Canada by the end of the forecast period, and the CMC trended slightly slower with the front and surface low across the northern Plains. Same model preferences apply with 12Z guidance. Broad trof developing over the Gulf of AK will continue to press southeastward with upstream shortwave energy swinging through to press height-falls/triple point and cold front through Vancouver Island and the Pac NW by early Monday. This undercutting shortwave will swing across the Northern Rockies and the southern Canadian Rockies by early Tuesday, supporting a western High Plains surface cyclone. Here the 12z NAM is very strong/quick as it rotates eastward. This will overall shift the core of the upper low farther east than has been/currently is suggested by the ensemble suite and trends. The 12z GFS and 00z UKMET are as fast as the NAM but much weaker overall, so these would be generally be favored over the NAM. The slower solutions of the ECMWF/CMC, by late Tuesday/Wed, both suggest a greater influence/tug on the shortwave lifting across the Canadian Rockies from the Arctic stream, drawing both north and west, and strengthening them. The ECMWF is more so, including against its ensemble suite members. This does not seem to play much of a role in the remaining upstream energy/jet streak that presses through the Pacific NW late Tuesday into Wed; so from a CONUS perspective, the ECMWF is viable in preferred blends. The CMC is a tad weaker but may be viable as well if desired. So all in all, a Non-NAM solution is preferred across the Northwest/Northern Rockies and the Northern Plains, though with greater weighting to the GFS/UKMET. Confidence in the overall pattern is high, but with solid differences in timing/strength of the key internal shortwaves and their influence on surface cyclone depth/orientation, confidence remains average in this blend. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina/Hamrick