Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Fri May 15 2020 Valid May 15/1200 UTC thru May 19/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 19Z Update: The models remain in excellent agreement after the arrival of the non-NCEP guidance for the West Coast region in regards to the synoptic scale upper level trough building in by Sunday and into Monday. With the downstream ridge across the Plains, the 12Z GFS remains slightly weaker with the ridge axis and the 12Z CMC is slightly stronger across the southern Plains by Monday evening with the ridge. The 12Z UKMET trended faster with the disturbance crossing the Gulf Coast region, however there is still some ensemble support from both the ECENS and GEFS for a slower solution here. Across the Great Lakes and Ohio Valley region for Day 3, the 12Z NAM remains the most pronounced with the evolving upper level cut-off low, whilst the UKMET and CMC are more indicative of an open wave at 500mb at the end of the forecast period. Finally for the disturbance off the Southeast coast, the 12Z ECMWF is initially displaced to the northeast with the low near the northern Bahamas and another near South Florida tonight, and then is a bit farther offshore as it tracks north and then northeastward. In contrast, the GFS and CMC are farther west and thus closer to the coast, with none of the guidance currently indicating a landfall over North Carolina at this time. Given the differences noted with the ECMWF, a greater emphasis will now be placed on the 00Z ECENS mean. Otherwise no big changes to the existing model preferences. Shortwave and surface low tracking across the Northeast through tonight ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: High The models and ensemble means are in very good agreement with this system and thus a general deterministic model blend can be incorporated. Shortwave and surface low tracking across the northern tier states ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 00Z Monday, then 12Z ECMWF/00Z EC mean/12Z GFS blend Confidence: Moderate A shortwave perturbation initially over the northern Rockies Friday will evolve into a closed upper low as it tracks across the northern Plains and Upper Midwest over the upcoming weekend. This low is expected to become cut-off from the northern stream westerlies by early in the week across the lower Great Lakes and Ohio Valley region, and then slow down in its forward progress. This will support surface cyclogenesis across the Midwest and track eastward across the lower Great Lakes along a frontal zone. There is decent overall model agreement through Sunday evening, after which the NAM becomes more amplified with the closed low dropping farther south, CMC weaker, and the UKMET more progressive. The 12Z GFS is noted as being weaker with the building upstream ridge across the Plains by late in the forecast period. Deep upper level trough developing along the West Coast by Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Moderate-High A large closed mid/upper level low is forecast to move from the eastern Pacific toward the Pacific Northwest coast through the period. Given the highly amplified pattern, the model guidance shows above average agreement, with only some minor differences at 500 mb on day 3 noted with the slightly stronger 12Z NAM solution. At the surface a low will approach the coast and then wash out by day 3 as the overall system slows down off the coast. This synoptic scale feature and thus the western third of the U.S. has the best overall model agreement compared to the other weather systems across the nation, and therefore a general model blend can be incorporated. Southern stream trough tracking from Texas and along the Gulf Coast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/12Z ECMWF Confidence: Moderate A relatively weak southern stream trough will move across the southern Plains through early Saturday, and gradually the mid level low crosses the Gulf Coast states by late in the weekend. The 12Z NAM is a little farther north with the 700mb low across Texas, and this results in less QPF compared to the other models through Saturday evening. The UKMET is noted as a slower solution by Monday, and therefore heavier QPF across the central Gulf Coast region. A blend of the ECMWF/GFS would serve as a good starting point since there is decent ensemble support for this initial blend. Low pressure developing near the Bahamas ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/12Z GEFS/00Z ECENS blend Confidence: Moderate The National Hurricane Center is currently monitoring an area of disturbed weather over the western Bahamas in association with a weak area of low pressure. This is forecast to develop further off the southern Florida coast through Friday evening and gradually move toward the Bahamas Saturday morning before turning northward this weekend. Forecast guidance agreement and confidence remains average at best and begins to diverge beyond 48 hours. The 00Z UKMET remains southeast of the model consensus as the low lifts northward from the Bahamas. By 12Z Monday, the ECMWF and its mean favor a slightly eastward track, whereas the GFS and the GEFS mean are favoring a slightly more westward track, with similar intensity levels. Although the 12Z NAM is closest to the model consensus, it is also the strongest solution at this time. Taking the above factors into account and to maintain previous model preference continuity, the favored blend incorporates the 12Z GFS, 00Z ECMWF, 6Z GEFS, and 00Z ECENS. For more information on this system and potential development, see the latest from the National Hurricane Center. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick