Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1205 AM EST Thu Feb 28 2019 Valid Feb 28/0000 UTC thru Mar 3/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Initial Model Evaluation...with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Closed low/upper trough over the Pacific Northwest ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/18Z GEFS mean Confidence: Near average An upper level low and its surface low initially parked off the Washington coast will be quite slow to move through Friday morning, after which it begins evolving into an open wave over Washington and Oregon and then tracks inland across the northern Rockies. The models are in good agreement on this system through Friday evening, with some slight differences noted in the CMC regarding surface low placement. The 00Z GFS becomes slightly faster with the trough progression whilst the 12Z UKMET is slightly slower than the model consensus. In terms of the upper low dissipation, a few of the euro ensemble members keep the low distinct whereas other model ensemble members have quickly transitioned to an open wave. A blend of the 12Z euro and 18Z GEFS mean provide a reasonable starting point for the forecast here. Closed upper low over southern Canada ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through Saturday morning, then non-NAM consensus Confidence: Above average An elongated upper low tracking southward from central Canada is forecast to consolidate into one main low centered over Manitoba and Ontario by this weekend with a trough axis extending westward near the Canadian border. This will send a strong arctic surface high southward across the Plains by Sunday with well below normal temperatures expected. The models are relatively well clustered, and the main difference noted is that the NAM is not quite as far south as the model consensus concerning upper low placement by Saturday night and into Sunday. Low pressure reaching California on Saturday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/18Z GEFS mean Confidence: Near average A rather potent southern stream shortwave and surface low tracking eastward across the Pacific is progged to reach the California around Saturday afternoon, bringing more additional heavy precipitation to this region. Although this shortwave dampens some by Sunday morning, it should continue its track across the southern Rockies and then be the next big weather story for the south-central U.S. just beyond this forecast period. The GFS keeps the low amplified a little longer than the other models by 12Z Saturday, and the UKMET and GEFS mean have a slightly northern track with the surface low, and the NAM slightly faster than the consensus. The 12Z euro and 18Z GEFS mean provide a reasonable depiction of the storm track evolution for now. Shortwave perturbations and low pressure systems across the eastern U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/00Z GFS/00Z NAM blend Confidence: Near average Quasi-zonal flow with low-amplitude perturbations will be present from the Midwest to the Southeast coast through this forecast period. The upper level pattern shows decent model agreement through Friday afternoon. The UKMET has 500mb heights that are likely too high over the Midwest by Saturday, and the CMC appears too low with these heights over the southern tier of the U.S. With no major differences noted among the NAM, GFS, and euro, a blend of those models should suffice for now across 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