Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 155 PM EST Wed Mar 07 2018 Valid Mar 07/1200 UTC thru Mar 11/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation with final preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Series of shortwaves in the vicinity of the Northeast U.S. closed low... ...Coastal low in the vicinity of New England through Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average There has been convergence with the surface low compared to yesterday's models, with the GFS moving east overall and the ECMWF generally moving west. Some minor flip flopping has been observed and can continue to be expected through Thursday night as the surface low tracks up the New England coast tonight and tomorrow before wrapping back inland to the west by Friday morning. However, the latest deterministic models and ensemble scatter low plots show good agreement, enough to support a general model blend for the preference. ...Secondary surface low near Nova Scotia Friday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS / 12Z ECMWF blend Confidence: Slightly above average A fair degree of spread remains in the ensemble with another surface low forecast to develop in the western Atlantic early Friday morning, about 500 mi south of Nova Scotia, with the low expected to track north before tracking west towards New Brunswick on Saturday. While the ensemble spread remains, the 00Z CMC is a deterministic outlier with its low position on Saturday, well to the east of the 12Z NAM, 12Z GFS, 00Z ECMWF and 00Z UKMET. The 12Z CMC adjusted west toward the remaining model clustering, but still lies farther north and east Saturday evening. Among the non-CMC guidance, the 12Z NAM and 12Z UKMET are similar to one another lying west of the latest ensemble spread which has not shown movement toward the west as seen in the 12Z NAM/UKMET, so a 12Z GFS and 12Z ECMWF blend is preferred at this time. The 12Z ECMWF did not deviate much from its previous cycle. ...Pair of shortwaves converging on the Northwest Thursday/Thursday night... ...Cold front(s) impacting the western U.S. through Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z CMC blend Confidence: Slightly above average The first in a pair of shortwave to impact the U.S. is expected to weaken as it reaches the Pacific Northwest on Thursday. There is good agreement with this idea in the latest model guidance with a general model blend preferred. The next shortwave is forecast to drop south down through British Columbia Friday morning. Only the 12/00Z CMC differs significantly here with its handling of vorticity within the base of the shortwave trough over Washington, which lags the northern portion of the trough advancing through south-central Canada. A non 12Z CMC blend is preferred with this system, and further impacts from this feature are discussed in the below section. ...Shortwave trough moving into the central/northern Plains at some point on Saturday... ...Possible surface low developing over the southern Plains late Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/ECMWF/CMC blend or 12Z GEFS mean/00Z ECMWF mean blend Confidence: Below average As the shortwave above advances southeastward from the northwestern U.S. Friday night into Saturday (referenced above), model differences become large. The 12Z GFS has been consistent with a stronger shortwave over the OK/AR border by Saturday night and some trends have moved in this direction from the 12Z ECMWF, UKMET and CMC. Taking a look at spaghetti heights from the ensembles, spread over the past 4 12/00Z cycles has only narrowly reduced, but the 12Z UKMET and 12Z GFS appear toward the stronger side of the ensemble spread, while the 12Z ECMWF/NAM and are flatter over the South valid 00Z/11. The 12Z CMC splits the difference to some degree with this feature. Valid 00Z/11, ensemble scatter low plots show a broad envelope of lows extending from west-central Texas into central Arkansas, with a mid-point near the eastern Texas/Oklahoma border. It is here that the preference lies, which is between the stronger 12Z GFS and weaker 12Z ECMWF along with the 12Z CMC. The thinking is that verification will be somewhere in the middle, which is supported relatively well by the 12Z GEFS mean and 00Z ECMWF mean. ...Weak shortwave approaching California from the west Saturday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non-12Z UKMET blend Confidence: Average The past 3 ensemble spaghetti plots have meandered about a common timing with this feature as a flat shortwave trough approaches the West Coast Saturday evening. The 12Z models have come into better agreement on the timing of the wave, but differ on the amplitude. For example, the 12Z UKMET is weaker with this feature but the remaining deterministic/ensemble consensus shows better support for a non 12Z UKMET blend. The 00Z ECMWF mean appears to fit the middle of the trends and latest members well, which is best supported by a blend of the 12Z GFS/GFS/ECWMF and 12Z CMC. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto