Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1210 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 NAM/GFS evaluation with preliminary 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 and the ECMWF 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 / 00Z 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. Among the non-00Z CMC guidance, the 12Z NAM and 00Z UKMET are west of the latest ensemble spread which has not shown movement toward the west as seen in the 12Z NAM and 00Z UKMET, so a 12Z GFS and 00Z ECMWF blend is preferred at this time. ...Pair of shortwaves converging on the Northwest Thursday/Thursday night... ...Cold front(s) impacting the western U.S. through Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 00Z 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 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 00Z 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 / 00Z ECMWF blend or 06Z GEFS mean/00Z ECMWF mean blend Confidence: Below average As the shortwave above advances southeastward from the northwestern U.S. Friday night into Saturday, model differences become large. The 12Z GFS has been consistent with a stronger shortwave over the OK/AR border by Saturday night. Taking a look at spaghetti heights from the ensembles, spread over the past 4 12/00Z cycles has only narrowly reduced, but the 00Z UKMET and 12Z GFS appear toward the stronger side of the ensemble spread, while the 00Z ECMWF and 00Z CMC are flatter over the South valid 00Z/11. 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 00Z ECMWF. The thinking is that verification will be somewhere in the middle, which is supported relatively well by the 06Z GEFS mean and 00Z ECMWF mean. ...Weak shortwave approaching California from the west Saturday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z GFS, 00Z ECMWF 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 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, 12Z NAM and 00Z ECMWF. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto