Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Mar 12 2018 Valid Mar 12/0000 UTC thru Mar 15/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation...including model preferences and confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwave energy crossing the TN Valley... ...Surface low tracking across the Gulf Coast States on Sunday... ...Strong surface low development offshore the East Coast... ...Phasing with strong Great Lakes height falls/closed low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z GFS, 00Z UKMET and 00Z ECMWF Confidence: Average The models take the vigorous shortwave trough currently crossing the TN Valley down to the east-southeast and then offshore the Southeast U.S. coastline by late Monday. As this occurs, strong cyclogenesis will be taking place off the Mid-Atlantic coast, with low pressure then expected to lift north-northeast and offshore New England through Tuesday as the shortwave trough pivots northeast ahead of the strong northern stream height falls/closed low dropping down across the Great Lakes region. Complete phasing of this energy will occur by Wednesday as the Great Lakes closed eventually captures the shortwave ejecting off the East Coast. By around 36 hours, and beyond, the 00Z NAM becomes a bit of a stronger outlier with the evolution of cyclogenesis off the East Coast with stronger 500/700 mb low evolution and surface reflection as the system approaches southern New England, although there is a bit better clustering/agreement with the models by Wednesday night and Thursday with the large scale closed low over the Northeast and with what is left of the coastal low that wraps north across Nova Scotia and then cyclonically looping back to the northwest near far northern ME. The concern is really in the 36 to 60 hour time frame relative to the New England as the NAM is not only a deeper outlier, but it tends to tuck its surface low a little to the left of the model consensus and closer into the coast. The 00Z GFS with this run trended a little farther east and has rather strong support from the 00Z UKMET and 00Z ECMWF which have both trended stronger compared to their respective 12Z runs. None of these models take the low quite as far west as the NAM and in time are not as strong as the NAM with their respective 500/700 mb low centers. The 00Z CMC is just a little east of the NAM and is west of the GFS/UKMET/ECMWF cluster. The 00Z GEFS mean has come in a little east of the operational GFS and certainly gives more support to the idea of the NAM being too far west for the 36 to 60 hour time period. WPC will prefer a blend of the GFS, UKMET and ECMWF at this time based on the latest ensemble clustering and deterministic trends. ...Shortwave digging over the Great Lakes by Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models all take another northern stream shortwave and dig it southeastward across the Great Lakes region by Thursday. the model mass field spread with this is rather minor, and so a general model blend will be preferred at this time. ...Shortwave over the Great Basin/northern Rockies... ...Energy shearing southeast across the High Plains by Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models all take a shortwave trough out of the Great Basin and then over the northern Rockies through early Monday. Thereafter, the guidance takes the energy around the crest of the large scale ridge axis developing over the West and drops the energy down over the High Plains where it will gradually shear out. The models show decent agreement with timing and depth and a general model blend will be preferred at this point. ...Large closed low/trough approaching the West... ...Surface cyclones offshore the West Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend...through 36 hours 00Z GFS/00Z GEFS mean/12Z ECENS mean...after 36 hours Confidence: Average...becoming below average after 36 hours A large deep layer closed low offshore the West Coast will be moving east over the next few days and will approach the West Coast by Tuesday and then begin to spread height falls inland on Wednesday as multiple shortwaves rotate around the base of the trough and lift northeast. This will bring surface low pressure up off the Pacific Northwest on Monday, and then with a new area of low pressure attempting to consolidate by Wednesday offshore as additional energy digs into the base of the trough and arrives across the West Coast. Aside from the weaker NAM, the guidance is reasonably well clustered through 36 hours with the first low offshore which the global models agree in deepening rather rapidly as it lifts north up off the West Coast. Thereafter, the model spread increases as the 00Z UKMET/00Z CMC and 00Z ECMWF solutions all suggest a lack of secondary low pressure organizing offshore. The 00Z GFS and especially the 00Z NAM have a well-defined low center by the end of the period offshore northern CA. The NAM is a strong outlier, but the 00Z GEFS mean and 12Z ECENS mean both favor the GFS solution. Confidence becomes quite low by the day 3 time frame with the mass field details offshore the West Coast, but will lean toward the GFS solution given the ensemble means and incorporate these as part of a GFS led blend for the period beyond 36 hours. A non-NAM blend will be preferred for the first 36 hours of the period. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison