Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1217 AM EDT Sat Mar 17 2018 Valid Mar 17/0000 UTC thru Mar 20/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Northwesterly flow regime across New England/SE Canada with numerous shortwave passages along southwest side of persistent upper low center... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GEFS Mean, 00Z EC ENS Mean Confidence: Average Large scale vortex remains locked across the northeastern portion of the North American continent and Labrador Sea for the forecast period leading to persistent northwesterly flow with embedded strong shortwave features with subsequent cold air surges. The lead very deep wave amplifies Sat and while the GFS/ECMWF have been very locked...the remaining guidance strongly agrees in totality for this wave. In its wake...smaller waves break through as a weaker but important center of upper low develops pushing a first wave through ME on Sunday with main core of height falls moving through Monday. Here the GEFS mean/ECENS mean continue to serve the forecast well washing out run to run smaller detail variations for a more consistent forecast...however the 00z NAM joins with the operational 00z GFS and 12z ECMWF to have solid enough agreement to bring out the smaller scale enhancements with some increased confidence. The 12z UKMET/CMC are not terribly off but are a bit more amplified and slow as or too weak and fast respectfully to match the ensemble suite. A 00z NAM/GFS and 12z ECMWF blend will represent the means well enough to have average confidence in it. ...MO Valley cyclone weakening as it Shifts east into the Ohio Valley and Carolinas this weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above Average GOES-16 WV suite and national radar mosaic depict a banded closed low elongating NW to SE through the MO valley starting to shift toward the Ohio Valley currently. The eastern vorticity center and associated surface wave will slide ESE today into the Carolinas with enhanced surface development along the lingering frontal boundary. Guidance has been very good in coming to a common solution though small variations especially in the mid-upper level circulation are noted with a deeper 00z NAM run making it a bit slower but not significantly so. As such a general model blend remains supported at above average confidence. ...Large Closed Low along OR/CA boarder over the Northwest U.S breaking down and interacting with northwest moving shortwave energy from high plains deformation zone...melding into weak broad trof across northern high plains/Dakotas by Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/12z ECMWF blend Confidence: Slightly below average Large scale upper low breaking down across the interior west over the next day or so remains particularly complex especially with respect to interaction with other stream flows/shortwave energy. Currently the closed low is centered along the OR/CA boarder with undercutting s/w and jet energy. This wave will lift northeast and slowly stretch NNE to SSW across W MT into the Great Basin by Sunday interacting with sheared shortwave energy from the NW portion of the wave over the central Plains. A small binary interaction/combination will evolve across the High Plains of MT into S central Canada by Monday where northern stream shortwave weak baggy troughing will also interact leading to a weak broad trof over the Northern Plains by Tues. Here the timing and general evolution is conceptually best represented by the 00z GFS and 12z ECMWF which are center to ensemble suite consensus. The 12z UKMET is much too strong with the internal s/w features during an overall filling setup...that by Tuesday is much too amplified in the final re-development/consolidation phase of the combined streams. The 00z NAM while timed well and generally weaker with the features than expected (from its know bias) shows a broader binary interaction with the shortwave features that looks out of sync with the ensemble suite and expected conceptual evolution. The 12z CMC shows a very strong wave in the northern stream potentially showing some bias as well. All considered a 00z GFS/12z ECMWF blend should suffice though given the complexity of the evolution confidence is slightly below average. ...Strong shortwave across central Canadian Rockies rotates SW through Vancouver Island and off Pacific NW by remaining static while shearing/drawing toward approaching central Pacific closed low Mon/Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Average GOES-W WV depicts a very strong shortwave moving west through the central Canadian Rockies along the northern periphery of the larger scale deep cyclone that dominating the western US at this time. This wave will continue to drift west before sliding offshore Vancouver Island and amplifying slightly by early Sunday. The wave will then stall off the WA coast through Monday when it will start to elongate/shear under the influence of the approaching closed low in the central Pacific. Given the weakness of the mean large scale flow...there is a fair amount of spread in timing/placement of this elongation...with the 12z CMC breaking first from the consensus shifting west under stronger/broader ridging lifting north between 120-130W and becomes a clear outlier. The remaining guidance depicts further slow dissipation of the remaining energy with ever so slight southward migration with time. Given the remaining agreement of the GFS/NAM/UKMET/ECMWF will favor this blend. Confidence is average given the complexity of the evolution given multiple streams/approach of closed low. ...Stronger wave on the western periphery of the closed low moves into California Saturday ejecting into the Plains on Sunday with Lee Cyclogenesis and eventual eastward progress into the Mid-Mississippi Valley on Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 1/4 00z GFS and 3/4 12z ECMWF blend Confidence: Average GOES-W WV depicts a warm zone along 130W along the western periphery of mean large scale western upper low. This wave descends in latitude and rounds the base of the upper low into S CA/AZ by early Sunday. The wave accelerates east and under very favorable upper level diffluent pattern and rapidly deepens/wraps up across NM into the Southern Plains by Monday with strong surface cyclogenesis. Run to run ensemble spaghetti analysis shows a reduction in model cohesion breaking from strong agreement yesterday to differences in latitude of ejection into the Plains as well as strength of influencing upstream shortwave/jet energy. The 12z UKMET evolution is typical of its bias...slightly faster and quite wound up emerging into OK Mon maintaining some strength of the inner core moving through the Ohio valley. The 12z ECMWF remains a southern solution and a bit stronger/wound up than some other guidance but also is the most consistent solution in timing/strength and shape showing some eventual filling/weakening east of the MS River into Tues, which seems conceptually consistent moving into confluent flow. The 12z CMC like the ECMWF is south and wound up but unlike the ECMWF remains compact not resolving the upstream confluence as well (note section one above). As for the newest guidance, the 00z NAM ejects most northerly; and typical of Day 2.5-3 bias is very strong/wound up eventually slowing in timing relative to the ensemble mean and by Tuesday is a clear outlier in mass fields. The GFS which had shifted north over the last few cycles has slipped south relative to the 18z run but remains north of the ECMWF/CMC emerging into the Plains. The wave is also a bit faster typical of its bias...both with respect to the main shortwave through the Ohio Valley to WV by 12z Tues but also with upstream stronger/faster amplification from fast moving wave originating from the Pacific across the lower Red River valley at the same time. All considered the 12z ECMWF is favored heavily but will include a portion of the GFS (3/4 to 1/4 GFS) to account for some variability in the ensemble suite. Confidence is average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina