Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1243 PM EDT Sun Mar 11 2018 Valid Mar 11/1200 UTC thru Mar 15/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including model preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwave energy crossing the MS/TN Valley through Sunday with surface low tracking across the Gulf Coast States today with Strong surface redevelopment offshore the East Coast Mon heading toward SW Nova Scotia by Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/NAM throughout with 00z CMC from 0-36hrs (00z Tuesday) 00z ECMWF from 60-84hrs (00z Wed to 00z Thurs) Confidence: Slightly below average As expected models are trending toward a common solution in the larger scale but there remains substantial differences particularly in the smaller scale especially in the spacing between the digging northern stream and the undercutting short-wave pivoting through the Carolinas angling toward offshore New England. The largest difference at this time remains in the 00z ECMWF with this phasing/interaction across the Mid-Atlantic early Tues; where the northern stream remains slow (typical of the EC) reducing the interaction/spacing of the waves and therefore slowing the rapid-developing phase of the lower level cyclone along the eastern seaboard (and leading it to be east initially). This slowness has been a negative bias at this time step for the last year or so...eventually trending toward the coast. The 00z ECENS mean helps to depict this being a bit stronger and to the west. After 36hrs and the arrival of the northern stream...the rapid development shifts the track west and more in line with the better consolidating surface cluster (including ensemble solutions) making it a bit more favorable to include in the preference. The 12Z NAM is much more aggressive (typical of its bias) showing tighter phasing/interaction through the Mid-Atlantic toward New England...there may be too much latent heat release with this given QPF magnitudes manifesting a bit higher. The 12Z GFS like the NAM trended a bit faster as well driving the inner core of the northern stream/pivot point a bit further east toward Long Island by 00z Wed...sharpening the FGEN/deformation zone near/along southeast New England eventually slowing the forward progress of the low as well into ME/SE Canada compared to prior GFS runs. The 00z UKMET while better timed to the GFS/NAM with the southern stream wave...shows some initial interaction but is slower lifting north which leads to a separation of the northern stream and southern stream waves after Tues south and east of the main clustering. The 00z CMC shows very strong agreement with the timing of both streams initially through the southeast and off the Carolina coast...but stronger phasing allows for the known negative bias of cyclogenesis to manifest by midday Tues leading to a much slower surface low especially now that the 12z NAM/GFS show a bit faster shift with the 12z runs. All considered a 12z GFS/NAM and 00z CMC is good through 36hrs (00z Tues) to a GFS/NAM solution Tuesday and eventually a 12z GFS/NAM and 00z ECMWF blend after 00z Wed (60hrs). Confidence remains slightly below average given large scale agreement but continued run to run variations with large impact differences in the smaller scale. ...Deep closed low dropping down over the Great Lakes region arriving over the Northeast through Tuesday/Wednesday...supporting weaker surface low along Carolina coast Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM and 00z ECMWF/UKMET blend Confidence: Average The leading height falls from expanding upper low through SE Ontario into the Great Lakes Tuesday are better associated with the Coastal low section above (please see preferences above); the upstream shortwave lobe will round out the upper low over the central southern Great Lakes by 18z Tuesday...where there is already some modest timing/strength differences. This was particularly the case with the 12z GFS as upstream shortwave ridging was a bit slow to sever connection with the Arctic Jet streak...allowing increased shortwave energy in inject into the southern stream. This delays the southward timing of the wave and now sticks out within the ensemble suite a bit and is in line with known negative GFS fast bias. The 00z CMC is also a bit weaker with the wave allowing for it to kick around the base of the upper low through the Mid-Atlantic/Carolinas too fast. As such a 12z NAM and 00z ECMWF/UKMET blend should suffice at average confidence. ...Shortwaves crossing the Intermountain West through mean large scale ridge into the High Plains by Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average GOES-WV mosaic depict a shortwave over ID and central AZ both moving through a larger scale mean ridge that is generally parked on the eastern side of MT to NM. This will delay the northern wave a bit as it tries to over-top the ridge late Monday into Tues. while the southern wave squirts through the ridge later today. Both features quickly shear into the NW flow across the central plains. Guidance is strongly agreeable in the timing/placement given its placement in larger scale ridging...and as such will support a general model blend at above average confidence. ...Large closed low off of the West Coast Monday night with surface cyclone strengthening off of the Pacific Northwest coast Monday night. Internal main wave rotates through closed low toward CA/OR coast with surface reflection Wed into Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: NON-CMC blend Confidence: Below average A large deep layer vortex exists in the eastern Pacific with an internal shortwave moving from the central Pacific east toward the West Coast by Tues. This wave and surface reflection will reach 130W before shifting due north under influence of the main inner core of the upper low itself. There is modest spread in this timing with the operational 00z ECMWF a bit faster than the 00z UKMET and 12z GFS and 12z NAM... the leading solutions are flatter overall vs the GFS/NAM being more wound up. At this point it is very difficult to favor one over the other given each ensemble suite supports their operational guidance. As the shortwave lifts north only the CMC breaks from the other guidance, as it merges the inner core and this wave well north leading to a very strong manifestation and becomes a clear outlier. As such a non-CMC blend is favored but shifting weighting toward ensemble means (GEFS/ECENS) after 60hrs. Confidence remains below average given the large spread internally. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina