Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1255 PM EDT Tue Mar 20 2018 Valid Mar 20/1200 UTC thru Mar 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Lower Ohio Valley strong shortwave and associated surface wave in the TN valley transferring to a coastal low near VA by late Tuesday... reinforced by rapidly amplifying upstream shortwave evolving into deep eastern US trof...producing a secondary sfc low in the Carolinas late Tues becoming a Nor'easter paralleling the coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Average Compared to its 00Z run, the NAM is faster with the initial surface wave moving off of the Mid-Atlantic coast on Tue. Then with respect to the second low developing off of the Mid-Atlantic coast late Tue into Wed, the NAM becomes faster than its previous run as the upper low moves off the Mid-Atlantic coast and begins to track northeast Wed night. This brings the surface low more quickly into the Canadian Maritimes by Thu evening. The GFS has remained fairly consistent with its 00Z run until Thu morning when it becomes a little more amplified with the upper low and in turn has shifted its surface low a little further west as it approaches Nova Scotia on Thu. Even with the shift in the GFS, the NAM is a little more amplified than the model consensus with the 700-500mb low as it tracks northeast from off of the northern Mid-Atlantic coast to east of New England on Thu, and in-turn tucks the surface low closer to the New England coast than the remainder of the deterministic guidance. ...Deep trough/closed low offshore the Northwest by Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly below average Overall, the NAM has remained fairly consistent with its 00Z run with this system. Compared its overnight run, the GFS digs a more amplified shortwave south of the low toward the Pacific Northwest Thu evening. This not only results in lower surface pressures along the associated frontal band as it moves across the northwestern U.S., but also appears to influence the timing and amplitude of the shortwave described below. At this point, the GFS is on its own. While awaiting the remainder of the 12Z deterministic guidance and the GEFS Mean, recommend leaning away from this solution toward one more similar to the NAM and 00Z ECMWF for now. ...Closed Pacific low near 34N/140W drifting east breaking down under influence of digging/northern stream closed low providing deep layer southwest flow/atmospheric river into Southern CA with associated surface low and frontal zone.... ...Remnant shortwave trough moving into the central U.S. on Fri... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly below average Overall, the NAM has remained fairly consistent with its 00Z run with this system. The GFS however has made a notable shift toward a more progressive solution. As the remnant shortwave associated with the low approaches California, it appears to come under greater influence of the more amplified shortwave digging south of the previously described upper low to the north -- ushering a less-amplified shortwave more quickly to the east into California on Thu. The GFS remains more progressive through the remainder of the period, in-turn developing a low more quickly in the lee of the central Rockies and then tracking it east into central Kansas by late Fri. Given the concern that the GFS may be too amplified with previously described shortwave to the north, recommend leaning away from the GFS here as well for now. As with the system to the north, will reassess as newer guidance continues to arrive. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Pereira