Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1257 AM EDT Thu May 17 2018 Valid May 17/0000 UTC thru May 20/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model evaluation...with latest preferences and confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper troughing/cold front crossing the Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models bring a relatively broad northern stream trough across the Northeast on Thursday which will allow a cold front to drop in across the region as a wave of low pressure also crosses southern Quebec. The guidance is in very good agreement overall with the evolution of this, and a general model blend will be preferred. ...Broad upper trough over the Southeast third of the CONUS... ...Upstream MCVs digging into the Southeast U.S. trough... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models take multiple vorts/MCVs over the lower MO and lower MS Valley region and dig them toward the base of the main upper trough over the Southeast third of the nation over the next couple of days. By this weekend, the upper trough will begin to weaken though and lift north through the OH Valley in response to approaching upstream shortwave energy. On the larger scale, the models are in good agreement on the mass field evolution. Will prefer a general model blend. ...Large upper trough/closed lows over the West... ...Ejecting east across the central Plains this weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z UKMET and 12Z ECMWF Confidence: Below average The models take the broad upper trough and embedded closed lows over the West and eject this energy toward the central Plains by this weekend where it should begin to weaken and gain a bit more progression toward the Midwest by the end of the period. By late Friday, the 00Z NAM begins to edge stronger with its height fall evolution and has a stronger upper trough that crosses the central Rockies. It also has a stronger closed low and associated deformation zone out across the High Plains on Saturday compared to the global models. Of the global models, the 00Z GFS in time becomes slower than the non-NCEP guidance. The GFS ultimately by this weekend ends up being the slowest with its surface low evolution. The GFS is also exhibiting some evidence of convective feedback. The 12Z UKMET and 12Z ECMWF are both reasonably well clustered on a relatively more progressive system that is arriving toward the Midwest by Sunday. The ensemble means are split as well, as the 18Z GEFS mean is slow like the operational GFS and the 12Z ECENS mean faster like the UKMET/ECMWF camp. The 12Z CMC is faster than the GFS, but is slower than the UKMET/ECMWF solutions. Will lean right now toward the majority camp which is more progressive and thus a blend of the UKMET and ECMWF. ...Upper trough crossing the upper Midwest this weekend... ...Cold front/surface wave crossing the region... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average The guidance agrees in bring another large scale northern stream trough down across southern Canada by late Friday which will clip the upper Midwest and upper Great Lakes region on Saturday. This will bring a cold front down across the region as a wave of low pressure also develops over the Dakotas and crosses northern MN by early Saturday. The 00Z NAM is a little slower than the global models with the progression of the upper trough. Will prefer a non-NAM blend at this point given the reasonably good global model clustering. ...Upper trough digging into the West on Sunday... ...Energy lifting north out of the northern Rockies... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z GFS, 12Z CMC and 12Z ECMWF Confidence: Slightly above average A new upper trough is expected to dig into toward the West by Sunday which will help to eject an upper trough lingering over the northern Rockies up to the north and into southwest Canada. The 00Z NAM is the strongest with the energy over the northern Rockies that lifts off to the north. The 12Z UKMET though is the strongest with the upstream troughing crossing the West Coast. Generally the better model/ensemble clustering favors the 00Z GFS, 12Z CMC and 12Z ECMWF and so a blend of these solutions will be preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison