Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1230 AM EDT Wed Apr 18 2018 Valid Apr 18/0000 UTC thru Apr 21/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS Evaluation...With Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep occluded cyclone over the Northeast Wed, gradually exiting through the Canadian Maritimes Thu... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Both the 00Z NAM/GFS were close to the consensus with the surface low that tracks from northern ME early Wed to a position over the Maritimes Thu into Thu night. After that time, the 00Z NAM becomes a bit faster with the surface low, as the mid level closed low starts to accelerate. However, the main effects on the US are over by that time, so a general model blend is preferred, with above average confidence. ...Closed mid level low over NE/IA this morning tracking across the OH Valley and Northeast Thu and Thu night... ...Surface low moving across the OH Valley, with secondary development off the New England coast Thu... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The 00Z NAM and 00Z GFS are close to the consensus with the placement of the closed mid level low Wed, which opens into a positively tilted long wave trough over the OH Valley Thu. Short wave energy dropping into the long wave trough from northern Ontario allows the long wave trough to evolve into a broad closed mid level low over northern New England Fri and the Maritimes Sat. The 00Z GFS now has the primary surface low over the OH Valley Wed night/early Thu weakening as a secondary low forms off the NJ coast Thu afternoon, before it tracks over Nova Scotia Fri and the Maritimes Sat. Earlier runs of the GFS kept the primary low moving across southern and central New England. By doing this, the 00Z GFS joins the model consensus (which includes the 18Z GEFS mean and 12Z ECMWF Ensemble mean). This results in a tight clustering of solutions with the track of the surface low moving into Newfoundland (though the 12Z ECMWF is a bit slower than the consensus at this point). Based on the change in the 00Z GFS, a general model blend is preferred. ...Long wave trough crossing the CA coast Wed night, which closes off over the Great Basin/Southwest Thu into Fri... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The 00Z NAM is close to the consensus as the long wave trough crosses the CA coast Wed night, then closes off across the Great Basin and Southwest states Thu into Fri. The 00Z GFS is similar with its evolution, but ends up a bit faster/further north the the closed mid level system over CO/NM/western KS into the northern Panhandle of TX. However, the difference in placement is not enough to take it out of the preferred blend. ...Strong short wave energy approaching the Pacific Northwest/British Columbia late Fri into early Sat... ...Associated surface cold front crossing the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-00Z GFS blend Confidence: Average The 00Z NAM was initially slower than the consensus with strong short wave near 43N 143W early Fri morning, but ends up close to the consensus over western WA/Vancouver Island by 21/12z. The 00Z GFS is initially close to the consensus with the short wave, but moves ahead of the pack in the 12/24 hours before 21/12z. It becomes the fastest piece of guidance by that time (even faster than the 18Z GEFS mean, which is closer to the consensus and 12Z ECMWF Ensemble mean). Because of this, the 00z GFS was not included in the preferred. However, due to the speed of the feature, there is some potential for model differences in subsequent runs, so the confidence is only average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hayes