Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1217 PM EDT Mon Apr 30 2018 Valid Apr 30/1200 UTC thru May 04/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low moving through the Northeast today with associated surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Stacked closed low over the Northeast will continue to fill the occlusion at the surface and slip northeast into Southeast Canada over the next day or so with very little model difference. Enough to support a general model blend at above average confidence. ...Upper level trough over the Intermountain West today, breaking down as it lifts into confluent flow over Northern Plains through late Tues into Southern Canada Wed ...Elongated surface trof with multiple surface centers over northern Plains to NE CO, with main low lifting toward Lake Superior by late Tues. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average GOES-16 WV suite depict the center of a shortwave currently over northern Utah supporting a broad surface trof from northeast CO to the Northern Red River valley with a few closed centers dotted along it. The wave will lift out into the northern High Plains by tomorrow with southwesterly flow height falls helping to shed a stronger surface tonight across the Dakotas to MN/Lake Superior by later tomorrow. However, the wave will rapidly weaken shifting into confluent flow in the Northern Plains as well as larger scale ridging building in the area due to upstream digging trof. This flow will further elongate the surface trof as the wave moves toward E Ontario/Quebec...stringing from Quebec across the northern Great Lakes back toward KS. There is a bit of model spread as each deterministic model favors its localized/convectively induced/maintained surface inflection...but overall the orientation/depth and translation of the mass fields look comparatively agreeable to support a general model blend for this feature and the lingering frontal zone before the next wave emerges (see section below). Additionally, please note, that though the mass fields support a general model blend please refer to QPFPFD and WPC graphical QPF products for additional as convective details may not be best reflected in this blend. Confidence in the mass field blend is increasing as the ensemble suite tightens toward a common agreement matching the preferred blend. ...Shortwave digging southward off of the West Coast today, reinforcing/re-centering western trof with a closed low aloft by Wednesday morning in the Lower Colorado River Valley. Eventually lifts to Colorado Thursday with deepening surface low in Central High Plains and lingering mid-level troughing across AZ/NM/NW Mexico... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly above average 00z Ensemble spaghetti analysis and cycle to cycle trends suggest a common solution is beginning to manifest as the shortwave off the West Coast currently refocuses the larger scale trof eventually leading to a cut-off low across the Lower Colorado River Valley by Wed. The only significant detail difference noted through the early forecast period is the 00z CMC showing a stronger and slightly slower secondary shortwave dropping south out of the northern stream late Tuesday into Wed just inside of the Cascades, this small detail keeps the CMC a tad slower through the remainder of the forecast period. There is a bit of splitting of deterministic models as the main core of the upper low lifts northeast into the CO Rockies and into the central Plains by Thursday. Uncharacteristically, it is the 00z ECMWF trending faster than the 12z GFS/NAM and lesser so the 00z UKMET. This leads to a slight southward track of the developing surface wave/frontal zone by 00z Fri, but only slightly slow. This may be in response to slightly faster/stronger and south focus of jet crossing the Northern Canadian Rockies. The increased depth/phasing supports a faster/stronger ejection than the GFS/NAM/GEFS that all suggest a weaker/slower northern stream wave. The 00z UKMET splits the difference in the northern stream, so it splits the difference south too. The CMC is the only stronger faster northern stream, but with the delay in the southern stream initially, leads to a different evolution across the Plains than the ECMWF. The 00z ECMWF compared to the prior 12z run, would suggest the northern stream is perhaps a bit too aggressive to favor it more that the other tight clustering but also not remove it from preference at this time. As such, preference is a non-CMC blend and given the increased ensemble clustering, confidence is increased to slightly above average. Like stated above, the mass spread is reduced, though convective influences/timing with instability/moisture/convergence may support a different blend in QPF...please refer to QPFPFD for additional details. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina