Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Wed May 16 2018

Valid May 16/1200 UTC thru May 20/0000 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...

12Z Model Evaluation...with Final Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Overall Preference: Blend of 12Z GFS, 12Z ECMWF (greater weight)
Overall Confidence: Slightly Above Average

---18Z UPDATE---
No significant changes with the rest of the 12Z model cycle means
that the preliminary preference remains unchanged. A blend of the
ECMWF and GFS in general is preferred, with slightly greater
weight given to the ECMWF mass fields. For details on
precipitation, refer to the QPF discussion.

---PREVIOUS DISCUSSION---
The pattern over the CONUS will consist of relatively weak flow,
and numerous shortwaves slowly meandering in that weak flow. In
fact, by Friday the 500mb heights may be almost entirely within a
range of about 150 meters, with most of the strong flow contained
well to the north in Canada.

Models tend to all show the large scale features in a similar
fashion, with a broad trough across the West, and another trough
situated across the Southeast. However, they do show differences
with individual waves.

The mass field agreement between the 12Z GFS and 00Z ECMWF is
fairly good across the CONUS, and not too dissimilar from multiple
ensemble means either. The primary concern with the 12Z GFS is the
possibility of some convectively intensified vorticity maxima from
the Front Range into the north-central Plains on Friday into
Saturday. The GFS significantly increases mid-upper level
vorticity in conjunction with developing convection, and
concentrates this into a relatively potent wave aloft. The 12Z NAM
and 00Z UKMET also do this, albeit to a lesser extent. Therefore,
the 00Z ECMWF and 00Z CMC may be preferred to trend away from
those tendencies in that region.

Otherwise, the 00Z UKMET and 00Z CMC seem to show more ridging
over the top of the Ohio Valley upper level low on Friday and
Saturday, which leads to a generally weaker depiction of the low
overall.

Given the generally good agreement between the GFS and ECMWF, the
preference is to lean in the direction of a blend of those two
models, with a slight weight toward the ECMWF.

Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Lamers