Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Tue Dec 25 2018

Valid Dec 25/1200 UTC thru Dec 29/0000 UTC

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

12z Model Evaluation...with Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend (12z ECMWF/ECENS, CMC, GFS)
Confidence: Slightly above average

19z update: The rest of the 12z guidance (CMC, UKMET, ECMWF) shows
very good agreement in the synoptic pattern through 84 hours
across the central US with the mid/upper level wave coming out
into the southern/central Plains and the surface low tracking
through the Upper Midwest. The NAM continues to be an outlier. The
ECMWF/CMC remain the slowest with the system pushing out of the
Upper Midwest on Friday. As a result, the WPC preferred blend will
remain the same (ECMWF/ECENS, CMC, GFS).

---previous discussion---
Through the forecast period, mid/upper level low currently over
southern California will push east into Southwest US and southern
Plains. Surface cyclogenesis developing in the lee of the Rockies
will deepen as it lifts toward the Upper Midwest. Moisture surging
ahead of it and strong upper level divergence will lead to
widespread precipitation, with deformation band snow event across
portions of the central/northern Plains and Upper Midwest. Heavy
rainfall will also be possible on the warm side of this system
across the Southern Plains into the Lower Mississippi River
Valley.

There has been growing consensus in the model guidance, converging
on similar solutions with the synoptic pattern. Aloft, there are
not large differences, though the 00z ECMWF is a bit
stronger/wound up compared to the GFS/UKMET/CMC as it lifts
through the central Plains. At the surface, the NAM is the fastest
solution on Wednesday/Thursday and was not included much in this
cycle. The rest of the guidance shows similar solutions, with the
latest GFS coming more into line. The ECMWF might be a tad too
slow, but overall it has shown the most consistency with this
storm system. Most significant QPF differences deal with how far
north the deformation band precipitation lifts Thursday
afternoon/evening. With the NAM having its surface low on the
northern side of the spread, its deformation band is several
hundred miles north compared to a more southern solution
(GFS/CMC). Here, the ECMWF seems to be a good compromise solution
as well. By the end of the forecast period, the NAM/UKMET and to
some degree the GFS are too fast lifting the low across the Great
Lakes, compared to the ECMWF/CMC which have the low center over
southwest Wisconsin at the same time. As such WPC CONUS blend is a
00z ECMWF/ECENS, 12z GFS, 00z CMC blend.

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

Taylor