Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
143 PM EST Sat Dec 22 2018

Valid Dec 22/1200 UTC thru Dec 26/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS, 00Z ECMWF, 12Z CMC, 12Z UKMET
Confidence: Slightly above average

---19Z UPDATE---
The 12Z ECMWF trended slower with its trough moving into the West
Coast on Tuesday, and is now slower than the model consensus and
the 00Z ECMWF Ensemble mean. Therefore, the final preference will
be weighted toward the previous (00Z) run of the ECMWF. The 12Z
UKMET also moved into better alignment with the preliminary
preference, so that is now included.

---PREVIOUS DISCUSSION---
Over the next several days, the GFS, ECMWF and CMC models provide
a good consensus forecast that is closely matched by ensemble
means, and the preference is for a blend of these models. The 12Z
NAM and 00Z UKMET, by contrast, show some important differences
that lead to notable forecast differences. These variations
emanate from fast-moving waves in relatively zonal flow over the
next couple days.

For example, the NAM shows a stronger and more amplified wave
pushing through the Rockies on Sunday Night, and emerging into the
Plains on Monday. This eventually leads to a well-developed
surface low in the Missouri Valley, while most other models just
show an inverted trough, and it also leads to a very different
QPF. Looking upstream from there, the NAM is also far less
amplified than the other models with the subsequent shortwave
ridge, and the next trough moving into the West Coast. These
differences do not have significant ensemble support.

The 00Z UKMET maintains flatter flow over the Plains and Midwest
on Sunday and Monday, with a considerably more amplified shortwave
pushing into the Northwest on Sunday Night, and a more amplified
and sharper primary trough pushing into the West on Tuesday. The
trough axis on the UKMET is out of phase with model consensus
(even with the NAM) and thus it also does not have significant
ensemble support.


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

Lamers