Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
153 AM EST Sun Dec 23 2018

Valid Dec 23/0000 UTC thru Dec 26/1200 UTC

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

00Z Model Evaluation...with Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend
Confidence: Slightly above average

07z update: There is a bit of increased spread through the Desert
Southwest, now that the ECMWF remains a bit more compact and
slow/digs a bit west of the faster GFS/NAM and 00z CMC.  The
UKMET, like the ECMWF is stronger, but shows less roundness to the
base is is more elongated N-S and narrower, given the still
stronger downstream ridge.  Trends also were a bit faster with the
approaching wave nearing the Pacific NW coast by 12z Wed with all
three non-NCEP deterministic solutions, likely due to sharpening
of the ridging across the Coastal Ranges/Sierras as the wavelength
shortens with the downstream closed off low.  Still believe a
non-NAM blend is prudent and well within the bulk of ensemble
solutions but slightly reduced overall confidence.

---Prior Discussion---
CONUS flow pattern continues to generally flatten over the next
few days with exception of the closed low beginning to weakening
through the Great Lakes into New England through Monday with good
model agreement.   Small, low amplitude waves will cross out of
the Rockies through the late weekend and early holiday week...here
the ECMWF is a bit flatter and faster while the 00z GFS and NAM
trended slower and deeper. The 00z GFS remains better timed
crossing the Central Plains into the Ohio River valley but only
the 00z NAM looks most out of place, ie too strong, too wet to
support.

This weak wave is in advance of the more imposing weather feature
amplifying out of the Pacific into desert Southwest Tuesday into
Wed.  Here timing and differences in latitude seem the largest
spread...especially over the Pacific into Northern/Central CA. 
The CMC/ECMWF are a bit more compact/mature and therefore slow and
north compared to the flatter/faster NAM/GFS, though both trend
toward a common solution, and by Tues/Wed, only small differences
in the strength of the downstream ridge over the Central Plains/MO
River valley seem to pose the largest variation, with the 12z
UKMET and NAM most out of phase with the ensemble suite. 

The upstream ridge amplification supports a slowing progression of
the next broader cyclonic circulation into the southern Gulf of AK
supporting a fairly agreeable eastward progression of the next
frontal passage (just after the forecast period, 26/12z), while
this is good, the 00z NAM has a stronger internal shortwave and
secondary surface trof that does not jive with the fairly solid
consensus... so this appears to be three strikes against the NAM
and with other strong solutions... a non-NAM blend is suggested at
slightly above to above average confidence throughout the CONUS
and upstream Pacific in the short-term forecast.

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

Gallina