Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1131 PM EST Mon Dec 17 2018

Valid Dec 18/0000 UTC thru Dec 21/1200 UTC

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

00Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
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...Overall Pattern Across the CONUS...
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Preference: Non-12z CMC Model Blend
Confidence: Slightly Above Average

Model agreement remain slightly above average across the CONUS
through late Thursday. Across the East, there is very good
agreement as a northern stream short wave from the Pacific
Northwest/Northern Rockies drops into a developing long wave
trough over the Plains by 20/12z. Both the 00z GFS/NAM are close
to the consensus forming a closed mid level system over GA by the
end of Day 3. The 12z CMC is close to the consensus with the
trough through 20/12z, but then appears to be too fast with the
evolution of the trough and associated closed mid level system by
21/12z. For this reason, the 12z CMC was not included in the
preferred model preference across the East.

Out West, the flow becomes less amplified as the fast mid level
flow is shunted back across the Pacific Northwest into British
Columbia by a strengthening mid level ridge west of Baja
California, and there is fairly good model agreement with the
evolution of the ridging. After 20/12z, as the mid level ridge
starts breaking down ahead of a positively tilted long wave trough
slowly approaching the West Coast, the 12z CMC becomes much slower
with the long wave trough, as additional short wave energy drops
into its base. The long wave trough weakens as it crosses the
Pacific Northwest and eventually the Northern Rockies by 21/12z.

Because 12z CMC remains slower than the consensus, it was withheld
from the preferred preference. Conversely, the 12z GFS becomes
increasingly fast with the translation of the remaining long wave
energy reaching the Northern Rockies (the 18z GEFS mean was faster
as well). While there are some timing issues with the mid level
energy reaching the Northern Rockies by 21/12z, a non-12z CMC
blend is preferred across the West.

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

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