Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
155 AM EST Tue Dec 18 2018

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

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

00Z Model Evaluation...with Final Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: 00z ECMWF/00z GFS/00z NAM Model Blend
Confidence: Average

Model agreement is about 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. However,
the non-NCEP 00z guidance showed more variability with the
placement of the closed mid level low associated with the long
wave trough as it takes on a negative tilt. The 00z CMC position
of the trough was closer to the consensus, but the 00z UKMET
slowed with the short wave energy dropping into the base of the
long wave trough. Due to the inconsistency with the 00z UKMET/00z
CMC solutions, they were removed from the 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 00z CMC remains 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 00z CMC remains slower than the consensus, it was withheld
from the preferred preference. Conversely, the 00z GFS becomes
increasingly fast with the translation of the remaining long wave
energy reaching the Northern Rockies (the 00z 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-00z 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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