Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1124 PM EST Sun Dec 16 2018

Valid Dec 17/0000 UTC thru Dec 20/1200 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: General Model Blend through 20/00z
            Non-00z NAM Model Blend across the West after 20/00z
Confidence: Slightly Above Average

Model agreement continues to be above average across the CONUS
through late Wednesday. Across the East, the 00z NAM may be too
far east with the track of the surface low initially (as is the
12z UKMET), but catches up to the remainder of the guidance by
28/12z over the northern tip of Nova Scotia. Otherwise, there is
very good model agreement with the synoptic scale systems through
the period.

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. Toward the end of the period, the mid level ridge
starts breaking down as a positively tilted long wave trough well
west of the US West Coast slowly approaches. The 00z NAM is the
slowest solution across the northern portion of the trough,
closing off a mid level system well west of the model consensus,
which could delay the arrival of precipitation across the Pacific
Northwest toward 20/12z. With this in mind, the 00z NAM was
removed from the preferred model blend across the Pacific
Northwest late on Day 3.

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

Hayes