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

Valid Dec 17/1200 UTC thru Dec 21/0000 UTC

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

12Z Model Evaluation...with Final Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: General Model Blend through 20/00z
            12Z ECMWF/GFS 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 12Z NAM and UKMET
appear to be staying on the eastern side of the guidance with
respect to the track of the surface low initially,  but catches up
to the remainder of the guidance by 18/12z over the northern tip
of Nova Scotia. Otherwise, there remained 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 NAM was 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. The 17/12Z ECWMF and GFS have shown much
better continuity with this system, and are in good agreement with
their respective ensemble means. Based on this, the preference for
the West has been changed to a blend of these two solutions.

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

Bann