Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1124 AM EST Tue Jan 01 2019

Valid Jan 01/1200 UTC thru Jan 05/0000 UTC

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

12Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
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...Overall pattern across the CONUS...
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Preference: Non 12Z NAM blend
Confidence: Average

There are two main systems to track across the CONUS and environs
through 05/00Z: a closed mid level low and its surface system
moving from the Southwest states into the TN Valley, and a short
wave and developing surface low that affects portions of New
England.

The 12Z NAM remains close to the consensus with the closed mid
level low tracking from northern AZ into far West TX by 03/00Z.
After that time, the NAM slows the mid level system down (as it
has done for the past three cycles), bringing it to a position
over far southern IL by 05/00Z. This is north and west of the
consensus at that time. The 12Z GFS remained close to the
consensus throughout the time period with the closed mid level
system, as well as its surface system over central KY.  Based on
this, the 12Z NAM was excluded from the preferred blend due to its
slowing of the mid level low, though it should be noted that the
ECMWF has been slowing with this system over the past three model
cycles as well (which could be signaling a general slowing of the
mid level system as it approaches the TN Valley by 05/00Z).

The 12Z NAM is close to the consensus with the short wave moving
southeast out of Manitoba into the Upper Great Lakes by 03/00Z.
Thereafter, the short wave is a bit slower and deeper than the
consensus (though the 12Z GFS is close to the NAM with the slowing
short wave). However, the 12Z NAM is a bit further west and much
deeper with the surface low that forms in response to the short
wave east of the southern tip of Nova Scotia (especially after
04/00Z). The 12Z GFS position is much closer to the consensus (as
well as the 00z GEFS mean position). Because the 12Z NAM is much
deeper with the surface low, it was not included in the preferred
blend with this system.

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

Hayes