Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
145 PM 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 Model Evaluation...with Final Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Overall pattern across the CONUS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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. The 12Z
ECMWF has sped up its timing of the closed mid level system as it
crosses into the Lower MS Valley, bringing it into better
agreement with the 12Z GFS/UKMET/CMC.  Outside of the slower 12Z
NAM, there was good clustering concerning the surface low over
TN/KY, though the 12Z ECMWF continues to be the deepest with the
low.  Based on this, the 12Z NAM was excluded from the preferred
blend due to its slowing of the mid level low.

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 non-NCEP guidance overall is faster/flatter with the
short wave, resulting in the developing surface low exiting east
of Nova Scotia more quickly. The 12Z GFS position is closer to the
consensus with the surface low (as well as the 12Z 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