Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
155 AM EST Mon Dec 31 2018

Valid Dec 31/0000 UTC thru Jan 03/1200 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: 00Z NAM/GFS/ECMWF blend
Confidence: Average

...07Z update...

Across the South/Southwest, the 00Z UKMET/CMC converged closer to
the middle but still are a bit slow/fast respectively compared to
the blended ensemble mean position and the agreeable 00Z
NAM/GFS/ECMWF. The other significant note is with a shortwave
amplifying toward Manitoba/Ontario Wednesday morning, where better
agreement is in place with the 00Z deterministic model suite for a
deeper trough axis with a general model blend preferred as better
agreement is now in place despite some modest ensemble amplitude
differences. Across the West, offshore, a 00Z NAM/GFS/ECMWF
continues to fit closest to the ensemble means with the 00Z
UKMET/CMC displaced with the location of the upper trough.
Confidence across the Pacific Northwest is lowest relative to the
rest of the nation.

...previous discussion follows...

The mid/upper flow pattern across the nation starts off with
ridging off of the Florida coast and western U.S. with mean
troughing in the west-central U.S. Over the next 3 days, a slow
shifting toward the east of features is expected with a slow
moving closed low nearing the Texas panhandle while a number of
shortwaves track through the northern stream.

With the southern stream closed low slowly moving east toward the
Texas Panhandle, the trend in the ensembles from Wednesday into
Thursday has been slower, with an average GEFS/ECMWF ensemble mean
position just ahead of the agreeable 00Z NAM, 00Z GFS and 12Z
ECMWF. While there is certainly room for the 00Z NAM, 00Z GFS and
12Z ECMWF preference to be a little too slow, the 12Z UKMET is
significantly slower and the 12Z CMC is significantly faster
(edges of the ensemble spread).

For the northern stream systems, a blend of the 00Z NAM, 00Z GFS
and 12Z ECMWF is nearest to the ensemble mean position/timing with
the shortwaves tracking near the U.S./Canadian border and offshore
of the Pacific Northwest. Despite some minor differences, this
3-way blend is closer than including the 12Z UKMET or 12Z CMC.
There are larger differences offshore of the Pacific Northwest by
12Z/03 with a cyclone several hundred miles west of
Washington/Oregon but at least through the end of the short range
(12Z/03) a NAM/GFS/ECMWF blend is best for Washington and Oregon..

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

Otto