Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Thu Dec 13 2018

Valid Dec 13/1200 UTC thru Dec 17/0000 UTC

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

12Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Overall Pattern Across the CONUS...

Preference: Blend of the 12Z GFS, 12Z UKMET and 12Z ECMWF
Confidence: Average

...19Z update...

The 12Z ECMWF/CMC continued a northward/quicker trend with the
closed low moving toward the eastern U.S. through Sunday but
continue to lag the farther north/quicker 12Z NAM/GFS/UKMET with
the 12Z UKMET fairly consistent with its previous run. Thinking
remains the same with the preference to be between the two camps
but closer to the GFS/UKMET timing.

The 12Z ECMWF also trended away from its slower upper
trough/surface low progression across the Pacific Northwest on
Friday. Regarding the West Coast for Sunday, only minor timing
adjustments were noted in the 12Z ECMWF/UKMET/CMC relative to
their previous cycles.

The only changes for the final preference from the previous one
were to replace 00Z with 12Z.

...previous discussion follows...

The upper pattern over the lower 48 stays rather amplified through
the weekend with three major systems to note. The first system is
a large closed low in the southern Plains that moves east and
eventually lifts north across the northern Mid-Atlantic region on
Sunday. Moderate model differences remain with this system with
the deterministic/ensemble ECMWF guidance slower/farther south
compared to the northern GFS/GEFS members. Most trends have been
toward the north and the more northern UKMET has had good
consistency with recent previous cycles. The 00Z CMC is a bit
farther south like the ECMWF, but overall, there are more signs
pointing toward the quicker/northern track than the
slower/southern track. This equates to a 12Z GFS, 00Z UKMET and
00Z ECMWF blend to give some support for the strong ECMWF
clustering, with the thinking that verification will be somewhere
in the middle of the two camps but more weighted toward the
northern/faster camp (GFS/UKMET).

This preference also holds true with the other two systems. A
potent upper trough will reach the Pacific Northwest on Friday
along with a cold front. The 00Z ECMWF a bit of an outlier with a
slower progression and stronger surface low hanging on longer than
the remaining guidance offshore during the day on Friday while the
remaining models are in good agreement. A second system will reach
the West Coast on Sunday and given the amplified nature of the
upper pattern, a slower/deeper mid-level trough axis is preferred
and supported in the latest ensemble guidance off of the West
Coast 00Z/17. This idea and the earlier trough on Friday are best
characterized by a blend of the 12Z GFS, 00Z UKMET and 00Z
ECMWF...with the least weight on the ECMWF.

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

Otto