Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
133 PM EST Fri Dec 07 2018

Valid Dec 07/1200 UTC thru Dec 11/0000 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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...Pattern across South Central US/Southeast...
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Preference: 12z ECMWF/UKMET/GEFS and 00z ECMWF blend
Confidence: Slightly above average

19z update: As alluded to with initial preference, the trend
toward faster solution with the lead wave was noted with the 12z
ECMWF/UKMET and CMC as well.  All three continue to lag the GFS
and NAM but much less so now, and line up well with 12z GEFS mean.
 The 12z CMC shifted a bit less showing slowest crossing of the
south into the southeast and is a tad further north than the
guidance/closer to the coast...not a tremendous difference but
will reduce the overall weighting in the blend.  With the upstream
wave, crossing the Plains into the lower MS River valley, the
ECMWF/UKMET are faster and adjusted favorably to match the GFS/NAM
to have good agreement with the lingering coastal front and
secondary surface cyclogenesis; in fact the UKMET even pressed the
height-falls and surface wave a bit east of the other guidance,
but enough to support a general model blend for that wave... but
still a 12z ECMWF/UKMET and 12z GEFS (00z ECENS mean) blend for
the lead wave.  Confidence is slightly above average given the
late guidance shifts.

---Prior Discussion---
The 12z GFS while slightly slower than the 06z GFS continues to be
the flattest solution and along with the 12z NAM in the fastest
solution camp too.  While there has been a slow run to run trend
in a slightly faster/flatter solution, the GFS does not appear to
be as viable within the solution.  As alluded to, the 12z NAM is
fast, but with a stronger cold surge relative to the other
guidance, it deepens the cyclone off the southeast coast a bit too
much lending toward the northernmost solution, which is not
favored at this time.  Both the NAM/GFS are on the outer fringe of
the GEFS member solutions and while the GEFS is equally flat
compared to the bulk of ECENS members...there is better overall
timing and could be incorporated with the blend to compensate the
loss of other NCEP solutions.   The 00z ECMWF, CMC and UKMET are
much slower than the GFS/NAM and appear to be behind the
aforementioned guidance trend in the run to run ensemble suite, as
such will incorporate the GEFS/ECENS mean in the preferred blend. 

As a secondary compact shortwave descends out of the northern
stream late in the weekend, the timing difference between the
GFS/NAM and the ECMWF/CMC/UKMET remains apparent, yet the
lingering front and slight differences in depth, allow for a
tightening of the GFS/NAM to the ECMWF/UKMET/CMC toward the end of
the forecast period into the Southeast.  As such, confidence
slightly increases in the preferred blend (still with GFS/NAM
removed) through Monday to 00z Tuesday.


...Pacific Northwest...
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Preference: non-NAM
Confidence: Average

19z update: There is greater overall agreement, in timing with the
non-NCEP guidance and while there is greater positive tilt with
the base of the trof off Central CA by 00z Tuesday than the GFS.
The variance in run to run adjustments with so many internal waves
would suggest inclusion of these members with the 12z
GFS...leading to a non-NAM preference.  Confidence remains low,
but slightly increased to average to account for these run to run
variance.

---Prior Discussion---
A global vortex sets up across Alaska with slow eastward
progression and therefore parades shortwaves along the southeast
periphery with an elongated trof clipping the Northwest as it
shears out Saturday with good model agreement.  However, the next
broader trof with numerous embedded shortwaves/jet streaks
approaches Sunday into Monday with models all over the place with
each feature in depth/placement and even orientation, leading to
moderate to high spread with internal features that lead to
moderate spread in eastward timing of the larger scale wave,
frontal zone and even the breadth of the longer-wave pattern as
well.   The 12z NAM advances fastest and slowly expands in width
as it makes landfall into the Northwest by late Sunday/early
Monday. Next the UKMET/GFS and CMC shift ashore, lagged with
ECMWF.  The UKMET is less sharp and does not extend as far south
as the sharper ECMWF/GFS.  Ensemble suite would favor the latter,
and so a GFS/ECMWF blend is preferred for this system, with some
CMC inclusion at lower weighting.  Confidence is slightly below to
average.


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

Gallina