Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1113 AM EST Sat Jan 12 2019

Valid Jan 12/1200 UTC thru Jan 16/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
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...Eastern US System and lingering trof over TN/OH valleys...
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Preference: Non-NAM blend
Confidence: Average

Guidance has been trending a slower southern stream wave/elongated
vorticity sheet from the MS Valley to the East Coast allowing the
northern stream amplification to pass by a bit more.  Yet, this
slowing allows for slightly increased smaller scale compact
shortwaves in the parent shearing trof.  No more clearer example
of this is the 12z NAM, which like prior runs is trending toward a
stronger, slower development especially with the Coastal wave,
presenting a very impressive deformation zone QPF swath shifting
north into the central Mid-Atlantic.  Yet, the 12z analysis
compared to RAOBs and VWP networks suggest the initial guess was
about 5-10 kts stronger through both the northern branch and
southern branch of the 250mb jet as well as about 5-15 degrees of
increased diffluence supporting a stronger wave even in the first
6-12 hours...well earlier than the typical over amplification NAM
bias seen around Day 2 into 3.  The NAM-Co-Nest seems a more
reasonable representation of the NAM family, especially given the
NAM family becomes more statistically relevant for day 1 even over
more traditional GFS/ECMWF and their ensemble families. 

The aforementioned northward trend continues in the 12z
GFS...shifting the elongated trof axis northward a few miles with
each 00z to 06z run.  It is per bias a bit wetter and a shade
faster than the ECMWF/ECENS mean and other guidance.  These small
adjustments northward overall support a non-NAM blend.  Given the
overall flat flow in importance to smaller scale features that
continue to shift/modulate from run to run.  Confidence is
average.

Overall the lingering trof energy across the OH/TN valleys remains
highly tied to the exiting wave and traditional timing and
amplification biases in the guidance, a general model blend is
likely sufficient through the end of day 3.  The next clipper
system entering the far Northern US Tuesday, shows the ECMWF is a
bit slow relative to the remaining guidance, but not severely so. 
The UKMET and CMC are a bit more amplified as well, but overall
the general model blend seems appropriate at average confidence
here as well.


...Western Cyclones entering California, incl. Southern Stream
affecting S TX...
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Preference: 12z NAM/GFS, 06z GEFS/GFS and 00z UKMET/ECENS blend
Confidence: Average to slightly below average

Models seem to handle the remaining shortwave entering southern CA
at this time even with the binary interaction with western side of
elongated eastern North American across the four-corners
region...so here a general model blend can be used with good
confidence.

The next digging trof along the west coast continues to have
moderate to large model spread.  The 00z CMC is the first to over
amplify and consolidate the upper low even by Monday, well north
and shifting it north of the NW US blocking ridge, a clear
outlier.  While the other guidance all support a strong compact
system, the 00z operational ECMWF finds the situation that feeds
its known negative bias, supporting a very strong/concentric
system. Given the flow regime, a strong anticyclone but also a
fairly sizable weakness into S CA, should support an
elongation/stretching toward the SE into the weakness and
therefore leaves the operational 00z ECMWF very suspect for
inclusion.  The ECENS mean members do not generally support the
operational and hedge closer to 00/06z GFS/GEFS, 00z UKMET and 12z
NAM.  The 12z GFS typical of bias, was a bit faster shifting SE
and broke from its continuity, eventually becoming more concentric
like the ECMWF, but given its placement to the SW of the
anticyclone block...was able to slowly weaken with time.  This
leads to a reduced agreement in the mass spread but at least the
moisture axis/timing of shortwave pulses in the southwesterly flow
still jive to support QPF placement/magnitudes on par with prior
runs and favored NAM/UKMET solutions. 

It is the southwesterly flow that draws out shortwave energy and
mid-level moisture plume axis to support QPF across N Mexico into
S TX by the end of day 3.  These features are misplaced by the
operational EC and CMC given their northern stream issues, but
match with the remaining guidance.  As such, a blend including the
NAM/UKMET and GFS with some ECENS mean to help handle some of the
uncertainty is suggested at average to slightly below average
confidence.

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

Gallina