Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1115 AM EST Sun Dec 09 2018

Valid Dec 09/1200 UTC thru Dec 13/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
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...Southeast storm/Lingering frontal zone...
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Preference: Non-NAM blend
Confidence: Above average

Models have come into solid agreement with the system in the
Southeast, with exception of the 12z NAM which continues to be a
bit stronger and further north with the surface wave, and then
again being a bit too slow with the secondary shortwave dropping
out of the Missouri River Valley currently.  Even with that
stated, the spread between the NAM and the remaining guidance is
fairly good, and provides above average confidence in supporting a
non-NAM blend.

...North, Great Lakes and Northeast Tues/Wed...
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Preference: Non-CMC blend
Confidence: Slightly above average

Along the southwest periphery of the larger scale global trof in
northeast Canada, the ECMWF continues to resolve a very strong
compact vort center with the digging wave across the Great Lakes
Tuesday, though the overall shape of the cold air and mid-level
trof look more on par with the remaining guidance, with exception
of the 00z CMC.  The CMC is uncharacteristically fast with the
wave favoring a stronger core to the global trof and enhanced jet
speeds rounding the base and therefore would remove it from the
preference.   This system is dry, but small details of
timing/amplification affect the upstream ridge (and therefore the
approaching elongated trof out of the Rockies by the end of the
forecast period (Wed)...here is where the stronger internal wave
with the ECMWF may have negative consequences upstream, but
specific for this feature a non-CMC blend is supported at slightly
above average confidence.


...Pair of troughs through the West shifting into the Plains by
late Tues/Wed...
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Preference: 06z GFS-FV3/00z UKMET blend for lead wave(s)
Confidence: Below average

While there continues to be solid agreement with the lead
height-falls/elongated trof entering the Pacific coast Sunday in
Monday, the very strong fairly flat Pacific Jet screams in and
further narrows the width of the trof and stretches it further
south and enters a more progressive pattern overall.  The strength
of the downstream ridge and precise placement of strongest vort
centers and jet speed maxima have lead to tremendous model spread
and run to run variability as the trof cross the Intermountain
West and emerges into the Plains by early Wed, as well as the
strength/timing with the base of the trof in the Southwest.  For a
QPF perspective, the system is quite dry, which should help with
some WPC decision processes, but the mass difference provides very
low confidence in any particular solution.   The good news is the
more meteorological impact wave with prolonged orthogonal moisture
flux into the Pacific Northwest midday Tues into Wed, shows fairly
strong agreement in placement/strength and orientation into E
ID/UT and by 13/00z Thursday, starting the process of developing
the closed low (see WPC PMDEPD for medium-range uncertainty)
through the South.

As the trof crosses the Rockies, certain models such as the 12z
NAM/00z ECMWF strengthen and close off the northern portion of the
elongated trof across the Midwest by Wed.  This was also hinted at
by a significant shift from the 00z run to the 06z run of the
FV3-GFS, but also seems to be the most center solution both here
in the northern stream as well as the base of the trof.  The 12z
GFS and 00z UKMET remain a bit more elongated and given the
shape/orientation of the slowly progressing downstream ridge in
the Great Lakes, seems a bit more reasonable.   However, the 12z
GFS and 12z NAM remain very fast with the base of the trof, which
given the weaker flow further south (relative to the orientation
of the jet) seems much less plausible.  The 00z ECMWF continues to
be very amplified/strong and therefore slow with this portion of
the wave in northern Mexico.  Here, the medium speed, UKMET and
FV3-GFS seem much more sensible in placement/timing and fit more
into the middle of the ensemble spaghetti analysis.  As such, will
favor the FV3-GFS and UKMET as preference for this lead wave(s),
though more inclusion of the NAM/ECMWF is likely helpful with the
Atmospheric River and translation of the second wave into the
Intermountain West late Wed.

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

Gallina