Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1149 AM EST Thu Feb 21 2019

Valid Feb 21/1200 UTC thru Feb 25/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
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...Closed low ejecting out of Southwest tracking through
Plains/Midwest/Great Lakes Sat/Sun...
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Preference: Non-CMC blend
    Exclude 12z GFS:  23/00z through 24/00z
    Exclude 00z ECMWF: after 24/06z.
Confidence: Slightly above average

Closed low over the Southwest is fairly well agreed upon in the
large scale, but typical timing biases start to manifest as it
ejects into the Southern Plains by 12z Sat.  The 12z GFS continues
to show this slightly faster ejection shifting a bit faster north
and east into Sunday but as it intersects with deeper
moisture/instability late Sunday, the system receives some
feedback to rapidly occlude, this slows the northeast timing and
increase TROWAL/deformation QPF band to the NW of the surface
cyclone, and leads the GFS to slow compared to prior runs and
start matching in track/placement of the cyclone toward day 3
across the Midwest into the Great Lakes, providing some confidence
in more of its inclusion to the preference.  The 12z NAM has
remained consistent and while slightly slower initially ejecting
compared to the GFS, it also shows upscale enhancement on late
Sunday for a strong/deep occlusion.  The 00z UKMET also falls
generally in line with this solution, but is a bit east and
deeper/fast through the end of Day 3, typical of its negative
known bias;  some lower weighting/incorporation to the mass fields
seems warranted.

The 00z ECMWF, is a slow member of the 00z ECENS solutions
suggesting some concern for falling into its known slow bias;
however, it is fairly well timed with the NAM/UKMET through the
Plains to have some confidence in its inclusion initially.  A
stronger northern stream shortwave/cold plunge out of the Arctic
develops a southern Prairie trof that is clearly out of place in
the ensemble suite, with only the CMC showing some vague
similarity.  As the cyclone occludes and the TROWAL enhances by
Sunday, this trof has negative influences, maintaining the depth
of the cyclone as well as drawing it back toward the West and
delaying its weakening across the Great Lakes into Ontario by 00z
Monday...as such the ECMWF is included in preference but
eventually becomes eliminated/removed by Day 3.    The 00z CMC,
like the ECMWF is slow, but is also the weakest solution with very
limited TROWAL enhancement and little upscale enhancement that is
expected by late Sat into Sunday...as such it is not included in
the preference.

As such the overall preference is a non-CMC blend but initially no
12z GFS (until 24/00z) transitioning to no ECMWF after 24/12z with
inclusion of the 12z GFS.  The overall mass spread is fairly small
to build slightly above average confidence; however, it is the
higher impact, small scale details that show moderate spread in
the Day 2/3 period that lead to the time specific
inclusion/exclusion within this blend.


...Pacific Northwest Trof Sat/Sun...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00z ECMWF/UKMET blend with lower weighting to 12z GFS
Confidence: Average to slightly above average

A trof will be descending along the BC coast on the eastern side
of an amplifying Gulf of AK ridge by the weekend.   The initial
lead wave will bleed through the North US/Southern Canadian
Rockies Sat/Sun and act as a kicker wave to the developing deep
cyclone described above, this appears fairly well handled with
some depth concerns mainly after affecting the Northwest (so
please refer to section above).  As flow under-cuts the developing
ridge, a weak Omega block pattern develops over the Northwest
aiding slowing of the trof dropping south as well as tapping some
subtropical moisture stream for developing an Atmospheric River
toward Day 3.  The 12z NAM and 00z CMC are clearer outliers with
respect to the elongating/amplifying trof, as they both shift the
shortwave energy onshore and press height-falls into the Northern
Rockies.  The 12z GFS has maintained a bit of increased
enhancement to the trof which delays the southward press of the
moisture stream into the Pacific Northwest, but given the overall
setup (upstream ridge) and the uncertainty in the precise latitude
of the under-cutting moisture, it still remains somewhat viable in
the blend at the Day 3 time-period, though the UKMET/ECMWF are a
bit better agreed upon and have some better ensemble support.  At
this point, a 00z ECMWF/UKMET preference with some inclusion of
the 12z GFS is preferred at average to slightly above 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