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FXUS10 KWNH 190631
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
231 AM EDT Thu Apr 19 2018

Valid Apr 19/0000 UTC thru Apr 22/1200 UTC

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

12Z Model Evaluation...With Final Preferences and Confidence

...Closed mid level low over southern Great Lakes tracking across
the Ohio Valley and Northeast Thu and Thu night with surface low
in central Ohio Valley, with secondary development off the New
England coast Thu...
Preference: General model blend
Confidence: Above average

GOES-16 WV loop shows center of upper low nearing the southern
portion of Lake Michigan at this time with the northern stream
shortwave amplifying and starting to enter northern L.P. of
Michigan.  Guidance appears to have a much better agreement on the
timing/spacing as the two waves interact and meld into a broader
trof across the northern Mid-Atlantic into New England Thursday.
The surface low has nearly fully filled across the central Ohio
River Valley/Appalachians currently before it starts the transfer
to a developing coastal low in the next 6-12hrs.  Modest deepening
and track look fairly tight in the deterministic guidance as well
as ensemble suite to suggest a general model blend will suffice as
WPC preference.  Confidence is also above average in this blended

07z update: Slight shift of the ECMWF faster leads to very tight
clustering to further provide confidence in a general model blend.

...Long wave trough crossing the California coast, which closes
off over the Great Basin and Southwest Thu into Fri...shifting
slowly east reaching Red River Valley Sun...
Preference: 00z GFS/ECMWF/UKMET blend
Confidence: Average

GOES-WV suite shows a large scale trof centered over SW Oregon
with a tight smaller scale shortwave just southwest of San
Francisco Bay likely to reach Central to S CA early this morning.
The combination of features further sink southwest into the Lower
Colorado River Valley by late Thurs/early Fri with fairly tight
model agreement.  The ECMWF/CMC and their ensemble suites lag the
GFS/NAM and GEFS ever so slightly through midday Sat; where it
starts to increase slightly.  The downstream ridge/surface high
are quite strong to support the slower solutions initially.
However, as the closed low exits and induces deep convection along
return Gulf moisture/instability into Sunday...should allow for
increased forward propagation under influence of the latent heat
release in the convective complex(es).  A surface inverted trof
will translate across TX into the Lower MS River Valley with the
00z GFS and NAM both closing a surface low in LA by 12z Sunday.
Here the 00z ECMWF`s inverted trof is much slower even within its
ensemble suite.  The 00z NAM also shifts the low further south and
east likely in response to the convection but also in response to
the stronger high to the northeast; this does not seem implausible
though it also feeds into the known Day3 bias of being a bit too
strong (which it is in comparison to the remaining guidance).
The 00z GFS slowed a bit and looks similar to the 00z NAM though a
bit weaker overall and closer in strength to the more middle
ground CMC and UKMET.     As such a general model blend blend
weighted to the UKMET/CMC/ECMWF is preferred early (in the
Southwest through 21/12z Sat) but shifts to a Non-ECMWF solution
by 22/12z.   Confidence is average through increasing given
reduction in ensemble spread and deterministic guidance better
representing the middle of said spread.

07z update: The 00z ECMWF showed marked increase in speed
especially on day 3 where it is a bit deeper too matching the
slowed 00z GFS...providing a bit stronger confidence, though the
00z UKMET remained a bit faster and generally weaker it still gels
well within the ensemble suite.  The 00z CMC slowed relative to
its prior run increasing spread from the CMCE mean making it stand
out a bit too much.  As such a 00z GFS/ECMWF blend with some small
percentage of UKMET seems a good mix of continuity, new data and
representation of the ensemble suite as a whole.  Confidence
remains average.

...Strong shortwave approaching the Pacific Northwest and British
Columbia late Fri into early Sat before shifting north of the US
border Sat into Sun...
...Associated surface cold front crossing the Pacific Northwest...
Preference: General Model Blend
Confidence: Slightly above average

A potent shortwave trof quickly approaches the Pacific Northwest
coast by Sat with an attendant cold front.  Last 4 cycles of
ensemble spaghetti analysis depicts the 12z UKMET a bit offset
along with the 12z CMC (faster/slower) but the UKMET has
significantly come back toward the growing consensus.  The CMC was
a bit slow and perhaps a bit south with the base of the trof,
showing more severing with the top of the larger scale trof.  As
the wave swings a bit more negatively through the US/Canadian
Rockies along the border...the CMC`s base finally breaks from the
slower northern portion of the trof leading to a dual center of
the mid-level wave on Sunday.  Both the differences do not seem to
have a great affect on CONUS weather but Canadian interests may
support something closer to the 00z GFS/NAM and 12z ECWMF;
otherwise WPC will support a general model blend at slightly above
average confidence.

07z update: While the 00z ECMWF remained well timed with the
GFS/NAM...there was less strength and so the ECMWF begins to roll
up into an occluded cyclone a bit quicker and further north.  Its
placement in the cluster still remains tight enough to continue a
general model blend...especially with very little variation in the

...Weak subtle shortwave with slight amplification as it reaches
Pacific Northwest Coast by early Sun...
Preference: Non-UKMET blend
Confidence: Average

In the wake of the prior potent shortwave a fairly flat zonal
pattern remains upstream...but with the downstream amplification
as well as rapidly northeast moving sharp wave pumping downstream
ridging around 130W on late Sat into early Sun.  A weak shortwave
amplifies at the Oregon coast.   Is effects are fairly limited by
the end of the Day 3 forecast period.   The 12z UKMET is
uncharacteristically slow; about 5 degrees too slow.  The 00z GFS,
typical of its bias it a bit fast and therefore most sharp by 12z
Sunday but at least is timed well with the 00z NAM and 12z
ECMWF/CMC as well as the ensemble suite means.  While the GFS will
be weighted slightly less to match more of the will
remain in WPC preference: Non-UKMET.  Given the
amplification/evolution appears more influenced external to this
wave; confidence is only average and has the potential to vary as
the upstream and downstream waves evolve.

07z update:  As the 12z GFS trended a bit slower than its 00z/06z
runs...the 00z ECMWF/CMC both trended a bit faster and look
remarkably similar to the 06z GFS (a bit stronger/wound up).  The
00z UKMET picked up pace but was more positively tilted through
the trof than the other more neutral guidance.   As such will
continue initial non-UKMET blend preference.

Model trends at
500 mb forecasts at


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