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FXUS10 KWNH 230435
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1234 AM EDT Mon Apr 23 2018

Valid Apr 23/0000 UTC thru Apr 26/1200 UTC

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

00Z model evaluation including preferences and forecast confidence

...Strong closed low over the Lower MS Valley today, gradually
opening into a trough and lifting into the Mid Atlantic and
Northeast, phasing with northern-stream shortwave digging through
the Great Lakes Tuesday Night into the Northeast...
...Weak surface low over the interior Southeast, redeveloping
along the Atlantic coast by mid-week...
Preference: 00z GFS/12z CMC blend
Confidence: Slightly below average

Models continue to show solid agreement through the Southeast at
least through early Wed when the trof opens up and there is
increasing influence of the digging trof crossing the Great Lakes.
 By late Wed into Thursday, the timing/amplification and spacing
of the two features lead to large model differences across the
Mid-Atlantic into the Eastern Great Lakes/Northeast.  The 12z
UKMET is a clear outlier depicting a much sharp but much further
south trof advancing through the northern Ohio Valley.  This wave
undercuts the weakening/opening shortwave through the Mid-Atlantic
resulting in a broader developing closed low feature with
maximized DPVA through the Mid-Atlantic coast Thursday resulting
in a deep surface reflection and strong moisture conveyor into ME.
 The 00z NAM shows a similar evolution but rather than the
upstream wave being strong, its the coastal wave that dominates
the interacting pair.  This is well timed and matches a solid
clustering of ECENS members but stronger wind fields/moisture and
deep surface low show signs of typical day 3 NAM
over-amplification issues.

The 00z GFS has shown a solid shift in timing out of the
Cumberland plateau toward the Carolinas with the opening trof.
This slowing here leads to a southeastward shift with the track as
it lifts northeast.  Additionally, this matches timing as the
Great Lakes trof starts to swing negative tilt and close off into
a weak concentric low, which is generally favorable in the mean
timing of the ensemble suite.  The 00z CMC, while a bit stronger
with the upstream wave than the GFS shows a similar evolution in
timing and seems a good compromise over the faster/stronger
NAM/UKMET.   The 12z ECWMF is very slow and strong with the Great
Lakes wave, and closes off, helping to sling the opening wave
through the Mid-Atlantic and presenting a very deep surface
reflection.  This is not even supported well in the
about half to 2/3rds are not this robust with closing off the
mid-level low.  As such the ECMWF cannot be supported in the blend
(especially after Wed), though the ECENS mean could help account
for some of the possibilities.   Still, a 00z GFS/12z CMC blend is
preferred at slightly below average confidence.

...Shortwave entering northern Great Basin, amplifying into the
northern Rockies through Monday Night, and then digging through
the Central Plains Wed/Lower MS Valley by Thurs...
Preference: 00z GFS and 12z ECMWF/CMC blend
Confidence: Slightly above average

Global guidance shows a solid agreement with the amplification of
the norther Great Basin/Oregon Desert shortwave across the
Northern Rockies tonight and sliding south and east along the high
terrain as the northern stream wave lifts away reducing influence.
 The GFS finally backed away from the faster/stronger more
concentric wave through the MO river valley and now tracks
similarly to the ECMWF/CMC Wed into Thurs.  The wave will amplify
nearing the base of the larger scale trof  across the eastern
third of the US as it enters the Lower MS Valley.  The ECMWF/CMC
both are bit broader and south, intersecting moisture return off
the Gulf a bit more readily than the GFS but still very close.
The 00z NAM while much better than the slower/closed low with the
18z run by 12z Thursday, still is a bit too slow to the better
consensus and remains a bit more wound up.  The 12z UKMET in the
most connected with the northern stream trof, so the top of the
trof digs deeply across the Great Lakes/Ohio River Valley while
leaving the base of the trof languishing across the Colorado
Rockies.  This has little to no backing support from other
guidance and ensemble is not considered for a blend.
 WPC preference will favor a 00z GFS and 12z ECMWF/CMC blend at
slightly above average confidence.

...Northern stream portion of Gulf of Alaska trough breaking off
and digging through the Canadian Rockies Wed, continuing into the
northern Plains Thursday, with associated surface cold front
pushing southeast...
Preference: 00z GFS and 12z UKMET/ECMWF blend.
Confidence: Slightly below average

A weak trof connecting a deep low over the Bering Sea and the
developing closed low near 35N 135W (section below)...will
progress out of the eastern Gulf of AK under the influence of an
approaching strong upper-level jet and shorten the wavelength.  An
amplified ridge pattern across the Western US will help deflect
the energy through BC though as it over tops the ridge breaking it
down slightly on Wed...trof will descend into the Northern US
Rockies and progress into the High and Northern Plains toward
Thursday.  The 00z NAM is a bit stronger and broader with the wave
hinting at some over-amplification/compact cyclogenesis issues
typical of day 3 bias.  The 12z CMC is also a bit broader and
therefore does not extend south (southwest) into the North Central
Rockies.  The 12z ECMWF which shows a bit stronger overall ridging
is therefore a bit faster emerging out of the Pacific though also
with further west ridging does extend into the Rockies more than
other guidance.  The GFS had been very weak with the ridge showing
significant suppression as the trof passed, thought the 00z
iteration was less so and therefore extended into the terrain
more.  The GFS while not as broad as the ECWMF shows good
agreement with a narrower/similarly timed 12z UKMET.  As such will
favor a 00z GFS and 12z ECMWF/UKMET blend though modest spread and
recent moderate shifts in timing/strength lead to average
confidence in this blend.

...Southern stream portion of Gulf of Alaska trough breaking off
and developing into a closed low around 35N/135W by Tuesday...
Preference: 12z UKMET/CMC blend
Confidence: Slightly below average

Strong downstream amplification of western US ridge will support
the base of the larger scale trof to dig south and close off a
broad low out near 35N and 135W by Tuesday.  Ensemble trends
suggest a strong ridge with good consistency.  The 12z ECMWF/ECENS
mean are a bit more bullish and shifted a bit west of the ensemble
consensus but are supported by the UKMET/CMC and CMCE members
though GFS/GEFS members are out of phase suggesting the northern
stream shortwave will suppress the ridge allowing for eastward
migration of the closed low as the internal shortwave features are
a bit more robust and set wider to the center of the center of the
parent closed low.  Conceptually, with orientation of jet axis and
location to higher terrain...this appears a bit too aggressive.
The 00z NAM is similar to the 18z GFS but supports a slightly
enhanced ridge than the GFS. The 00z GFS did strengthen the ridge
slightly and shift a bit west with the closed low but still
remains on the eastern side of ensemble suite.  The UKMET and CMC
are more central to the ensemble suite and run to run continuity
though also closer to the ECMWF.  WPC will prefer a 12z UKMET/CMC
blend to best represent the ensemble trends/mean.   The breadth of
the spread leads to slightly below average confidence at this

Model trends at
500 mb forecasts at


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