Prognostic Meteorological Discussion
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FXUS10 KWNH 220646
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Sun Apr 22 2018

Valid Apr 22/0000 UTC thru Apr 25/1200 UTC

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

00Z Model Evaluation including preferences and forecast confidence

...Lingering trough over the Northeast US, with progressive
shortwave moving it through the Northeast by Monday...
Preference: General model blend
Confidence: Above average

General consensus appears to be made with the shortwave crossing
the Northeast on Sunday/Monday...sharpening a bit into the
Northwest Atlantic.  As such general model blend is supported at
above average confidence.

...Strong closed low over the southern Plains, pushing into the
Southeast by Monday and then opening into a trough through Central
Ohio Valley Wed...
...Inverted surface trough in the western Gulf Coast region, with
weak surface low developing in the central Gulf Coast states
Sunday...GA Tuesday and Carolinas Wed...
Preference: General model blend through 24/12z
            Non-ECMWF afterward
Confidence: Average

There is fairly good agreement with the mass fields through 23/12Z
Monday, as the 12z ECMWF/CMC shift a bit south and east likely
under influence of upscale growth/enhancement from stronger latent
heat release.  This also shifts the surface low/triple point south
compared to the NAM/GFS which favor the deeper occluded sfc low
across TN.  Eventually, there is a transition back toward better
consensus by late Tuesday as the surface low lifts north through
the Carolinas.  Spread aloft is increases a bit as the ECMWF slows
under greater influence of a slightly faster/sharper upstream trof
through the Great Lakes.  This likely directs stronger moisture
flux further south and slows northward progression by the end of
the forecast period; this while the 12z UKMET/CMC and 00z GFS/NAM
both lift faster through the central Appalachians into the
southern Mid-Atlantic.  While each have a small issue, a general
model blend appears a good compromise to handle the smaller
spread/timing least mass-wise.  Confidence is average
in this blend as the blending likely flattens/smooths out the
features a bit, so shifting the features while retaining the
magnitude of the sensible weather to that central position would
be best to keep integrity of said magnitude.

07z update: The 00z ECMWF trended a bit stronger which delays its
north-northeast progression and evolving into an open trof
compared to the other guidance and is particularly slow lifting
the surface wave/associated qpf north through the Mid-Atlantic.
Though with the slowing of the upstream sharp/shortwave trof in
the Great Lakes the track is more favorable overall compared to
the 12z run.  The 00z CMC and UKMET while not as fast as the
GFS/NAM are much closer to those than the ECMWF.  As such a
general model blend is favored through day 2 but by day 3...a
Non-ECMWF blend is supported, especially given the spread from the
12z ECENS mean (matches 00z UKMET best).

...Secondary shortwave reaching the Pacific Northwest early
Sunday; amplifying and pushing into the Northern Rockies Monday
and northern Plains Tuesday before sliding south in Central Plains
Preference: 00z CMC/UKMET/ECMWF blend
Confidence: Slightly below average

A fast/subtle shortwave approaches the OR/N CA coast by 12z Sunday
and starts to amplify and broaden across the Northern US Rockies.
A growing consensus is emerging toward this broader more
cyclonically symmetric solution toward midday Monday providing
confidence through this time period.  However, as the wave exits
the terrain, into the high plains, a shortwave in the northern
stream inserts influence in either amplifying or sliding past
leading to a split in evolutionary paths.  The 00z GFS/NAM both
are stronger further north initially (likely due to bias of being
initially fast), this allows for increased amplification of the
wave crossing into the Northern Plains before dropping south
through the MO River Valley.  The 00z GFS in fact even trended
more amplified and faster translating through the Midwest into the
lower MO Valley by Wed. The ECMWF/CMC both are less influenced and
the shortwave is "back loaded" toward the SW portion of the wave
through the Rockies, and as such the shortwave is slow to exit the
terrain and slides southwest with increased positive tilt and
stretching through the Central Plains by Wed.  The UKMET while
more wrapped up exiting Absaroka Range Monday or "front loaded"
with the shortwave...quickly shears/stretches under influence of
the northern stream wave...allowing for a more ECMWF/CMC evolution
to manifest with the wave by Day 3.  These features appear well
agreed upon to exist and are fairly timed well in the guidance
suite, but it is clear small differences in strength/amplification
lead to large end output, so confidence is reduced.
Unfortunately, the ensemble suite generally favors their parent
deterministic solutions and an overall trend is not apparently
clear in the last 4 cycles in spaghetti analysis.  Placement of
jet streak/overall flow regime and conceptual meteorological model
would suggest a faster northern stream and less influence,
favoring something close to the 12z ECMWF/UKMET.  As such a 12z
UKMET/ECMWF blend will be the initial WPC preference, but reserve
the right to shift toward any growing consensus with the 00z

07z update: The 00z UKMET/CMC show similar evolution of the
shearing trof through the central Plains by Wed in line with 12z
UKMET and initial preference.  The 00z ECMWF remains a bit faster
than the UKMET but shows a similar evolution/timing to the 12z run
(initial preference) even though there was a bit more interaction
with the northern stream shortwave.  As such will favor a 00z
ECMWF/UKMET/CMC blend at slightly below average confidence.

...Sharpening/digging shortwave in the middle section of the
larger scale trof in the Great Lakes by Wed...
Preference: 12z UKMET and 00z ECMWF blend
Confidence: Slightly above average

In the wake of strong central Canadian cyclone lifting north and
east toward the southern Hudson Bay/James bay by
upstream shortwave digs southeast splitting and sharpens as it
reaches the axis of the larger scale global trof over Ontario to
TN Valley.  The bulk of shortwave energy will be focused around
the L.P. of Michigan by the end of the forecast period.
Uncharacteristically, the 12z ECMWF/CMC are both faster with the
shortwave feature likely under reduced influence of the Pacific
shortwave across the central US/Midwest (see section above).  The
12z UKMET is also less influenced by this wave but also a bit
slower than the ECMWF/CMC, more in line with the 00z NAM/GFS.  The
12z ECENS members suggest the ECMWF may be a bit too fast as well.
  The 18z GEFS mean is also a bit sharper and faster than the 00z
GFS looking very similar to the UKMET providing additional
confidence toward it over the deterministic 00z GFS.  As such
preference will be the 12z UKMET anchored by the 18z GEFS/12z
ECENS means.  Inclusion of the other deterministic models may be
ok, but would likely lead to a broader/weaker wave, that is not
likely to verify given agreement in the sharp nature of the trof.
Confidence is slightly above average, as the differences are small
and the UKMET is supported by the ensemble averages.

07z update: To be expected, the 00z GEFS mean slowed a bit to
better match the timing/amplification of the 00z NAM but still not
as far west as the 00z GFS. The 00z ECMWF slowed and shows a bit
more consolidation of the base of the wave across the Great Lakes
in alignment with other guidance.  The UKMET trended faster, more
typical of its bias switching places with the ECMWF.  The CMC
remained fairly shade fast of the initial preference.
  As such will favor a 00z ECMWF blend with the *12z* UKMET.

Model trends at
500 mb forecasts at


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