Prognostic Meteorological Discussion
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FXUS10 KWNH 250641
Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
241 AM EDT Wed Apr 25 2018

Valid Apr 25/0000 UTC thru Apr 28/1200 UTC

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

00Z model evaluation including preferences and forecast confidence

...Closed low over the Cumberland Plateau, gradually opening into
a trough and lifting into the Mid Atlantic and Northeast while
phasing with northern-stream shortwave closing off through the
Great Lakes late Wednesday before lifting into the Northeast
...Surface low over Carolinas translating into
Mid-Atlantic/Northeast later today into Thurs...
Preference: General model blend
Confidence: Above average

After days of consternation over model preference with respect to
timing/phasing of northern stream closed/nearly closed shortwave
and the decaying TN/OH valley upper low...there appears to be a
growing common solution.  The 12z UKMET remains a clear outlier in
the tightening ensemble spaghetti analysis plots, deeply
closed/west of the remaining solution.  The 12z ECMWF/CMC continue
to be a bit more wound up with the wave into the Eastern Great
Lakes and suggest a faster acceleration of the decaying Southeast
wave into the northeast...while the 00z NAM/GFS are more
aligned/phased with a greater negative tilt orientation to the
combined trof through the Northeast.  The 00z GFS shifted slightly
deeper and slowed compared to the NAM/GEFS/18z GFS to further
tighten the cluster.  At this point a Non-UKMET blend is
supported, leaning ECMWF/CMC if subtly so.  Confidence is slightly
above average.

07z update: The 00z UKMET shifted toward the common ground
solution near the initial preference.  The 00z ECMWF/CMC only
further tightened the overall agreement with the 00z GFS/NAM to
support a general model blend at this point at above average

...Elongated trough from Central Rockies to Mid-MO Valley drops
southeast to the lower MS Valley Thursday before weakening through
the Southeast by Friday with surface low crossing South early
Thurs to Mid-Atlantic by late Friday...
Preference: 00z NAM/ECMWF/CMC blend
Confidence: Average

Energy crossing the northern Rockies and High Plains will drop
southeast over the next couple of days and dig in toward the
southeast U.S. by Friday. The 12Z UKMET becomes generally a weaker
and faster outlier in digging the energy across the South, with
the best model clustering generally residing with the 00Z NAM, 00Z
GFS and 12Z ECMWF.  All of this energy is expected to drive an
area of low pressure across the interior Southeast by Thursday and
then northeast up toward the Mid-Atlantic region by Friday.
However, there is some notable spread with respect to the
placement of low pressure at the end of the period as the guidance
differs with degree of possible northern stream interaction at
that time.  The 00z NAM and GFS are a bit deeper rounding the base
of the larger scale trof through the interior Southeast and so are
a shade north with a tighter radius of rotation; however both
(more so GFS) did trend wider and further southeast.  The 12z
ECMWF is also quite quick to break down the upper low and even
though it is moving into larger scale confluence/ridging in the
Carolinas/ may be uncharacteristically too fast
in doing so.  The 12z CMC is not too bad showing a similar track
to the ECMWF, but is as usual a bit too slow to be considered at
this time.

07z update: A slight jog north with a slightly deeper solution
brings the 00z ECMWF into a better agreement with the 00z
NAM...and while the 00z CMC is a bit faster and further southeast,
there is enough similarity (especially in the lowest levels) to
have better confidence in this growing consensus.  This also makes
the 00z GFS appear much too deep, especially with the negative
tilt orientation and fairly deep surface result to suggest either
elimination or significant reduction in weighting in the
preference.  At this point will favor a 00z NAM/CMC and ECMWF
blend at average confidence.

...Northern stream portion of Gulf of Alaska trough breaking off
and sliding southeast through Northern Rockies early Thursday then
starting to anchor the Global scale trough from the Great Lakes to
the lower MS Valley late Friday with base swinging off southeast
Coast Sat.  ...Frontal zone through northern Plains Wed/Thurs
stalling in Great Lakes Fri...
Preference: 00z NAM/UKMET/ECMWF blend
Confidence: Average

GOES-WV mosaic depicts the incipient development of the next trof
over-topping the western ridge in the Canadian Rockies.  By
tomorrow, the wave will expand in a full-latitude but narrowing
trof that slides southeast from the northern Plains to Central
Plains.  With time, the northern and southern portion of the trof
have stretched to the point of breaking with the base of the trof
beginning to collect into a cyclonic/consolidated wave, especially
by early Friday entering the Lower MS valley.  The 12z UKMET,
however was earliest to do so, and is nearly mature by reaching NE
TX by 12z Fri.  The 00z CMC on the other side of the spectrum is
timed ok, but remains more stretched/elongated with positive tilt
orientation...which further compounds late Fri into Sat in the
Southeast.   The 00z NAM/GFS and 12z ECMWF remained timed well
until this time late Fri at the base of the parent trof.  The 00z
GFS/NAM are both north but the GFS shows typical fast bias by the
end of day 3, exiting the SE coast, while the NAM/ECWMF are
slowed.  The ECMWF was always south of the other guidance cluster,
and so may be too far south into the far NE Gulf, but would prefer
this timing with the NAM over the GFS.   As such a 00z GFS/NAM and
12 ECMWF blend is preferred at slightly above average confidence
up to 28/00z shifting to NAM/ECMWF after at average confidence.

07z update:  The 00z UKMET trended a bit slower to wrapping up the
base of the trof through the Red River valley; and while it is a
bit deeper than the NAM and more so the ECWMF, there is good
timing agreement and lower level mass field response to consider
it within the blend.  The 00z CMC trended stronger and less flat
through the Southern Plains into the lower MS Valley, but like the
GFS becomes a shade too fast rounding the base in comparison.  The
00z ECMWF is a bit weaker and remains south compared to the
NAM/UKMET, but still timed well with initial thinking...enough to
supports a 00z NAM/UKMET/ECMWF blend at average confidence.

...Closed low evolution toward the West Coast by Friday...
Preference: General model blend
Confidence: Slightly above average

Now that the closed low has matured with the main shortwave
placement relative to the center of the main vortex...timing,
strength and overall shape of the upper low have come into starkly
better agreement even by day 3.  While there remains smaller scale
internal details yet to be resolved, many of those details are
typical of known guidance biases and are best handled by blending
them to a common solution.  As such a general model blend is
preferred at slightly above average confidence.

07z update: A slightly increase spread manifests at the end of day
3 as the closed low reaches N CA/SW OR, a slight slowing in the
ECMWF and more significant slowing by the UKMET at the same time
frame the NAM shifted a bit faster to increase this spread.  At
this point, the spread is modest enough to keep initial, general
model blend support at slightly above average confidence.

...Fast moving clipper wave across south central Canada toward N
MN by early Friday...amplifies to anchor the northern portion of
the Long Wave trof over Ontario Sat...transfer of surface wave to
deep low over Ontario/Quebec late Fri/Sat...
Preference: General model blend weighted toward ECMWF
Confidence: Average

A strong fast moving jet will over-top the larger scale ridge in
north central Canada Thursday and rapidly accelerate SE supporting
a subtle surface wave that reaches N MN early Friday.  The wave
eventually amplifies to anchor the larger scale broad trof over
Ontario into the weekend.  As such the stalled surface front from
the prior trof (2 sections above) will buckle and support a
deepening low across SE Ontario and SW Quebec as well as congeal
with the surface low lifting out the Mid-Atlantic (see 3 sections
above).  The 12z is uncharacteristically fast with the
shortwave/nose of the jet into the western Great Lakes but this is
likely due to a delayed/slower downstream trof that is also much
stronger/deeper by Sat.  This leads the CMC to have significant
differences in the surface pattern and makes it less favorable
overall.   The 12z ECMWF is initially a bit south of the remaining
guidance and is a bit faster reaching the CONUS with a bulk of the
ensemble suite cluster (including some CMCE/GEFS).  The 12z UKMET
and 00z GFS/NAM are all a bit deeper at the surface but also
further north.    Still, the ECMWF slows at the base of the large
scale trof while the GFS/UKMET and lesser so NAM transfer to the
developing SE Ontario low a bit quicker and northeast.  While the
spread is modest...the overall shape, evolution look similar
enough to support a Non-CMC blend at this point from validation.
Confidence is average to slightly below average due to the spread
within the blend.

07z update:  A significant shift south by the 00z CMC brings it
into alignment with the ECMWF especially after early Friday.  This
while the UKMET also shifted south. The 00z GEFS also trended
south and faster with the wave and transition across to the
eastern side of the large scale trof on Sat, even more than the
00z GFS.   This increased agreement puts the northern focused
NAM/GFS on the outside of the spread but not significantly so.  As
such will favor a general model blend though weighted toward 00z
ECMWF.  The reduction in spread increases confidence to average.

Model trends at
500 mb forecasts at


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