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Model Diagnostic Discussion

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Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
249 AM EDT Tue Apr 24 2018

Valid Apr 24/0000 UTC thru Apr 27/1200 UTC

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

00Z model evaluation including preferences and forecast confidence

...Closed low over the lower OH valley today, gradually opening
into a trough and lifting into the Mid Atlantic and Northeast,
phasing with northern-stream shortwave closing off through the
Great Lakes late Wed before lifting into the Northeast Thursday.
...Weak surface low over the interior Southeast transition to
Carolinas Wed to northeast Thurs
Preference: 00z ECMWF/CMC blend
Confidence: Average to slightly below average

Large spread remains mainly after 25/12z, as the compact upper low
over the Carolinas starts to weaken and lift north under influence
of a digging/closing off closed low in the Central to Eastern
Great Lakes.  The 12z UKMET is a clear outlier having very little
semblance to any other ensemble member, presenting a stronger,
earlier, further west closed low over the SW Great Lakes...aiding
a rapid northeast acceleration of the Southeast closed low.   The
00z GFS continues to be weakest and therefore latest in closing
off the low in the far eastern Great Lakes region and so is more
phased in time/spacing with the closed low lifting northeast.  
The 12z ECMWF trended a bit east and while it seems reasonable is
still remains on the western side of the ECENS member suite, which
remains centered about half way between the 00z GFS and 12z ECMWF.
 GEFS members are more tight to the GFS comparatively than the
ECENS/ECWMF however, they lag the GFS.  The 12z CMC is now slower
and while a bit weaker than the ECMWF looks unfavorable given WPC
preference continuity.  The 00z NAM, shows a interesting shift
having slowed across the Northeast, it is timed favorably between
the GFS/ECMWF but is also stronger (typical bias of tightening the
low too much) and digs southeast a bit too much and supports a
deeper sfc low and moisture flux into eastern New England. 

Ideally, the NAM would be best representative to the ensemble mean
and general continuity of WPC preferences...but it must be
tempered in strength.  As much as this may squash the need signal,
a blend of the 00z GFS/NAM and 12z ECWMF may be a good compromise
in this mess of possibilities.  Confidence remains below average
after 25/12z.

07z update: The 00z ECMWF and CMC both shifted further east and
now seem to represent the ideal initial preference in
placement/strength and timing looking very similar to the 12z
ECENS Mean and a weaker (slightly north and west) version of the
00z NAM.  The 12z UKMET remains equally slow and is even deeper
across the Great Lakes, but has shifted north of the prior
solution...and remains unfavorable.  The 00z GEFS continued to be
fast compared to initial preference but also lags the operational
GFS supporting the fact that likely the GFS remains too fast. As
such will favor a 00z ECMWF/CMC blend at average to slightly below
average confidence. 

...Elongated trof with embedded shortwaves across northern
Rockies/Black Hills drops to Low MS Valley Thurs before weakening
through Southeast Fri
Preference: 00z GFS/12z ECMWF blend
Confidence: Average

GOES-16 WV loop shows elongated trof with strong lead
shortwave/vort center near the Black Hills and a secondary/warming
vort center over Eastern OR/SW ID.  The lead shortwave is shifting
toward the blocking flow across the Upper Midwest and the bulk of
guidance continues to shed a very minimum of energy north of the
block into the digging trof (see section above)...with the bulk
shifting south across the Central Plains and eventually toward the
Lower MS Valley early Thursday.   The UKMET is off somewhere not
in any vague semblance to other deterministic or even the most
radical of ensemble members acting more as a kicker wave to the
closed low over the TN Valley than a blocking force.   The
remaining guidance is fairly well timed through Thursday, though
as the shortwave consolidates and rounds the base of the larger
scale developing trof late Thurs...the CMC is a bit broader and
forward.  The 00z NAM is uncharacteristically weakening and
generally north in the cluster and further separates from
continuity/ensemble suite weakening/accelerating up the East Coast
Fri.    The 12z ECWMF is better centered to the 18z GEFS/12z ECENS
mean solution, and is more consistent moving into larger scale
ridging over the East Coast, by weakening and sliding south and
east compared to the ensembles.  The 00z GFS remains a bit more
compact and slower to weaken late Thurs/early Fri.  Still, it is a
good compromise with the ECWMF to best represent the middle ground
presented by the ensemble suite.  Confidence is average overall in
this 00z GFS/12z ECMWF blend.

07z update: The 00z ECMWF trended a bit faster but also a bit less
sharp rounding the base before weakening through the Carolinas. 
While this is trending faster away from the center of the ECENS
suite, it still remains within sight of the initial preference.  
The 00z CMC remains faster but also shifts a bit south and east of
the cluster after the shortwave rounds the base of the larger
scale trof.  While the UKMET has trended a bit slower/more still remains a clear outlier.  As such will keep
initial preference of 00z GFS/ECMWF blend at average confidence.

...Northern stream portion of Gulf of Alaska trough breaking off
and sliding southeast through Northern Rockies early Thurs then
starting to anchor the Global scale trof from Great Lakes to Lower
MS Valley late Fri...
Preference: General model blend
Confidence: Average

The broad trof over the eastern Gulf of AK will start to slide
Southeast into the Canadian and eventually N US Rockies Wed into
Thurs...and while there remains a bit of difference in the
strength of the ridging it is over-topping, there is growing
agreement that the upstream jet will narrow the trof wavelength
across the Central Plains...though the strong flow north will lead
to increasing stretching eventual disconnection across the Great
Lakes as the remaining narrow trof extends from the Midwest to Red
River Region.  There are small timing differences and each model
tries to consolidate vorticity centers along the elongated trof by
Fri...but a general model blend should account to smooth this out
in a more realistic manner (especially this far out in time, to
focus/favor on one small vort center over another).  Confidence is
slightly above average as the ensemble/deterministic suite
tightens in width of the wavelength as well as timing.  

07z update: While prior few runs hinted that the ECMWF favored
loading the trof more to the base, the new 00z run goes further
leading to increased severance of the northern and southern
portion of the trof by midday Thursday through the Central Plains;
eventually leading to a more rounded/symmetric mid-level low into
NE Texas by 12z Friday.  This has increased support from the 00z
UKMET though the symmetric wave is a shade further north in E OK,
matching closer to the GFS/NAM which are substantially weaker. 
The 00z CMC appears a good compromise in intensity between the
UKMET and GFS/NAM and placement.  Given the agreement in timing
and suggestion of severing the two streams by Fri...will continue
a general model blend as WPC preference.  Though increased spread
at the end of day 3 reduces overall confidence in the blend coming
to fruition, given the trends starting to manifest.

...Southern stream portion of Gulf of Alaska trough breaking off
and developing into a closed low around 35N/135W by
Tuesday...slowly wobbles toward Central Pacific Coast toward Fri
Preference: 00z ECMWF/CMC blend
Confidence: Average

The 00Z GFS continues to strengthen the ridge slowly downstream
but remains much narrower and east of the ensemble solutions.  The
00z GFS/NAM both continue to suggest the shortwave rounding the
incipient center of the upper low will be at a wider distance of
rotation leading to an eastward wobble.  This wobble along with
the weaker ridging will support stronger stretching/elongation of
the upper low with time.  While this is possible the trends of the
larger/broader wobbling solutions continue to trend toward the
more consistent/compact solutions presented by the 12z ECWMF/CMC
and the bulk of ECENS members.  The 00z GFS also starts to be much
weaker than even its "wobbling" partners of the 00z NAM/12z UKMET
providing further reduction in confidence toward its inclusion.  
As such will favor a 12z ECWMF/CMC blend at average confidence.

07z update: No significant changes were noted with the 00z
ECMWF/CMC or UKMET with the former in good agreement and the
latter favoring the NAM.  So there is no change in preference.

...Fast moving clipper out of North Central Canada clips N
Minnesota by midday Fri...
Preference: 50% 00z ECMWF with 25/25% 00z GFS/UKMET
Confidence: Slightly below average

In the wake of the amplifying large scale trof across Hudson Bay
to the Great Lakes...strong supergeostrophic jet/tight height
packing north of the ridge in Canada support a strong but flat
fast moving Clipper system that drops south out of the Canadian
Territories clipping northern Minnesota by midday Friday.  To be
expected, given differences in strength of the ridge, there would
be placement differences.  The greatest standouts from the
ensemble suite are: the 00z NAM which is quite strong but also
most northerly and the 00z CMC which is very slow as well.   While
the 00z GFS and 12z UKMET are further north than the ECWMF, both
are on the northern edge of the ECENS/GEFS ensemble suite (though
on the south side of the CMCE suite).  The 12z ECMWF is south and
fast within the ensemble suite too.  A compromise to the 18z
GEFS/12z ECENS means would be to favor the ECMWF in a blend with
the GFS/UKMET, perhaps 50 to 25/25.   Confidence is only average
given the spread and high variance in timing in fast flow regime. 

07z update: The 00z GEFS mean continue to lag the operational GFS
providing no significant change in thinking from an ensemble
perspective.  The 00z ECMWF remains strongest and furthest south,
with the surface low just north of the US border in the Boundary
Waters vicinity by 12z Fri, further increasing the spread from the
GFS/NAM solutions.  The 00z trended even slower and as such ejects
over a stronger ridge (in time) and therefore a increased
latitude...even more so that the 00z NAM.  The UKMET also sped up,
but only matching the timing/longitude of the GFS/NAM further
north.  As such will keep initial preference and weighting but
confidence is reduced due to increased model variance to slightly
below average.

Model trends at
500 mb forecasts at


Last Updated: 249 AM EDT Tue Apr 24 2018