Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1239 AM EDT Mon May 07 2018

Valid May 07/0000 UTC thru May 10/1200 UTC

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

00Z Model Evaluation...with Preliminary Preferences and Confidence
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...Eastern US trough with digging wave passing through the
Northeast today, and lingering southern stream portion in the
Southeast...
...Deamplification of southern portion Tuesday and Wednesday...
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Preference: 12Z ECMWF; 00Z NAM; 18Z GEFS
Confidence: Average

Models are generally in good agreement with the northern stream
wave brushing by the Northeast today, but show detail differences
with the lingering trough over the Southeast that will slowly
eject toward the Mid Atlantic by Wednesday. The 00Z GFS
concentrates the mid-upper level vorticity into a very compact
wave, which is not something that many of the other models do. The
preference is to lean toward the remaining models, as the
expectation is that a weak, broad wave that is deamplifying would
not become more potent and compact as it ejects northeast. The 12Z
UKMET is on the other end of model scenarios with a much broader
wave that deamplifies and ejects faster. The preference is to lean
toward a blend of the 12Z ECMWF, 00Z NAM, and 18Z GEFS Mean which
are all relatively consistent.


...Shortwave ejecting through the Northern Rockies today, reaching
the Northern Plains Tuesday and Upper Midwest Wednesday...
...Lee cyclogenesis over the Plains with the surface low and
associated fronts eventually pushing east...
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Preference: Blend of 00Z GFS; 12Z ECMWF
Confidence: Average

The primary differences with this system appear to still originate
with how the models handle two separate shortwaves: (1) the
initial wave pushing into the Plains and Midwest, and (2) a
secondary shortwave digging ESE from British Columbia later today
and eventually phasing with the initial wave. The 00Z NAM and 12Z
UKMET tend to concentrate the mid-upper level vorticity with the
resulting phased shortwave further north near the Canadian border,
while the 00Z GFS is a little further south, and the 12Z ECMWF is
furthest south. Ensemble cluster analysis from the 12Z model cycle
tended to favor scenarios with a vort max further south than
depicted by the NAM and UKMET, but a split whether that would be
quite as far south as the ECMWF is depicting. The preference is to
blend the GFS and ECMWF, as those are the models with the greatest
ensemble support. The 12Z CMC has higher heights through much of
the region than other models, so it was not included in the
preference.


...Upper level trough approaching the Pacific Northwest Tuesday
and Tuesday Night...
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Preference: Blend of 00Z GFS; 12Z ECMWF; 12Z CMC
Confidence: Slightly above average

For this system, prefer to maintain continuity with the same model
blend favored by the day shift. The GFS, ECMWF, and CMC all show
relatively similar timing of the trough axis as it pushes into the
Pacific Northwest, and have a more compact open wave. The 12Z
UKMET and 00Z NAM show a closed mid-level low developing off
Vancouver Island in the absence of significant cyclogenesis, and
spread height falls a bit further into the Pacific Northwest. The
GFS, ECMWF, and CMC have greater ensemble support and that is the
preference.


...Increasingly zonal flow with several embedded, low-amplitude
shortwaves from the Great Basin into the southern Plains Wed-Thu...
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Preference: Blend of 12Z ECMWF; 00Z NAM
Confidence: Slightly above average

There are fairly notable model differences in the mass fields in
the increasingly zonal flow across the southern tier of states by
mid-week. The 00Z GFS, in particular, seems to show considerably
more mid-upper level vorticity associated with several shortwaves.
This does lead to sensible weather impacts with several of the
shortwaves. One particularly notable example is in the southern
Plains Wednesday Night. The GFS ramps up a vort max much more than
the other models, and this may be somewhat related to convection
that it develops. It also shows a more prominent southward
extension of the trough that will be pushing into the Upper
Midwest and Great Lakes. The 12Z UKMET, meanwhile, shows much
higher heights than the other models over much of the Southwest
and southern Plains. The preference for this region is to lean
toward the ECMWF and NAM models.


Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Lamers