Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1238 PM EDT Sat Apr 21 2018

Valid Apr 21/1200 UTC thru Apr 25/0000 UTC

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

12Z Model Evaluation...with Preliminary Preferences and Confidence
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...Lingering trough over the Northeast US, with progressive
shortwave moving it through the Northeast by Monday...
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Preference: General model blend
Confidence: Above average

Models show very good agreement with the mass fields for this
system and a general model blend is preferred with high confidence.


...Strong closed low over the southern Plains, pushing into the
Southeast by Monday and then opening into a trough...
...Inverted surface trough in the western Gulf Coast region, with
weak surface low developing in the central Gulf Coast states
Sunday...
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Preference: Blend of 12Z GFS; 00Z ECMWF
Confidence: Average

There is fairly good agreement with the mass fields through 23.12Z
(Monday morning), with increasing differences as the closed low
opens into a trough. The 12Z NAM and GFS, and 00Z ECMWF, show the
trough beginning to lift northeast and gain latitude as the closed
low opens up, while the 00Z UKMET and CMC keep the base of the
trough a bit further south. The NAM does lift the trough faster
than the ECMWF and GFS. The reasonable similarity between the GFS
and ECMWF, and their proximity to the ensemble means (GEFS, ECMWF,
NAEFS bias-corrected) suggests this may be the most likely
scenario, and the forecast preference is weighted in that
direction.


...Strong shortwave/attendant cold front nearing the Pacific
Northwest currently before shifting north of the US border this
weekend...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

Models are in very good agreement with the mass fields for this
system and a general model blend is preferred with high confidence.


...Secondary shortwave reaching the Pacific Northwest early
Sunday; amplifying and pushing into the Northern Rockies Monday
and northern Plains Tuesday...
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Preference: Blend of 12Z NAM; 00Z CMC; 00Z ECMWF
Confidence: Slightly below average

The preference for this system will be largely to maintain the
previous preference from overnight, which was a blend of the NAM,
CMC, and ECMWF. The 12Z GFS shows the vort max aloft to generally
be concentrated further northeast as the wave arrives in the
Plains, and even starts to close off a low at 500mb. Other models
keep the mid-upper level vorticity max generally concentrated in
the base of the trough further southwest. This seems to be a
reasonable scenario. The 00Z UKMET shows a very different
structure to the trough, with a stronger and larger northern
portion of the wave along the Canadian border, and a much more
elongated southern portion that lags the timing of the other
models. This leads to much more widespread precipitation than
depicted by other 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