Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1228 PM EDT Thu Apr 26 2018

Valid Apr 26/1200 UTC thru Apr 30/0000 UTC

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

12Z Model Evaluation...with Preferences and Forecast Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


...Evolution of the eastern CONUS trough through Sunday...
...Shortwave pushing through the Southeast today and Friday,
phasing with digging wave into the Great Lakes and Northeast by
Saturday...
...Weak East Coast surface low development Friday-Saturday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Average

The evolution of the eastern US trough continues to be relatively
complex as several shortwaves will be interacting. This introduces
some uncertainty in the details, such as precise placement of
precipitation bands. However, the models do generally show good
agreement with their mass fields, and there is no consistent bias
to warrant the exclusion of any specific model. Therefore, a
general model blend is preferred at this time to account for the
slight uncertainties originating from the detail differences.


...Positively tilted shortwave in the central Plains today,
digging to near Florida by Friday Night...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend
Confidence: Average

Models show reasonable similarity in their mass fields for this
system. The primary exception is with the 12Z NAM, which shows a
more amplified wave by 27.12Z (Friday morning) as it arrives in
the Lower Mississippi River Valley, and this eventually leads to a
slower progression along the Gulf coast. The preference is to lean
toward the broader model consensus.


...Closed low pushing to the West Coast by Friday...
...Opening into a trough over the Northwest on Sunday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of 12Z GFS; 00Z ECMWF; 00Z CMC
Confidence: Average

The GEFS, ECMWF, and bias-corrected NAEFS means have remained
relatively consistent over the past several model cycles, and with
the 00Z model cycle there was a convergence of ensemble members
toward these means. Therefore, the preference is to remain
relatively close to that timing and amplitude of the closed low
and associated trough as it pushes into the western US. The models
that most closely represent this are the GFS, ECMWF and CMC. The
12Z NAM and 00Z UKMET show the base of the trough further south as
compared to other models, and this has become less supported by
ensemble members over the past couple model cycles.


...Weak southern stream shortwave ejecting out toward the southern
High Plains by Saturday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of 12Z GFS; 00Z ECMWF
Confidence: Slightly above average

The progression and evolution of this wave may be affected by the
closed low to its northwest, and thus the preference is kept
fairly similar. The 00Z CMC and UKMET both show considerably more
amplitude to the height pattern in the vicinity of New Mexico on
Saturday. The preference is to lean toward the GFS and ECMWF which
have much greater ensemble support.


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

Lamers