Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Wed Aug 21 2019

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

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

12Z Final Model Evaluation...Including Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: Blend of the 12Z GFS/GEFS/ECMWF and 00Z ECENS mean
Confidence: Average

20Z Update: Note the 12Z CMC was not available for analysis.
Otherwise, the previous blend remains sufficient and no changes
are suggested other than to use the newest versions of the
deterministic and ensemble guidance.

Previous Discussion:
Flat ridging across the Southern tier will give way to a more
amplified mid-level pattern by the end of the week. This evolution
will be caused by dual longwave trough across the northern tier
and into Canada, which will gradually become deeper, driving a
shorter wavelength but higher amplitude flow. While the guidance
is in good agreement in this evolution, there are details that
differ considerably, primarily with timing of features across the
CONUS.

Thursday into Friday, a trough will dig through the PacNW and into
the Northern Plains. This trough will maintain a positive tilt
into Friday night before becoming absorbed in a large trough
centered over northern Canada, and taking on a subtle negative
tilt. The NAM is a fast outlier compared to the global consensus,
and should be used with caution despite its agreeable QPF output
across the Northern Plains. The UKMET is in general temporal
agreement with the evolution of this feature, but is weak as it
sheds a secondary piece of energy well southward into the ridge,
which appears unlikely, especially as it has limited ensemble
support.

Downstream of this trough, a shortwave ridge will amplify into the
Plains and Great Lakes, while a positively tilted trough persists
into New England. The CMC is fast driving this feature into the
Atlantic, leaving the GFS/ECMWF as the most reasonable solution
for the evolution as it begins to close off over the Mid-Atlantic
late in the forecast period.

Utilizing the ECMWF/GFS blend seems best, but including the
ensemble means is prudent to eliminate some of the finer scale
details in shortwaves rotating within the westerlies which
struggle to be resolved on these spatial and time scales.
Additionally, while there is some evidence of mid-level energy
lifting into the Northern Gulf of Mexico Saturday, the means are a
reasonable compromise for this feature due to the wide variety of
positions and intensities features in the deterministic guidance.


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

Weiss