Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
110 AM EDT Thu Jul 25 2019

Valid Jul 25/0000 UTC thru Jul 28/1200 UTC

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

00Z NAM/GFS Evaluation, Including Preliminary Preference
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...Northern Tier of the U.S...
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Preference: General model blend through 00Z/28
            non 12Z ECMWF blend 00Z/28-12Z/28
Confidence: Average
      
Two distinct shortwaves will drive cold fronts through the
northern U.S. this forecast period. The first shortwave moving
through Saskatchewan today shows no significant differences among
the models. However, the second shortwave, to enter British
Columbia Friday night, does have some model differences,
especially by Sunday morning. The 00Z GFS is unique in its
handling of the shortwave trough with the way it breaks it into
two parts, northern and southern components. However, impacts
across the northern U.S. are minimal based on this difference
through Sunday morning. The 12Z ECMWF shows larger differences
with a faster handling of the feature, not supported in the
remaining ensemble/deterministic guidance, which affects a surface
low over the Dakotas. Greater support across the Dakotas exists
for a non 12Z ECMWF blend.

...Southwest...
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Preference: General model blend
Confidence: Slightly above average

The 00Z GFS is slightly weaker with the 700-500 mb ridge centered
over the Southwest, but outside of that, there are no significant
mass field differences to mention.


...Remainder of the CONUS...
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Preference: General model blend
Confidence: Average

The biggest feature of note across the remainder of the CONUS has
to do with a lingering frontal boundary in the Gulf of Mexico.
There has been poor run to run continuity regarding surface low
development along this feature with possible heavy rain impacts to
locations along the Gulf Coast. There has been better clustering
of solutions around the idea of a weak low center developing along
the front south of LA and drifting southwest toward the western
Gulf of Mexico by Friday with the 12Z UKMET deviating the greatest
taking the feature more to the north before weakening. The
ensemble guidance is very unsupportive of a developed low Friday
night through Sunday morning in the vicinity of the Gulf Coast.
The latest 00Z and 12Z deterministic models do not show a well
developed low and while agreement is not too good between the
guidance with any weak feature that is forecast, a general model
blend is recommended to iron out these differences, not putting
too much weight into any one model.

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

Otto