Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Wed Jul 24 2019

Valid Jul 24/0000 UTC thru Jul 27/1200 UTC

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

00Z Final Model Evaluation, Including Preference and Confidence
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...Pacific Northwest into the Northern Plains...
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Preference: Non-NAM blend
Confidence: Slightly Above average

07Z Update: The NAM is a south outlier now compared to the rest of
the global suite, and the CMC has backed off on its speed as it
now features a stronger first shortwave, more in line with the
remaining guidance. Although the spatial differences are
relatively minor, the NAM suggests a far enough south track that
PVA would produce enhanced QPF across MN/WI day 3 which has little
support.

Previous Discussion:
Two distinct shortwaves will drive cold fronts through the NW into
the Northern Plains this forecast period. The first, Wednesday,
will move onto the Washington coast before lifting into Southern
Canada to continue its eastward trajectory. While the primary
forcing and shortwave will remain north of the border, the cold
front crossing the northern tier of the CONUS will bring some QPF
to the region. The second impulse, Friday, will follow a similar
trajectory. While the guidance is in good agreement in the large
scale evolution of these impulses shedding from a closed low to
the north, the CMC is a bit fast compared to the global consensus,
which is in otherwise very good agreement.


...Southwest...
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Preference: General Model Blend
Confidence: Slightly above average

07Z Update: The UKMET remains subtly strong both with its ridge
intensity and that of the shortwave moving into California Friday.
Despite that, its placement of features are temporally similar to
the consensus, so can be included in the blend which overall
features little spread through the forecast period.

Previous Discussion:
A mid-level ridge expanding across the Four Corners will be the
primary synoptic scale feature of concern. While the global suite
is in good agreement with placement and timing of the ridge, there
are some difference in the intensity, most notably with the UKMET
being the strongest/most expansive. Additionally, the guidance
shunts a shortwave around the southern periphery of the ridge and
off the California coast, before bringing it back onshore Northern
California Friday. There are, again, intensity differences with
this feature which may have impact on QPF due to the monsoonal
moisture in place. However, a general model blend still should
suffice for now as these small scale features are tough to resolve
on synoptic time scales, and since all the guidance has some
semblance of this occurring a blend with the means should somewhat
limit the differences noted for now.


...Remainder of the CONUS...
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Preference: General model blend
Confidence: Above average

07Z Update: The biggest difference continues to be in the
evolution of what may be a weak low pressure developing on the
tail end of the weakening baroclinic zone/front in the Gulf of
Mexico Friday. The UKMET and NAM are both strong with the
mid-level circulation lifting northward, while the remaining
guidance features weaker energy with a wide variety of motion.
Despite the intensity differences, there is good agreement still
that a wave will develop, and will maintain the general model
blend until more certainty can be gleaned in the intensity and
track of the low developing beneath the ridge expanding from the
west.

Previous Discussion:
The global suite is in good agreement with a longwave trough
persisting along the coast, being reinforced by a secondary
shortwave moving through the OH VLY/Mid-Atlantic Thursday. The
biggest question mark across the east involves potential low
pressure development on the tail end of the weakening cold front
across the Gulf of Mexico by day 3. All guidance shows some
vorticity spinning up across the Gulf, but thereafter the
intensity and position waver considerably. There appears to be
enough model agreement to support at least a wave in the Gulf
Friday, but for now will compromise with a general model
blend/means to wash out the deterministic differences.


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

Weiss