Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Thu Jul 25 2019

Valid Jul 25/1200 UTC thru Jul 29/0000 UTC

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

12Z Final Model Evaluation, Including Confidence and Preferences
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...Northern Tier of the U.S...
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Preference: 12Z ECMWF/12Z NAM/12Z GEFS
Confidence: Average

19Z Update: There exists little spread in the evolution of the
dual shortwaves and associated cold fronts moving across the
northern tier. The ECMWF shows good consistency and is in great
agreement with the NAM and GEFS mean. Subtle timing and intensity
differences remain in the CMC and UKMET, so no changes planned to
the blend.

Previous Discussion:
Two shortwaves will move onshore the PacNW and then race eastward
in the vicinity of the Canada/US border. While the primary forcing
for ascent with these features will remain north of the CONUS,
each one will drag a cold front across the northern tier. The
first shortwave features good model agreement in that it will race
eastward and shear out near the Northern Plains/Great Lakes
Saturday. The second has more uncertainty and discrepancy in
timing and placement. The CMC has a second piece of energy
associated with the primary shortwave which will move east across
the PacNW, and this has no model or ensemble agreement. The other
area of discrepancy is where the associated front will lie by day
3 across the Northern Plains. The 06Z GEFS/00Z ECMWF/12Z NAM are
in good agreement with a NW to SE oriented band of precip along
the front across MN, while the 12Z GFS is much further south with
this feature. The UKMET is in decent agreement as well, but is
very much an outlier elsewhere (see below) so is not included in
the preference here.


...Southwest...
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Preference: Non-UKMET blend
Confidence: Slightly above average


19Z Update: The UKMET remains a strong outlier with the ridge
intensity, but has toned down the important shortwave rotating
around the periphery. Still, it is a wet outlier with its QPF due
to monsoonal moisture interaction with the shortwave while the
other guidance matches better the consensus.

Previous Discussion:
High pressure over the Southwest will drive monsoonal moisture
into the region through the forecast period. The guidance features
a small spatial envelope for the large scale ridging, but varies
considerably in intensity, with the UKMET continuing to be a
strong outlier and the GFS bringing up the weak side of the
envelope. The recent runs of the GFS do show a strengthening trend
however, so the GFS is included in the preferences. The other
feature of note is a shortwave which is progged to lift onto the
CA coast and then ride eastward around the periphery of the ridge.
While all the guidance shows this, the UKMET is stronger with the
shortwave which causes enhancement in QPF across the Four Corners.
This is much different from the remaining suite which features
better consistency in the placement and intensity of this
shortwave.


...Remainder of the CONUS...
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Preference: Non-UKMET/Non-CMC blend
Confidence: Slightly above average

19Z Update: Biggest chance with the 12Z Non-NCEP suite was with
the UKMET handling of the possible wave of low pressure in the
Gulf. The UKMET is now in much better agreement with a weaker
solution lifting northeast at the tail end of the cold front, but
due to its fluctuation in solutions prefer to continue to leave it
out of the blend. The rest of the discrepancies noted below remain.

Previous Discussion:
Across the rest of the CONUS, the feature of note is a developing
wave of low pressure in the Gulf of Mexico along the tail end of a
weakening front/baroclinic zone. Once again, the models are
struggling with what to do as this feature develops, despite
nearly unanimous agreement in a feature existing. The UKMET is by
far the strongest with its mid level vorticity energy and QPF
along the Louisiana coast, and is excluded. The CMC is also quite
strong and wet compared to the ensemble means. While a wetter
solution is possible should this feature strengthen and drift
northward, the preferred blend all suggests a more northeast
trajectory of a weaker system in the northern Gulf by day 3.


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

Weiss