Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
104 PM EDT Wed Jul 24 2019

Valid Jul 24/1200 UTC thru Jul 28/0000 UTC

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

12Z Model Evaluation, Including Preference and Confidence
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...Pacific Northwest to the Northern Plains...
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Preference: Blend of the 12Z GFS, 00Z UKMET and 00Z ECMWF
Confidence: Slightly above average

Two distinct shortwaves will drive cold fronts through the
Northwest and across the northern Plains this forecast period. The
first shortwave today will cross Washington before lifting across
far southern Canada and advancing east toward Ontario by Saturday.
While the primary forcing and shortwave will remain north of the
border, an attendant cold front crossing the northern tier of the
CONUS will bring some QPF to the region. The second shortwave
arriving on Friday, will follow a similar trajectory, although
currently the 12Z NAM is a slow outlier with this next system,
with the 00Z CMC being a progressive outlier. In accounting for
the model spread especially with the second shortwave, a blend of
the 12Z GFS, 00Z UKMET and 00Z ECMWF will be preferred since those
models are well clustered with both systems.


...Southwest...
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Preference: General model blend...large scale ridge axis
            12Z NAM/GFS/ARW blend...700/500 mb vort center
Confidence: Slightly above average

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 broader
ridge axis, there are some differences in the handling of a 700 mb
vort center/trough axis lifting north up across the Desert
Southwest and portions of the Great Basin around the west side of
the ridge. The 12Z NAM/ARW solutions are a tad stronger than the
global models and remaining hires model guidance including the
ARW2/NMMB solutions. Morning GOES-17 visible satellite imagery
does confirm a weak vort center over western AZ that is advancing
west-northwest. The 12Z NAM/GFS solutions though both do have
initialized a 500/700 mb vort center across this region, and the
morning runs of the HRRR have this as well. However, the non-NCEP
guidance from 00Z including the UKMET/CMC and ECMWF do not have
this vort at all. Therefore, given the current satellite
observations and model initialization of the energy, a somewhat
stronger evolution of the vort center will be preferred. A general
model blend will be preferred with the larger scale ridge, but a
blend of the 12Z NAM/GFS and 12Z ARW will be preferred with the
mid-level vort energy/troughing lifting around the west side of
the ridge.


...Remainder of the CONUS...
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Preference: General model blend...large scale upper pattern
            Blend of the 12Z NAM and 00Z ECENS mean...Gulf low
Confidence: Above average...large scale upper pattern
            Below average...Gulf low

The global suite is in good agreement with a longwave trough
persisting in a gradually weakening and elongated fashion from the
Gulf Coast states northeast up across the Mid-Atlantic and
southern New England through the end of the period. It should be
noted that the 12Z NAM is a tad stronger with the trough axis
versus the global models. This will keep a front stalled out well
to the south across the northern Gulf of Mexico and the FL
Peninsula. The biggest question mark involving the front involves
potential low pressure development along it over the northern Gulf
of Mexico. All guidance again shows some low-level vorticity
spinning up across the Gulf along the boundary, but with differing
placement and ultimate intensity. The 12Z GFS, 00Z CMC and 00Z
ECMWF are the most ill-defined with the development versus the
stronger 12Z NAM and 00Z UKMET. The NAM would suggest a low center
approaching south Texas by Friday, whereas the UKMET would have a
low center coming into southeast LA. There appears to be enough
model agreement to support at least a wave somewhere over the Gulf
by Friday, and especially since there are a fair number of
European and Canadian ensemble members depicting a low center that
drifts southwest into the western Gulf of Mexico. A general model
blend will be preferred for the large scale pattern, but regarding
the evolution of any wave development over the Gulf of Mexico, the
model preference will be toward a blend of the 12Z NAM and 00Z
ECENS mean which would suggest a wave that at least is focused
somewhere over the western Gulf of Mexico by the end of the week.
Confidence though is quite low with the Gulf system evolution.


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

Orrison