Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
302 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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Pacific Northwest to the Northern Plains...
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Preference: Non-NAM blend
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 becomes a slow outlier with this next
system. In accounting for the model spread especially with the
second shortwave, a non-NAM blend will be preferred since the
global models are well clustered with both systems.


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

A mid-level ridge expanding across the Four Corners will be the
primary synoptic scale feature of concern. Additionally, there
will be modest 500/700 mb vort energy/troughing lifting north out
of the Desert Southwest and around the western periphery of the
deep layer ridge. There are some modest differences with the
models regarding the strength of this energy at this point, but
with the non-NCEP solutions now favorably initializing and
depicting this energy, a general model blend will be preferred
with the smaller scale vort energy and also the large scale
ridging over the region.


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

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 of the guidance again shows some low-level vorticity spinning
up across the Gulf along the boundary, but with some spread on
placement and intensity. As compared to the 00Z cycle of guidance,
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.
The 12Z NAM and 12Z UKMET are the strongest solutions, with the
12Z GFS, 12Z CMC and 12Z ECMWF all weaker, but still at least
supporting a wave. There are a fair number of European and
Canadian ensemble members depicting a low center that drifts
southwest into the western Gulf of Mexico as well. There is a
solid level of support from the 12Z HREF suite of guidance
including the NAM-conest, ARW, ARW2 and NMMB solutions. Of all of
the hires models, the NMMB and NAM-conest are the strongest
solutions. Given the convergence of solutions toward there being
at least a modest low center over the western Gulf of Mexico by
the end of the week, the model preference at this point will be a
general model blend.


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

Orrison