Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1249 AM EDT Sun Sep 02 2018

Valid Sep 02/0000 UTC thru Sep 05/1200 UTC

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

00Z model evaluation...with latest preferences and confidence
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...Overall Synoptic Pattern across the CONUS...
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Preference: Blend of the 00Z GFS/12Z ECMWF...across the CONUS
            00Z GFS/12Z ECENS mean blend...near the Gulf Coast
Confidence: Average

Ridging over much of the Eastern U.S. will remain in place going
through Wednesday, and the guidance again agrees strongly that the
subtropical ridge will strengthen with time over the Mid-Atlantic
region going through the middle of the week. Meanwhile, a mean
layer upper trough is expected to remain intact over the Northwest
U.S. as northern shortwave energy continues to reinforce height
falls across the region at least through Tuesday. On Wednesday,
the troughing over the Northwest does begin to lift out though as
height falls offshore of British Columbia allow a shortwave ridge
to advance inland. Meanwhile, there will also be a broad southern
stream weakness/trough focused over the Four Corners region which
will tend to retrograde westward a bit by the middle of the week.

Key features of note through the period will again be the ejection
of small scale vort centers/shortwaves that will be lifting north
and northeast around the western and northern periphery of the
aforementioned subtropical ridge over the Eastern U.S. and
interacting with a well-defined frontal zone over portions of the
central Plains and the Midwest which will also be influenced by
the northern stream energy crossing the Pacific Northwest and
northern High Plains. By Wednesday, this front will make at least
some attempt to drop farther southeast as there will be somewhat
stronger height falls crossing southern Canada. On the large
scale, the dynamical guidance is in good agreement with their mass
fields, although the 12Z CMC did exhibit a stronger low pressure
wave lifting northeast up along the front involving the upper
Midwest by early Tuesday. The 00Z NAM also tends to have some wave
activity a tad out of tolerance as compared to the 00Z GFS/12Z
UKMET and 12Z ECMWF. Overall, the GFS and ECMWF though do tend to
have the best model clustering, and so a blend of the GFS and
ECMWF will still be preferred across the CONUS.
 
Regarding the tropical wave and possible low development over the
northern Gulf of Mexico by Tuesday, the 12Z ECMWF is by far a deep
outlier with this system. The 00Z NAM is the weakest solution. The
00Z GFS, 12Z CMC and 12Z UKMET all suggest developing low centers
over the northern Gulf of Mexico, but the CMC and UKMET are on the
southwest side of the model spread. The ECMWF low track is
actually agreed upon rather closely by the GFS, but the intensity
is definitely not. The 12Z ECENS mean though is close to the GFS,
and supports a more tempered system approaching the central Gulf
Coast Tuesday night. A blend of the 00Z GFS and 12Z ECENS mean
will be preferred with this system. Please consult the latest
TWOAT from the NHC for more details on this system.

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

Orrison