Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sat Sep 01 2018

Valid Sep 01/0000 UTC thru Sep 04/1200 UTC

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

00Z model evaluation...with latest model preferences and confidence
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...Overall Synoptic Pattern across the CONUS...
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Preference: General model blend...led by 00Z GFS/00Z ECMWF
Confidence: Average

Ridging over much of the Eastern U.S. will remain in place going
through Tuesday, and the guidance again agrees strongly that the
subtropical ridge will strengthen with time over the Mid-Atlantic
region. 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 through the
period. There will also be a broad southern stream weakness/trough
focused over the Four Corners region in between the deep layer
ridge over the eastern half of the CONUS and a ridge offshore of
California.

Key features of note through the period will 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. On the large scale, the dynamical guidance
is in relatively decent agreement with their mass fields, but
there are some rather critical small scale differences with
respect to convectively driven vorts/waves and their resulting
QPF. This is especially the case later in the period with the 00Z
NAM and to an extent the 00Z CMC which are a bit more out of
tolerance compared to the 00Z GFS/00Z UKMET and 00Z ECMWF over
areas of the upper Midwest and south-central Canada. Will prefer a
multi-model consensus approach for the mass fields led by the 00Z
GFS and 00Z ECMWF across the CONUS.

One other area of interest relates to the tropical wave that will
be entering the eastern Gulf of Mexico on Monday and approaching
the central Gulf Coast on Tuesday. The 00Z non-NCEP guidance, and
especially as it relates to the ECMWF, have trended a bit stronger
with this system for this cycle. The ECMWF and CMC both support
closed circulations, with the UKMET, GFS and especially the NAM
all having a flatter wave representation. 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