Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
219 AM EDT Sat Aug 17 2019

Valid Aug 17/0000 UTC thru Aug 20/1200 UTC

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

00Z Model Evaluation...Including Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: Blend of the 00Z GFS/ECMWF, and GEFS/ECENS means
Confidence: Average

The large scale flow across the CONUS will remain broadly cyclonic
through the weekend, although gradual height rises are expected by
later Sunday and early next week across the central and southern
U.S. Shortwaves rotating through the flow will be the primary
sensible weather makers, and it is discrepancy among these
features that lead to the preferred blend.

The most notable differences involve the 00Z NAM which continues
to be a relatively strong outlier with several of these vort lobes
impacting the Plains/Midwest that are shedding around a complex of
upper lows in Canada. In fact, the NAM gradually becomes out of
tolerance with the global model consensus across much of
central/southern Canada by the end of the period. The 00Z UKMET
also is problematic with energy it has lifting up across the upper
Midwest through early Sunday as it is sharper and stronger with a
surface wave lifting up along a cold front crossing this region.
The UKMET also is out of tolerance across Canada versus the model
consensus, which is strongly supported at this point by the 00Z
GFS and 00Z ECMWF overall.

Across the Southeast and Mid-Atlantic coastal plain, a persistent
and quasi-stationary frontal zone is expected to be the focus for
multiple waves of low pressure. One wave of low pressure will
eject northeast off the Mid-Atlantic coast and out to sea on
Saturday. Meanwhile, a second wave of low pressure situated
already down across northern FL will lift northeast very close to
the Southeast and southern-Mid-Atlantic coastline through Sunday
before then exiting offshore and out to sea on Monday. The 00Z
NAM, 00Z CMC and especially the 00Z UKMET intensify this low
center as it lifts northeast up the coast, with the 00Z GFS and
00Z ECMWF showing more modest development. A large number of
Canadian ensemble members and a fair number of the GEFS members
favor at least a modest low center, with the vast majority of the
ECENS members more ill-defined. The 00Z NAM-conest, 00Z ARW/ARW2
solutions show a bit more development comparable to the NAM, but
the 00Z NMMB is a weak outlier.

There is strong support toward a GFS/ECMWF blend out of the latest
GEFS/ECENS means across the CONUS, and thus a blend of all of
these will be preferred at this point. However, a slightly
stronger version of the GFS/ECMWF solutions is suggested along the
Southeast and Mid-Atlantic coasts with the low center lifting
northeast across these areas on Sunday.


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

Orrison