Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1250 PM EDT Sat Aug 18 2018

Valid Aug 18/1200 UTC thru Aug 22/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
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...Upper trough crossing the Lower Great Lakes/Northeast...
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Preference: General model blend
Confidence: Slightly above average

Multiple shortwave troughs will evolve and tend to merge together
over the lower Great Lakes and Northeast through Sunday and Monday
with multiple waves of low pressure evolving along a cold front
settling slowly southeast across the Northeast and Mid-Atlantic.
Mass field differences are rather minor, and so a general model
blend will be preferred here.


...Upper trough/surface low amplifying across the Plains/Midwest...
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Preference: Blend of the 12Z GFS and 00Z ECMWF
Confidence: Average

The models amplify a shortwave trough down from the northern High
Plains to the central Plains tonight and Sunday, with low pressure
initially focusing over SD early Sunday along a frontal zone
before then redeveloping over eastern KS Sunday night. Thereafter,
the guidance takes the upper trough and associated surface low
northeast across the Midwest and Great Lakes region going through
Monday and Tuesday with a trailing cold front that will cross the
lower MS Valley. Overall, the 12Z GFS and 00Z ECMWF are a bit
better clustered and aligned with the ensemble means as compared
to the 12Z NAM, 00Z CMC and 00Z UKMET, which in time tend to
outpace the GFS/ECMWF camp as the system crosses the Great Lakes
on Tuesday. Will prefer a GFS/ECMWF blend for now.


...Trough digging across the Northwest through early next week...
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Preference: Non-NAM blend
Confidence: Above average

A new upper trough and associated closed low will dig into the
Northwest on Sunday, and then gradually cut off by Tuesday as
ridging noses up north of it across southwest Canada. The models
for most of the period are in good agreement with the closed low
evolution, although the 12Z NAM becomes a less defined with the
system by the end of the period versus the global models. Will
prefer a non-NAM blend as a result.

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

Orrison