Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1258 AM EDT Mon May 20 2019

Valid May 20/0000 UTC thru May 23/1200 UTC

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

00Z Model Evaluation...with Preferences and Forecast Confidence
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...Upper trough exiting the Great Lakes Monday...
...Strong height falls digging into the Northeast by Tuesday...
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Preference: General model blend
Confidence: Average

The models gradually take the upper trough over the Great Lakes
region off to the east on Monday and allow a cold front to sweep
down across the OH Valley, Mid-Atlantic and Northeast by Monday
night. On Tuesday, the guidance is in good agreement in
strengthening the height falls over the Northeast as additional
northern stream energy digs southeast across Ontario, and this
will result in a closed low evolution for northern New England and
a deepening of surface low pressure. The guidance is really in
good agreement with the details of the mass field evolution of
this system, and so a general model blend will be preferred.


...Deep closed low crossing the Southwest through Monday....
...Going negative tilt across the High Plains by Tuesday...
...Leeside cyclogenesis over the High Plains...
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Preference: 00Z HREF mean/12Z ECMWF blend...through 36 hours
            00Z GFS/12Z ECMWF blend...after 36 hours
Confidence: Slightly above average

An anomalous closed low moving across California will work through
the Four Corners region on Monday and then eject out into the
southern/central Plains Monday night through Tuesday, and taking
on a strong negative tilt as it does so. It is then expected to
slow as it lifts north toward the Dakotas by early Wednesday. At
the surface, leeside cyclogenesis will take place Monday, helping
to advance an anomalous plume of moisture and instability
northward. In the broad, large-scale sense, the latest models are
in rather good agreement with the set-up and evolution. However,
the differences lie in the details, with exact boundary placement,
and surface low position, especially Monday evening through
Tuesday. The trend has been for a more westward position of the
surface low and supporting energy aloft, but most of the
deterministic models now have a similar position through 36 hours.
Beyond that time, the NAM takes the low somewhat more off to the
northeast versus the global models. Meanwhile, the 12Z CMC
gradually becomes the slowest solution. The hires CAM (HREF)
guidance led by the 00Z ARW/ARW2/NMMB solutions tend to favor the
12Z ECMWF a bit more with low placement in the short-range
(through 36 hours), but the 00Z NAM-conest is a bit farther east.
Based on the latest clustering and trends, a blend of the 00Z HREF
mean and 12Z ECMWF will be preferred through 36 hours for the mass
fields, followed by a blend of the 00Z GFS and 12Z ECMWF
thereafter. Please consult the latest QPFERD for significant
concerns for excessive rainfall with this event over the next 36
to 48 hours out across the central and southern Plains.


...Vigorous Pacific closed low digging into the West Tuesday...
...Teleconnecting subtropical ridge over the Southeast...
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Preference: Non-CMC blend
Confidence: Above average

Another strong northern stream closed low will move into the
Pacific Northwest and California coast Monday and Tuesday. The
main PV anomaly will drop south through California, as it becomes
absorbed within the large scale troughing that will develop over
the Western U.S. In general, the models show fairly similar
solutions through the next 3 days with its mass fields, exhibiting
some of their typical biases. The 00Z GFS is a tad on the faster
spread of solutions, while the 12Z UKMET/ECMWF and to a somewhat
greater extent the 12Z CMC are on the slower end. The 00Z NAM is
actually fairly close to the GFS. The CMC is overall a periodic
outlier solution in being a tad too slow, so will prefer a non-CMC
blend at this point with this next system, but all of the models
regardless show a very large closed low feature over the Western
U.S. by the end of the period. This will also teleconnect to a
strong downstream subtropical ridge over the southeastern third of
the country by the middle to latter part of the week which all of
the guidance supports.


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

Orrison