Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
251 PM EDT Fri Jul 20 2018

Valid Jul 20/1200 UTC thru Jul 24/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
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***NOTE THE 12Z CMC IS CURRENTLY DELAYED WITH ITS ARRIVAL AND IS
UNAVAILABLE FOR THIS ISSUANCE***

...Amplifying upper trough/closed low moving into the East...
...Developing surface low along the Mid-Atlantic coastal plain...
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Preference: Blend of the 12Z GFS and 12Z ECMWF
Confidence: Slightly below average

The models are in good agreement with the details of the
amplifying upper trough and closed low evolution over the OH/TN
Valleys through Saturday and then with a gradual advance of the
energy down to the south across the interior of the Southeast by
Sunday. By Monday, the guidance is in good agreement on a closed
low center focused near the AL/GA border.

Meanwhile, this system will be interacting with a low to mid level
trough and developing wave of low pressure over the eastern
Carolinas. The digging height falls into the Southeast will help
to eject this energy northeast up across the Mid-Atlantic coastal
plain by Saturday and into the interior of the Northeast on
Sunday. The 12Z GFS remains the most progressive solution overall,
however, the 12Z ECMWF has come in a bit more progressive than its
previous run and is very close to the GFS solution at this point.
The GFS did also trend a little slower with this cycle, and so
there is definitely convergent trends seen among these two models.
Meanwhile, the 12Z NAM which trended more progressive with this
cycle is slower than the GFS/ECMWF camp, and also tracks its low a
little farther west and to the left of the model consensus. The
00Z CMC was also a bit left of the model consensus with its low
track. The 12Z UKMET though is the farthest east solution and is
right of the model consensus. Based on the latest model trends and
clustering, along with ensemble output, a blend of the GFS and
ECMWF will continue to be preferred.


...Upper trough focusing across the Northwest/Northern Plains...
...Cold front ejecting out across the High Plains this weekend...
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Preference: Blend of the 12Z GFS and 12Z ECMWF
Confidence: Slightly above average

The models generally show good agreement with the upper trough and
associated closed low impacting the Northwest which is forecast to
advance east and skirt across southern Canada while also impacting
the northern High Plains. A cold front associated with the energy
will eject out across the northern High Plains through the
weekend. The guidance is in good agreement aloft, but there are
some meaningful differences at the surface. The 12Z UKMET has a
notably stronger surface low crossing southern Canada, while the
12Z NAM and 00Z CMC were quite a bit weaker. The 12Z GFS and 12Z
ECMWF are well clustered with their low placement and intensity in
between the two camps, so a blend of the GFS and ECMWF will be
preferred.

Orrison