Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1254 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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...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 00Z 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 still appears to be a bit too progressive with
this system, although it has trended a little slower from its 00Z
cycle. The 12Z NAM which is a little slower than the GFS has
actually trended a bit faster compared to its 00Z run, but tends
to have better temporal support from the 00Z non-NCEP guidance
including the CMC, UKMET and ECMWF. One concern however with the
NAM and CMC is that they take the low pressure wave a bit farther
west than the model consensus. The UKMET though is likely too deep
with the system as it crosses the Delmarva. A blend of the GFS and
ECMWF will be preferred at this point as a compromise with timing
and depth, but also since this will reflect a rather good degree
of ensemble support from the GEFS and ECENS members.


...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 00Z 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 00Z 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 00Z
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