Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1244 AM EDT Fri May 18 2018

Valid May 18/0000 UTC thru May 21/1200 UTC

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

00Z model evaluation...with preferences and confidence
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...Broad upper trough and embedded MCVs over the Southeast...
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Preference: General model blend
Confidence: Above average

The 00Z NAM and 00Z GFS are just a tad faster then the non-NCEP
guidance with the gradual ejection of the large scale trough out
of the southeast CONUS going through this weekend. In general, the
model mass field differences are quite small at this time, and so
a general model blend will be preferred.


...Upper trough crossing the upper Midwest this weekend...
...Cold front/surface wave crossing the region...
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Preference: Non-NAM blend
Confidence: Above average

The 00Z NAM again stands out as being a somewhat slower and deeper
outlier solution with this system compared to the otherwise well
clustered global model and ensemble guidance. Will prefer a
non-NAM blend.


...Large upper trough/closed lows over the West...
...Ejecting east across the central Plains this weekend...
...Crossing the Midwest by Monday...
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Preference: Blend of the 12Z CMC, 12Z ECMWF and 18Z GEFS mean
Confidence: Below average

Multi-model trends with this system over the last 24 hours
continue to be generally slower and deeper as a large portion of
the Western U.S. trough axis and associated closed low energy
ejects out across the Plains this weekend. There is significant
disagreement between the NCEP and non-NCEP guidance on the details
of the energy as it crosses the central Rockies Saturday and edges
out into the central Plains on Sunday. The 00Z NAM and 00Z GFS
become both stronger and slower than the 12Z UKMET, 12Z CMC and
12Z ECMWF. Despite the slower overall trend in the guidance, there
are concerns with both the NAM and much more so the GFS as a
result of convective feedback. This feedback is likely resulting
in an unrealistically slow and deep solution. The non-NCEP models
however could still be a little too fast, but overall the non-NCEP
camp has better ensemble support right now as the 18Z GEFS suite
was more progressive and not at all supportive of the latest
NAM/GFS camp. Of the non-NCEP models, the UKMET is likely too far
north with its energy and associated low track going through
Monday and Tuesday. As a means of compromising, and finding a
meaningful model cluster, a blend of the 18Z GEFS mean, 12Z CMC
and 12Z ECMWF will be favored at this time.


...Upper trough digging into the West on Sunday...
...Energy lifting north out of the northern Rockies...
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Preference: Blend of the 00Z NAM, 00Z GFS and 12Z ECMWF
Confidence: Slightly above average

Overall trends in the models have moved toward a deeper trough
axis across the West Coast by Sunday morning, with all of the
models supporting a closed low dropping down through CA by Monday.
These height falls will be responsible for kicking the lingering
energy over the northern Rockies region up to the north and into
southwest Canada. Model spread with that system is fairly minimal,
but the 12Z UKMET is overall the weakest solution with that
feature. Regarding the closed low over CA by Monday, the 12Z UKMET
and 12Z CMC are the strongest solutions. The 00Z NAM, 00Z GFS and
12Z ECMWF are all well clustered on a slightly weaker system which
is well supported by the latest GEFS mean and ECENS mean.
Therefore will lean toward a blend of the NAM, GFS and ECMWF at
this time.


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

Orrison