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The Weather Prediction Center

Model Diagnostic Discussion

[Abbreviations and acronyms used in this product]
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Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Thu May 24 2018

Valid May 24/1200 UTC thru May 28/0000 UTC

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

12Z Model evaluation including final preferences and forecast

...Surface low development over the Gulf of Mexico into Sun...
Preference: 12Z GFS/ECMWF blend
Confidence: Average

Both the 12Z NAM/GFS are close to the model consensus early
concerning the tropical wave off the Yucatan Peninsula late Thu
morning as it tracks northward ahead of a strengthening mid level
ridge over the western Gulf of Mexico. However, the 12Z NAM slows
with time with the surface feature (as well as its mid level
circulation), and by Sun evening becomes much slower and further
west with the surface wave.

The 12Z GFS, in contrast, remained much more consistent with its
track of the surface system, maintaining a slower and further west
track over the last three cycles. This places the 12Z GFS very
close to the 12Z ECMWF/00Z ECMWF ensemble mean by Sun evening,
though the 12Z ECMWF has slowed its northward progress a bit. 

The 12Z CMC has slowed its approached with the surface system
(though there was not a corresponding change in the mid level
system), though it is not as slow as the 12Z NAM. Because of this,
the 12Z CMC has been dropped from the preference. The 12Z UKMET
remains the eastward outlier with the surface and mid level
systems (as its mid level ridging over the eastern Caribbean is
further east than the consensus.

Since the 12Z GFS is close to the 12Z ECMWF with the surface low,
these solutions comprise the favored blend. Because of the 12Z NAM
and 12Z UKMET maintaining seemingly outlier positions, and the 12Z
CMC/ECMWF has slowed their approaches, the forecast confidence
remains average.

...Shortwave and surface wave clipping New England late Fri/Sat...
Preference: General model blend
Confidence: Above average confidence

Both the 12Z NAM/GFS are close to the consensus with the short
wave energy crossing Ontario/Quebec late Fri into Sat. These
solutions are also close to the consensus concerning weak surface
lows that develop on a frontal boundary extending from the
northern Mid Atlantic states across southern New England, as
surface high pressure crossing northern New England. Given the
good model clustering with these systems, a general model blend is
preferred with above average confidence.

...Short waves tracking across the Northern Rockies along the
Canadian border Sat/Sun
...Associated surface low across Great Lakes Sat/Sun...
Preference: Non 12Z UKMET blend
Confidence: Slightly above average

Both the 12Z NAM/GFS are in close agreement with the short wave
energy over Alberta/Saskatchewan Thu afternoon, and remain close
to the consensus as the short waves cross the upper Great Lakes
sat into Sun. Only the 00Z UKMET is slower than the consensus at
this point with the short wave, as it becomes deeper and slower
with the short wave over the UP of MI. Interestingly, the 12Z
UKMET is not far from the consensus (held by the 12Z
NAM/GFS) with the surface wave that crosses MN/WI/UP of MI during
this time frame.

However, the slower timing of the 12Z UKMET does result in some
differences in mass fields over the upper Great lakes, so this
solution was excluded from the blend.

...Closed low off central West Coast reaches CA by early Fri
before closing off over the Great Basin by Sun...
Preference: General model blend
Confidence: Above average

Both the 12Z NAM/GFS are closer to the model consensus as they
close off a mid level low at the base of a long wave trough late
Thu into early Fri. After that time, there is very good model
agreement as the closed mid level low comes ashore over central CA
Fri afternoon, then crosses the Great Basin Fri night into Sat
night. Given the very good clustering of the operational/ensemble
means with the mid level system, a general model blend was
preferred with above average confidence.

Model trends at
500 mb forecasts at


Last Updated: 236 PM EDT Thu May 24 2018