Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1230 PM EDT Fri Apr 27 2018

Valid Apr 27/1200 UTC thru May 01/0000 UTC

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

12Z Model Evaluation...with Preliminary Preferences and Confidence
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...Evolving Eastern CONUS trough, with phasing and consolidation
of several shortwaves over the Great Lakes and Northeast...
...Mid-upper level closed low developing Sunday; slowly exiting
the Northeast by Monday Night...
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Preference: General model blend; weighted toward 00Z ECMWF
Confidence: Slightly above average

Trends in ensemble spaghetti plots over the past several model
cycles show good consistency on the progression and position of
the closed low from the ECMWF Ensemble. On the 00Z model cycle,
the NAEFS ensemble members began to converge toward that timing as
well, with the GFS model and GEFS mean following that trend as
well. The 12Z GFS remains on the fast end of model spread, while
the 00Z CMC and 00Z UKMET are positioned further north relative to
the ensemble means. The differences are minor enough overall that
none of the models are discounted, but the preference is for the
general blend to be weighted more substantially toward the 00Z
ECMWF which is closest to the consistent ECMWF Ensemble mean.


...Closed low reaching the West Coast today, opening into a trough
over the Intermountain West by Monday...
...Lee cyclogenesis over the Plains by Monday...
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Preference: Blend of 12Z GFS; 00Z ECMWF
Confidence: Average

The global models all generally show a similar forecast in the
synoptic sense, as the closed low opens into a trough and pushes
into the West. However, detail differences do exist. The 00Z CMC
shows a different structure to the trough as compared to other
models, while the 12Z NAM is on the fast end with the progression,
and the 00Z UKMET on the slow end. The preference is for a blend
of the GFS and ECMWF, as they generally have good ensemble support.


...Shortwave digging from the Aleutians tonight to off the
California coast by Monday...
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Preference: Blend of 12Z GFS; 00Z ECMWF Ensemble Mean
Confidence: Average

A reinforcing shortwave will dig down the West Coast and carve out
a broader trough over the Western US. The 00Z UKMET is far more
amplified with this wave, and is generally unsupported by other
models. Otherwise, the remaining models show reasonable
similarity. The 12Z GFS appears closest to both the 06Z GEFS Mean
and 00Z ECMWF Ensemble Mean, with the 00Z CMC and deterministic
ECMWF slightly slower. The preference is for a blend of the GFS
and ECMWF Ensemble Mean.


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

Lamers