Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1137 PM EST Thu Nov 29 2018

Valid Nov 30/0000 UTC thru Dec 03/1200 UTC

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

00Z Model Evaluation...with Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: Blend of 12Z ECMWF, ECMWF ENS and CMC
Confidence: Slightly above average

The pattern change across the CONUS is underway, and as such model
forecast spread into the weekend has generally narrowed. The
pattern will be shifting from zonal flow to a broad and highly
amplified trough over the western two-thirds of the CONUS. By
Monday, the normalized ensemble spread indicates a fairly high
degree of predictability for that time range, so there is broad
agreement from the models on the large-scale synoptic features. As
such, forecast confidence has been rated slightly above average,
even though local to regional scale differences will affect
placement and intensity of weather impacts.

Models have continued to strongly cluster close to the 12Z ECMWF
and 12Z ECMWF Ensemble with the evolution of the occluded cyclone
in the Central US. For deterministic models, the exceptions remain
the 00Z GFS and 12Z UKMET, which show a faster eastward motion of
the surface low and a flatter trajectory of the low overall.
However, NAEFS and ECMWF ensemble members are strongly clustered
further back, with a sharper low track into the Missouri Valley,
and this also includes the 12Z ECMWF, 00Z NAM and 12Z CMC. The
preference remains to lean close to the ECMWF ensemble mean, and
thus the deterministic ECMWF and CMC. The 00Z NAM lags the low
back much longer than the other models, though, and thus is
generally excluded from the preference beyond Day 2, or roughly
02/06Z.

The 00Z NAM also is showing noteworthy differences in the
Southeast and the West. Along the cold front in the Southeast, the
NAM keeps QPF further northwest than the other models, and has a
different orientation and timing to the front due to differences
in how it handles the exit of the occluded cyclone in the Great
Lakes and Northeast. In the West, its temperature profiles are
colder and would probably yield more snow. Therefore, the NAM is
generally excluded from the preference, although it is reasonable
with its depiction of the central US cyclone through Days 1 and 2.

Some of the largest spread in the ensemble distribution outside of
the central US cyclone is related to the amplitude of the broad
trough in the Western US by Sunday and Sunday Night. The CMC and
UKMET were flatter, less amplified, and yield a slightly warmer
forecast. The preference in the West, therefore, is to lean closer
to the ECMWF and ECMWF Ensemble Mean, which are very consistent
and even have support from the 00Z GFS now.


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

Lamers