Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
151 PM EST Wed Jan 2 2019

Valid Jan 2/1200 UTC thru Jan 6/0000 UTC

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

12Z Final Model Evaluation...with Preferences and Confidence
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***Overall pattern across the Continental U.S.***
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Preference: General model blend through 48 hours, then
UKMET/ECMWF/CMC/GEFS mean blend through Saturday morning
Confidence: Slightly above average

19Z Update: The 12Z ECMWF has trended slightly faster with the
southeast U.S. closed low, placing it between the 00Z EC mean and
the 12Z NAM, leaving the EC mean on the slower side of the model
guidance.  The 12Z UKMET is right in line with the model consensus
with the shortwave trough passing through the Northeast U.S. on
Thursday and therefore slightly less amplified.  There has also
been a trend for a slightly stronger Pacific low approaching
California this weekend, with the UKMET on the southern edge of
the guidance and the EC mean to the north of the model consensus. 
Given these considerations, the previous model preferences remain.


Previous discussion: The short range forecast period will be
represented with distinct northern and southern stream flow aloft.
There is good model agreement regarding the northern stream
shortwave and associated offshore surface low near the Northeast
U.S. Wednesday night and into Thursday morning, with the 00Z UKMET
slightly more amplified with the 500mb trough.  A general model
blend should suffice for this region.

The main storm system affecting the continental U.S. through the
end of the week will be a pronounced closed low tracking across
Texas on Thursday and then across the Deep South on Friday, with
an occluded surface low associated with it.  The 12Z NAM becomes
faster with the upper low by 12Z Friday and northeast of the model
consensus, and the 00Z ECMWF indicating a stronger low with
support from its 00Z EC mean.  The 12Z GFS is faster than the
CMC/ECMWF/EC mean by 12Z Saturday, but it has support from the 00Z
UKMET.  A non-NAM consensus is recommended beyond Friday morning.

Across the western U.S., an eastern Pacific shortwave is forecast
to reach the Pacific Northwest Friday morning with the 00Z UKMET
to the south of the model consensus with the surface low.  The 00Z
CMC is slower to the remaining deterministic consensus compared to
earlier runs and there is relatively good ensemble clustering. 
The second and more impactful shortwave approaches the California
coast by the end of the forecast period Saturday morning, with the
ensembles showing better agreement on timing of this feature, but
more spread is noted regarding amplitude.  The GFS is slightly
ahead of the other guidance and the ECMWF slower, with the GEFS
mean and 00Z UKMET being a suitable compromise.

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

Hamrick