Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1155 AM EST Fri Jan 18 2019

Valid Jan 18/1200 UTC thru Jan 22/0000 UTC

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

12Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A major winter storm will affect the central and eastern United
States as low pressure tracks from Colorado this morning to the
New England coast by Sunday followed by bitter cold temperatures
for the northeastern quadrant of the CONUS. Across the West, a
combination of shortwaves will impact the West Coast with  will
develop behind the storm in the central and east, while weather in
the West remains active.

...Central and Eastern CONUS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non 12Z NAM blend
Confidence: Slightly Above Average

Model clustering continues to slowly tighten with each cycle, such
that confidence is a bit above average for the track and evolution
of the upper trough and surface low moving from the central to
eastern U.S. this weekend.  However, the 12Z NAM stands out the
greatest from the relatively tight model clustering with its
surface low tracking faster/east compared to the remaining
deterministic and ensemble consensus. This is due to a slightly
flatter low-mid level wave in the NAM. The 12Z NAM also differs
with its 700 mb evolution across the Ohio Valley on Saturday with
a less consolidated wave. While the 12Z NAM is not a significant
outlier, there is enough difference to exclude it from the overall
preference across the central and eastern U.S.

...Western CONUS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12Z GFS and 00Z UKMET
Confidence: Average

There is good agreement with only minor detail differences with
the potent shortwave to track toward central British Columbia
tonight and with the trailing cold front and moisture plume into
the Oregon and northern California coast. Timing differences exist
with the next wave though, with the 00Z ECMWF nudging faster than
the remaining model consensus. Ensemble spaghetti heights show a
slowing trend over their past 3 cycles with the GFS/GEFS on the
slower edge of the envelope of solutions and the 00Z ECMWF faster.
With the 00Z ECMWF mean slower than the 00Z ECMWF, and considering
the trends, a middle ground represented by a 12Z GFS/00Z UKMET
blend is preferred. The 12Z NAM timing appears reasonable but its
latitude is a bit north of the model consensus across the
Intermountain West and with a weak surface wave in eastern Montana
early Monday.

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

Otto