Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1210 AM EST Sun Dec 30 2018

Valid Dec 30/0000 UTC thru Jan 02/1200 UTC

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

00Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: nearest to the 12Z ECMWF
Confidence: Average

The mid-upper level pattern will remain similar through Wednesday
morning with ridging off of the West Coast, mean troughing over
the west-central U.S. and ridging off of Florida. One of the
larger systems to affect the lower 48 over the next couple of days
will come from a closed low over northwestern Mexico today, as the
system ejects toward the northeast and spawns a surface low to
track from Arkansas toward the lower Great Lakes Monday into
Tuesday. On its heals will be a second upper trough to amplify
across the Intermountain West and close off a low near the Four
Corners on Tuesday. Another shortwave is forecast to crest the
eastern Pacific ridge and a cold front to near the Pacific
Northwest early Wednesday. Overall, the 12Z ECMWF is considered a
good match for all of these systems when blended with selected
other deterministic models depending on which area of the nation
under consideration.

There were some minor differences with the system approaching the
Great Lakes, with the 00Z NAM displaced a bit west with the
surface-850 mb low compared to the ensembles late Monday. The 00Z
GFS and 12Z ECMWF are a good fit but the 00Z GFS is faster than a
vast majority of guidance with an upstream northerly stream
mid-level trough entering the north-central U.S. Monday night
which has implications on precipitation west of the surface low
near the Great Lakes. The 12Z ECMWF is a reasonable fit when
blended slightly with the 00Z GFS.

Elsewhere, the 00Z GFS and 12Z CMC are faster to move eastward
with an upper trough that amplifies into the western U.S. Tuesday.
The 12Z UKMET is displaced westward with the 00Z NAM and 12Z ECMWF
a good fit to the tightly clustered ensemble means. There remains
some spread in the ensembles here, but trends seem to be pointing
closer to a 00Z NAM/12Z ECMWF blend than any other of the
deterministic guidance at this time.

Lastly, the 12Z ECMWF, 00Z NAM and 12Z UKMET are closest to the
better ensemble clustering regarding a shortwave to near Manitoba
and Ontario Wednesday morning, with the 00Z GFS more amplified
than the remaining deterministic guidance and toward the very edge
of 540 dam ensemble spaghetti heights at 500 mb valid 12Z/02.

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

Otto