Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1155 PM EST Wed Dec 26 2018

Valid Dec 27/0000 UTC thru Dec 30/1200 UTC

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

00Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00Z NAM, 00Z GFS, 12Z ECMWF blend
Confidence: Slightly above average

An amplified pattern will be in place over the next few days with
mean troughing over the west-central U.S. with two significant
waves to impact the central to eastern U.S. Across the Northwest,
ridging aloft will give way to a progressive, somewhat flattened,
shortwave for Saturday into Sunday. Model agreement is above
average likely owing to the amplified nature of the flow across
the CONUS but a couple of systems are worth noting.

Differences were noted with the second system forecast to amplify
across the Four Corners region on Friday and slowly translate east
through Sunday morning. Trends in the NAM/GFS/ECMWF have been to
slow the southern portion of the troughing across the Southwest
while the UKMET/CMC have been fairly steady on the slower side.
However, trends in the ensemble spaghetti heights show a
tightening toward the middle of the deterministic models best
represented by the 00Z GFS and 12Z ECMWF with the 12Z UKMET/CMC
slightly slower than preferred. The preference is to be toward the
middle of the spread for now.

Across the Northwest, ensemble trends have clearly shown a
flattening of the amplitude of the shortwave to reach the Pacific
Northwest Saturday over their past 4 runs. Most deterministic
models show the same trend but the 00Z GFS and 12Z UKMET remain a
bit more amplified compared to the consensus and the latest
spaghetti plots from the ensembles. A 00Z NAM, 12Z ECMWF blend
seems better here compared to the 00Z GFS but the 12Z ECMWF lies
on the southern side of most guidance with a surface low nearing
the central Canadian/U.S. border Sunday morning, with more weight
to the GFS/NAM preferred downstream into the northern Plains.

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

Otto