Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
204 PM EST Thu Dec 20 2018

Valid Dec 20/1200 UTC thru Dec 24/0000 UTC

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

12Z Model Evaluation...with Final Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The upper level flow pattern across the lower 48 through the
weekend will undergo changes with a transition from highly
amplified today (strong trough axis across the central U.S. and
anomalous ridging western U.S.) to more zonal-like for Sunday with
a series of progressive shortwaves tracking eastward from the West
Coast. confidence is a little below average given shifts noted
across the West since yesterday with a fair degree of spread in
the latest ensemble guidance remaining.

The best agreement is with the exiting longwave trough axis across
the eastern U.S. through Saturday with a general model blend
preferred given only minor differences remain. Remaining systems
across the lower 48 with more significant differences are
separated out below:

...Mid-level shortwave over the Great Basin Friday night and
tracking into the Ohio Valley on Sunday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS/12Z ECMWF blend
Confidence: Average

Run to run consistency is better than its been with this system
but the 12Z UKMET trended much deeper than its 00Z cycle. The 12Z
ECMWF trended faster compared to its 00Z cycle which was the
slowest and most amplified of the recent guidance. The 12Z UKMET
lies on the deeper edge of the ensemble spread but is not a
complete outlier. However, given the transition of flow to a more
progressive nature across the upstream Pacific with shorter
wavelength troughs, any of the slowest/deepest solutions are not
favored downstream. On the other hand, the more progressive 12Z
NAM/GFS could be too fast based on the latest suite of guidance.
The ensemble means support a middle ground but toward the faster
side of the guidance, best represented by a 12Z GFS and 12Z ECMWF
blend.

...Pacific Northwest Saturday into Sunday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS/12Z CMC/00Z ECMWF blend
Confidence: Below Average

There is low confidence across the Pacific Northwest given high
run to run variability and poor model agreement. The model
consensus is for an amplifying, negatively tilted shortwave to
approach the Pacific Northwest late Saturday with an accompanying
surface cyclone. Differences in the details are large though with
the 12Z GEFS mean showing troughing just offshore, the 00Z ECMWF
mean with shortwave ridging just offshore and 09Z SREF mean with
zonal flow across the coast. The deterministic guidance shows
various subsets of the solutions shown by the ensembles. Ensemble
scatter low plots and spaghetti height fields at 500 mb are large
with the low amplitude shortwave, but perhaps is best represented
by a 12Z GFS/12Z CMC/00Z ECMWF blend...also lying in the middle of
the latest deterministic guidance. The 12Z UKMET is a
deterministic outlier and the 12Z ECMWF trended a bit deeper with
troughing along the West Coast but prefer to stay toward the
middle ground.


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

Otto