Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1135 PM EST Thu Dec 20 2018

Valid Dec 21/0000 UTC thru Dec 24/1200 UTC

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

00Z Model Evaluation...with Final Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00z NAM and 12z ECMWF/ECENS mean/18z GEFS mean blend
           *00z GFS* included east of the Rockies
Confidence: Average to slightly below average (especially in the
Intermountain West)

Large full latitude trof is starting to lift out of the Southeast
later today exiting fully by midday Sunday across the Northeast
with much tighter model agreement including internal smaller scale
waves/interactions, to suggest a general model blend would be a
high confidence solution for this system.

With its exiting, the pattern will shift across the CONUS as
strong Pacific energy will be quite progressive across the West
into the Central US with a more zonal pattern look to the overall
flow.  The static larger scale trof across the Northern Gulf of AK
will shed shortwave/jet streak energy into the Pacific Northwest
12-24 hours or so.  This pattern is very chaotic and has low
fidelity in predictability mainly in timing of these wave features
but also some times in amplification.  We have seen this in the
last 4-5 cycles of the ensemble suite especially along the 546
through 552 dm spaghetti analysis, where each model and ensemble
suite members cannot latch onto specific timing/depth of the waves
particularly after crossing the Cascades into the Intermountain
West/Rockies and eventually into the Central Plains. 

The current system/tropical moisture plume is the best handled
system now fully into the best sampling network crossing the
Canadian Rockies into the Southern Prairies by early Sat.  Here
the 12z CMC is quick, south and generally weak an the 12z UKMET
appears a bit north.  The next wave shed out of the Gulf of AK
into the Pacific Northwest later today shows over amplification in
the UKMET, likely due to some spacing/enhanced outflow from the
prior system being too far north, as it treks through the Rockies.
 As such a Non-CMC/UKMET solution is preferred for this next pair
even through the Great Lakes/Ohio Valley in the late weekend/early
workweek. 

The next progressive wave (Sun/Mon into the NW) has trended
favorably in ensembles toward a common solution with a strong
surface cyclone reaching Vancouver island late Sat into Sunday and
directing broad moist/onshore flow into WA/OR/N CA terrain,
however begin to devolve after crossing the Cascades into the
Intermountain West. Here traditional negative model bias begin to
manifest a bit more with the 00z NAM/GFS fast, 12z ECMWF slow. 
The 12z UKMET is strong and quick continuing with less preference
here.  The 00z GFS has backed off a stronger/more amplified
trailing surface wave/moisture slug entering S OR/N CA toward 00z
Monday than the 12z/18z runs but this solution continues to be on
the fringes of the ensemble suite and away from the mean overall. 
Here, while possibly too slow the ECMWF and CMC and a mix of the
00z NAM seem a good compromise with inclusion of the ECENS/GEFS
mean.  Given strong but progressive and unstable run to run
continuity...confidence is below average for this wave.

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

Gallina