Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1122 PM EST Wed Dec 19 2018

Valid Dec 20/0000 UTC thru Dec 23/1200 UTC

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

00Z Model Evaluation...with Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Eastern third of the US through Sat/Northeast Sunday
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00z GFS/12z ECMWF blend
Confidence: Average

A full-latitude neutral tilt trof begins to mature by late tonight
across the eastern third of the CONUS loading heavily to the base
across the lower MS River valley into the Southeast; spurring
cyclogenesis out of the central Gulf and up the spine of the
Appalachians.  Some model differences start to unfold with this
initial height-fall surge/surface wave, before the secondary and
stronger surface wave lifts east of the mountains through the
Carolinas Friday into the Mid-Atlantic by Saturday. The 00z NAM
has trended favorably to a more stable/run to run consistent
solution presented by the GFS/ECMWF and their respecting ensemble
means; however, typical of bias is a bit stronger with the initial
wave/cold punch across the Great Lakes.  The 12z UKMET is similar
and less favorable to include in this portion of the country,
particularly with the QPF solutions on the colder side of the
overall cyclone.   Confidence in a GFS/ECMWF solution is fairly
good, but with such a large system, small scale internal
interactions are likely to remain moderately variable to have full
confidence especially toward the end of the forecast period as the
wave lifts into New England.


Northern Plains Fri/Sat...Great Lakes Sat/Sun
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00z GFS and 12z ECWMF/UKMET blend
Confidence: Slightly above average

Upstream, a less amplified shortwave slides through the northern
Rockies into the Central Plains/Southern Prairie Providences, and
with downstream blocking swings the base through remaining nearly
stationary with the fulcrum/pivoting vort center in Canada.  Here
guidance is fairly agreeable with exception of the CMC which is a
bit more aggressive sliding energy southeast into the shortwave
ridge into the Midwest/NW Great Lakes.  The 00z NAM becomes a bit
more amplified with a compact closed wave into late Saturday as
well. 


Great Basin/Central Rockies into Central Plains
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00z GFS/12z ECMWF blend
Confidence: Slightly above average

Further upstream, the progressive pattern emerging from the
Pacific continues, flattening the ridging across the Rockies with
the aforementioned trof and shedding some weak negative tilt jet
energy into this increasing zonal flow.  The 12z UKMET and CMC
both suggest greater shortwave amplification through the Great
Basin and Central Rockies into the Great Plains, while the
GFS/ECMWF agree with the existence of this wave, the greater
amplification seems less likely in the larger scale evolution
especially without more impressive downstream ridging.  As such
WPC gravitated toward the GFS/ECWMF solution for this portion of
the country and given agreement between these solutions and fairly
stable run to run ensemble suite with this blend; confidence is
slightly above average. 


Pacific Northwest Late Sat/Sun
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00z GFS/NAM and 18z GEFS/12z ECENS mean blend
Confidence: Below average

Progressive/strong Pacific pattern continues to try to establish a
solid zonal flow into the Pacific Northwest/SW Canada as strong
broad closed low takes up residence in the northern Gulf of
Alaska.  Models struggle with timing of next jet streak/shortwave
and how consolidated the streak is.  The 12z UKMET/CMC are
quickest with nose of the jet and support a flatter/longer
wave-length solution entering the area toward the end of the
forecast period.  On the other side of the ensemble suite, the 12z
ECMWF is once again suggesting a slow jet approach and significant
amplification to the wave and more importantly furthest south. 
Even within the ECENS suite, this is in the southern third of
solutions.  While, run to run ECENS members have been consistent
in the large variation, the operational keeps flip-flopping back
and forth of this amplified/southern track making it a very low
confidence solution especially considering the the GEFS/CMCE
members.  The 00z NAM and GFS both trended to a more amplified
solution, similar to the ECMWF, but further north directed toward
Vancouver Island/Olympic Peninsula, while the GFS also trended
much faster, typical of a negative bias.  The GEFS/ECENS mean
solutions appear a good compromise and the NAM/GFS seem a good but
not great representation of the solutions (think GFS is too fast).
 As such a 00z NAM/GFS and 18z GEFS/12z ECENS mean blend is
preferred but a below average confidence.


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

Gallina