Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
230 AM EST Wed Dec 19 2018

Valid Dec 19/0000 UTC thru Dec 22/1200 UTC

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

00Z Model Evaluation...with Final Preferences and Confidence
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...Central/Eastern US...
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Preference: A blend of the 00Z ECMWF and UKMet
Confidence: Below average (especially days 2/3)

The upper level pattern will become increasingly more amplified
over the central/eastern US through the forecast period. A
deepening trough over the central US will take on a negative tilt
by late Thursday-Thursday night, likely closing off a low near the
southern Gulf Coast, then become even more negatively tilted as it
lifts through the mid-Atlantic region. In general, the spread
among the guidance, both at the surface and aloft, is a bit
greater than desirable for a 48-84 hr forecast -- in large part
owing to the differences with the timing and degree of negative
tilt with the mid-upper trough. The 00Z operational GFS and CMC
are most negatively-tilted with the upper trough by Friday, thus
are farthest left (west) with the surface low track up the spine
of the Appalachians. The NAM meanwhile at 00Z continued to exhibit
poor run-to-run continuity from its previous few runs, and is a
notable outlier with an initial sub 992mb surface low lifting into
western PA/NY by 12Z Fri. Given such spread in the guidance,
forecast confidence is below normal here in the eastern CONUS. The
preference at this juncture is a blend of the 00Z ECMWF and UKMet,
which aligned more closely to a blend of the ECENS/GEFS means.

Over the central-northern Plains, with respect to the next upper
shortwave and surface trough migrating east of the
central-northern Rockies Friday night, a non-GFS/non-NAM blend is
preferred, as the operational 00Z GFS and NAM continue to be on
the faster edges of the guidance spread (including the ensemble
means), despite their amplification with the upper system.


...Pacific Northwest-Northern California into the Northern
Rockies...
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Preference: A blend of the 00Z GFS, UKMet, and ECMWF
Confidence: Average

A pair of fast-moving shortwaves will push quickly eastward toward
the end of day 2 and into day 3. As noted above, the NAM and GFS
are faster compared to the rest of the guidance with the initial
shortwave reaching the Central Plains by Friday night; however,
before that time a general model compromise appears reasonable as
the initial trough traverses the PAC NW - northern Intermountain
West - northern/central Rockies.

With respect to the second system coming ashore late Fri and
reaching the central Great Basin by 12Z Sat, a GFS/UKMet/ECMWF
blend was preferred, as the NAM was flatter with the wave into the
Great Basin compared to the rest of the guidance, while the CMC
was slowest and more amplified with the southern stream wave.
Compared to its 12Z run, the 00Z ECMWF compared more favorably to
the GFS and UKMet, along with yesterday's 00Z ECMWF, with a
notably flatter mid/upper shortwave into the Great Basin. As a
result, the ECMWF solution was included in the final model
preference.

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

Hurley