Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1238 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 NAM/GFS/UKMet Evaluation w/ Prelim Model Preferences and
Confidence
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...Central/Eastern US...
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Preference: A blend of the 00Z UKMet, 12 ECMWF, and 12Z CMC
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 is most
negatively-tilted with the upper trough by Friday, thus is the
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,
with at this point a lean toward the 00Z UKMet, 12Z ECMWF, and 12Z
CMC, which  parallel GFS (FV3) which seemed to align more closely
to a blend of the ECENS/GEFS means.

Over the central U.S., with 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 and UKMet
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 blend was
preferred, as the NAM was flatter with the wave into the Great
Basin compared to the rest of the guidance, while both the CMC and
ECMWF were on the more amplified edges of the guidance spread. In
addition, the 12Z ECMWF was fastest with the trough by 12Z Sat
(into the eastern Great Basin), while at the same time the CMC was
the slowest among the rest of the guidance.

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

Hurley