Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
303 PM EST Thu Nov 29 2018

Valid Nov 29/1200 UTC thru Dec 03/0000 UTC

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

12Z Model Evaluation...with Preferences and Confidence
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...Central U.S. Storm and Downstream Pattern into the East...
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Preference: A blend using any of the 12Z deterministic guidance,
but with primary emphasis on the ECMWF
Confidence: Average

A compact shortwave trough is expected to briefly take on a
negative tilt as it ejects from the Rockies. It will then go
through a fairly quick life cycle as a mature low - driving winter
weather impacts in the northern High Plains and central Plains to
Upper Midwest. The models are in seemingly good agreement,
especially through most of the impact period, Days 1 and 2.
Occasional runs of the ECMWF, Canadian, and NAM tuck the surface
reflection up under the low a little farther west and a little
stronger than other guidance. The NAM thermal fields are slightly
colder than other guidance, with cooling taking place at a more
rapid rate, such as over western Nebraska. These are very real
possibilities, however, with a dynamic negative-tilt wave, and one
in which even ensemble consensus takes the surface pressure down
under 990 mb. We would not be surprised to see the cyclone verify
toward the strong end of the envelope. The 12Z ECMWF did trend
back toward consensus, though, in an overall sense, including the
Day 3 phase when the system begins to fill. Given the ECMWF is
better preferred in handling the upstream flow patter over the
Western states, we place emphasis on ECMWF in our preference, but
a consensus blend may do well.


...Western U.S...
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Preference: Blend of 12Z ECMWF / UKMET / Canadian
Confidence: Slightly below average

Overall when considering the full CONUS and the eastern Pacific,
the forecast carries quite a bit of uncertainty with respect to
shortwave details. Consistency run to run has not been great. NCEP
guidance wants to flatten the flow on Days 2 and 3, with broad
cyclonic curvature over the west, and generally warmer heights
than the non-NCEP guidance. Looking at trends in the ensemble
spaghetti plots has not proven too helpful in this case, as the
ECMWF and Canadian have become more emphatic in their lower
heights in the Southwest on Day 3, whereas the GEFS has become
more emphatic on warmer heights and absence of a marked vorticity
center there at that time. This is all related to a somewhat
jumbled mess of shortwave interactions in multiple streams over
the Pacific. Based on synoptic thinking, we would argue for
continuity of the shortwave digging into the Southwest at 02/00Z
so that a distinct shortwave trough will dig into the 4-corners
region and eject to the southern Plains on the heels of the
departing Days 1-2 cyclone. The trend in the height field may be
slightly toward NCEP guidance, but the GFS certainly looks too
warm with flat flow in the western trough on Day 3, and the NAM is
also pretty solidly outside the non-NCEP consensus, in addition to
having a poor Day 3 track record for synoptic features.

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

Burke