Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Thu Oct 04 2018

Valid Oct 04/1200 UTC thru Oct 08/0000 UTC

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

12Z Model Evaluation...with Final Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: Blend of the 12Z GFS/UKMET/CMC/00Z ECENS mean/12Z GEFS
mean
Confidence: Slightly above average

19Z update: The 12Z ECMWF/UKMET/CMC all trended quicker with the
shortwave crossing the north-central U.S. Friday night, but with
larger shifts seen in the ECMWF. This adjustment from the ECMWF
brings the position of the cold front closer to the preferred
consensus but the 12Z ECMWF remains north of the better clustering
over Illinois/Indiana on Sunday morning with the frontal boundary.
Better clustering lies near the 12Z GFS/UKMET/CMC which agree best
with the latest ensemble means.

Across the West on Sunday, the latest 12Z deterministic guidance
shows only minor differences with timing and perhaps larger
differences with latitude by 00Z/08. The 12Z NAM/GFS are a bit
farther south with the core of the trough/closed low, with the 12Z
CMC farthest north and the 12Z ECMWF/UKMET in the middle. A
general blend here is preferred, but with the 12Z CMC given the
least weight given the amplified nature to the pattern and better
agreement for more troughing out West in the 12Z guidance.

...previous discussion follows...

A series of shortwave troughs will traverse the western U.S. over
the next few days and then eject/shear out across the
central/northern Plains. This will include an initial southern
stream shortwave near the Four Corners region Thursday afternoon,
which will continue to weaken as it nears the Great Lakes region
early Friday morning. Meanwhile, at least two northern stream
shortwaves will cross the Pacific Northwest with the latter piece
of energy expected to amplify Saturday and Sunday rather
vigorously over the Intermountain West and setting the stage for a
deep longwave trough over the western U.S. which will include a
closed low evolution by Sunday evening. Across the East, all of
the models show the persistence of a strong subtropical ridge,
currently over the Southeast but translating to the Mid-Atlantic
coast through Sunday.

There are two areas to mention regarding differences. The first is
with a northern stream shortwave expected to reach the northern
Plains Friday night and its related surface low/cold front. The
00Z ECMWF appears to be too slow here given trends in the
ensembles and their agreement for a somewhat faster timing, nearer
to the 12Z GFS. The related cold front, stretching from the Great
Lakes to the southern Plains on Saturday is slower/more-north in
the 00Z ECMWF compared to the remaining model consensus. The 12Z
NAM is faster with the north portion of the cold front into the
Northeast, and is considered an outlier. However, the latest
ensemble means are in good agreement and are slightly slower than
the 12Z GFS, so a blend of the 12Z GFS with the ensemble means is
preferred.

Out West, as a closed low sets up over the Intermountain West, the
latest ensemble means show good agreement with the ECMWF mean
rather consistent on timing valid Sunday, considering its past 4
cycles. The 00Z UKMET was quicker to take the low toward the east,
which is contrary to spaghetti height trends converging near the
ECMWF mean. The 12Z GFS aligns well with the means regarding the
closed low and also with thermals being slightly colder than the
00Z ECMWF over the central and northern section of the country.

Overall, a 12Z GFS/00Z ECMWF mean/06Z GEFS mean blend works best
for all portions of the country.

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

Otto