Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
240 PM EDT Wed Sep 26 2018

Valid Sep 26/1200 UTC thru Sep 30/0000 UTC

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

12Z Model Evaluation...with Preferences and Forecast Confidence
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...Overall Preference for the Entire CONUS...
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Preference: Non-NAM blend
Confidence: Slightly above average

19z Update: No major changes with respect to the mean longwave
trough across the central US, with slightly better agreement with
the 12z ECMWF/CMC/UKMET toward the 12z GFS; this makes the NAM
appear more amplified across the Upper Midwest and Great Lakes
with the tighter agreement elsewhere. So, overall a non-NAM blend
for central/eastern US.

For the West Coast, the CMC trended slower with respect to its
northern stream energy across BC and the 12z ECMWF trended slower
to the closed low moving into CA/Pac NW, more in line with the
other guidance. So, a non-NAM blend is preferred.

---Previous Discussion---
Overall, 12z model guidance continues to show a relatively static
pattern for the end of the week into the first part of the weekend
with the synoptic features across the CONUS. A broad longwave
trough will spread across the northern tier US with a Rex block
off the West Coast slowly breaking down by the end of Day 3
(Sunday 00z). At the surface, a lagging frontal boundary will wash
out along the Gulf Coast with periodic shortwaves riding up
through the Southeast US into the mid-Atlantic Friday.

Relatively good agreement among the guidance through the period
with just minor differences in the 500mb heights with the longwave
trough across the northern tier states. The 12z NAM continued the
trend with its over amplification (resulting in higher QPF) across
the Southeast US and Mid-Atlantic region and was not preferred for
this cycle.

Across the West Coast, the 00z ECMWF was a bit faster breaking
down the Rex block thus allowing the closed low moving inland and
being absorbed into northern stream energy by the end of Day 3
(Sun 00z).

So, overall, a general model blend is preferred across the bulk of
the CONUS through the short-term forecast period, with exclusion
of the 12z NAM blend across the southern US/Mid-Atlantic
27/18z-29/00z (Thurs-Fri night) as well as the excluding the 00z
ECMWF along the West Coast/Pacific NW 29/12z and after. 
Confidence is fairly high in this preference, ie. slightly above
average.

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

Taylor