Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1258 AM EDT Fri Sep 27 2019

Valid Sep 27/0000 UTC thru Sep 30/1200 UTC

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

00Z Preliminary Model Evaluation with Preferences and Forecast
Confidence
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...Overall Pattern Across the CONUS...
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Preference: General Model Blend, limited weight on the NAM D3
Confidence: Above average

Guidance is in exceptionally good agreement in the evolution of
the longwave pattern through the next 3 days. In the west, an
anomalously strong trough reaching as low as -3 standard
deviations with respect to 500mb heights will dig slowly southward
across WA/OR, while ridging is forced downstream to expand across
the eastern half of the CONUS. Across the east, differences among
the guidance in amplitude and small scale features are negligible.

With respect to the large scale trough in the west, subtle timing
and amplitude differences exist in shortwaves calving off the main
trough and ejecting northeastward through Saturday. However, the
differences in the first 60 hours are minimal and thus a general
model blend is acceptable. By day 3 /Sunday/ the upper trough
begins to fill and lift slowly northeast. The NAM is a bit fast
ejecting this feature out of the NW, which causes its associated
surface low and frontal positions to be well northeast of the
remaining suite and consensus by the end of the forecast period.
The trend has been for a subtly faster/northward solution with
this evolution, so the NAM cannot be entirely ruled out, but
should feature less weight Sunday due to its outpacing of the
remaining, well clustered, guidance.


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

Weiss