Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Sun Sep 29 2019

Valid Sep 29/0000 UTC thru Oct 02/1200 UTC

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

00Z Final Model Evaluation with Preferences and Forecast Confidence
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...Overall Pattern Across the CONUS...
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Preference: General model blend, lower weight on GFS/CMC Day 3
Confidence: Above average

07Z Update: After examination of the 00Z Non-NCEP suite, it
appears little change is needed for the preferred blend. The CMC
continues to be quick to shed energy through southern Canada and
the GFS is still subtly lower with its 500mb heights compared to
consensus.

Previous Discussion:
The guidance continues to be extremely well clustered, at least
through 60 hours, in the highly amplified pattern with a trough in
the west and downstream ridge in the east. Beginning day 3,
Tuesday, spread begins to emerge with respect to the weakening of
the upper trough in the west. Nearly the entire suite shows the
core of the trough beginning to lift northeast while shedding
vorticity impulses in fast flow into Canada. The CMC once again
becomes a very fast outlier with energy lifting north of the Great
Lakes, and while this has little impact to the sensible weather in
the CONUS, it should be noted as the CMC has featured a fast bias
recently. The GFS operational at 00Z/29 also begins to outrun the
consensus mean and is well ahead of its mean. This may be due to
subtly lower overall 1000-500mb thicknesses across the CONUS, so
by day 3 the GFS should be used with caution. However, the GEFS is
in good agreement with the ECMWF/ECENS/UKMET/NAM, and is preferred
to the operational run in the blend.

Regardless of the exact evolution of the upper trough, a period of
heavy rainfall is likely as anomalous moisture rotates around the
western periphery of the ridge and lift is driven through an
increasingly intense 300mb jet across the middle of the country.
However, the exact placement of this axis of heaviest QPF will be
determined by the thickness packing across the Plains, so even
small longitudinal differences in the trough axis could have
significant implications in the placement of the swath of QPF
beginning D2.


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

Weiss