Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Mon Sep 30 2019

Valid Sep 30/0000 UTC thru Oct 03/1200 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 through 48 hours,
ECMWF/ECENS/UKMET/GEFS 48-84 hours
Confidence: Slightly below average

07Z Update:
As seems to have been the case the past several days, the 00Z CMC
comes into much better agreement with the global consensus, but
its lack of consistency run-to-run keeps it out of the preferred
blend at this time. Differences in the mass fields become quite
significant by D3 across the west as a longwave trough
re-amplifies behind the D1-2 system lifting off towards the Great
Lakes. For consistency will maintain the previous blend, but
differences in the speed and amplitude of this second trough will
be important to timing a cold front and QPF moving onshore late in
the period.


Previous Discussion:
Guidance continues to feature little spread through 48 hours in
its evolution of a large scale trough in the west and downstream
amplified ridge in the east. Thereafter, spread increases quickly,
and the forecast confidence beyond D2 decreases rapidly.

The divergence in the mass fields begins with the deamplification
of the longwave trough in the west. As this features begins to
fill, it lifts northeast and weakens, but at the same time tilts
neutrally and eventually negatively. The speed and latitude at
which this occurs makes considerable difference in the sensible
weather, especially across the middle of the CONUS. The CMC lags
all the other guidance and the consensus mean in filling the upper
low, in some instances as much as 18-24 hours, and therefore its
downstream evolution is out of phase with the global suite. On the
other end of the spectrum, the operational 00Z/30 GFS is much
faster than the global mean, and begins to tilt its 500mb trough
negatively into the western Great Lakes D3. This is fast and
southeast of the consensus, but is not too far removed from the
00z/30 NAM. This appears to be in response to slightly lower
thicknesses within the ridge allowing for a faster and more
southern progression of this trough. The GFS is also far outpacing
its ensemble mean, which is better in line with the
UKMET/ECMWF/ECENS to create the preferred blend.

A heavy rain event still looks likely across the middle of the
country the next 3 days. There are some timing and spatial
differences in the guidance associated with the discrepancies
noted above, but in general there is increasing confidence in
significant QPF from New Mexico into the Great Lakes the next few
days.


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

Weiss