Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
321 AM EDT Thu Sep 05 2019

Valid Sep 05/0000 UTC thru Sep 08/1200 UTC

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

00Z Final Model Evaluation, Confidence, and Preferences
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...Overall Pattern Across the CONUS...
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Preference: GEFS/ECMWF
Confidence: Average through 60 hours, below average for day 3

07Z Update: The biggest challenge remains the considerable spread
in the evolution, both in timing and amplitude, of a trough
digging into the West on day 3. The 00Z Non-NCEP suite did little
to squelch the uncertainty, although the CMC and UKMET remain
strong outliers. Will maintain the previous preferences and just
update to the newest guidance, as there at least is run to run
consistency in these models.

Previous Discussion:
While the guidance shows considerable agreement in the synoptic
evolution through the first 60 hours, it diverges considerably
thereafter, most notably in the west.

Through 60 hours, a large ridge will persist across the Mountain
West, with a trough continuing in the east. A shortwave dropping
SE into the Great Lakes Thursday night into Friday will eventually
be the kicker to lift Hurricane Dorian quickly away from the
eastern seaboard. The guidance is in very good agreement in timing
and amplitude of this shortwave, with the only exception being a
subtly deeper/faster impulse noted by the 00Z GFS. This remains
within the ensemble envelope and has minimal spread overall, so a
general model blend can be used for this feature.

Two additional shortwaves will then traverse from the PacNW atop
the Mountain West ridge and into the Northern Plains. The first of
these deamplifies quickly and shows little amplitude by the time
it reaches the Plain. A much more significant feature follows
behind it as a mid-level low in the Gulf of Alaska drops southward
and opens into a longwave trough along the Pacific Coast. This is
where guidance begins to split significantly. The UKMET is
exceptionally fast weakening this feature and lifting it eastward
into the Plains, so much so that by day 3 it is 180 degrees out of
phase with the global consensus. The CMC is also quite fast, not
as much as the UKMET, but at the very edge of the ensemble
envelope. The GFS/ECMWF are in better agreement, and comparable to
their means, but still feature different amplitudes and
wavelengths of this trough as it opens to the east. The most
clustered guidance appears to be a blend of the GEFS mean and the
ECMWF, but confidence is below average. However, with the
agreement of these two into day 3, and minimal spread prior to 60
hours, a GEFS/ECMWF is suggested for consistency across the CONUS
for all 3 days.


...Hurricane Dorian...
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Please refer to the official National Hurricane Center forecast
track

With respect to the 03Z NHC Advisory track, a global consensus is
sufficient through about 36 hours. Thereafter, most of the
guidance is a bit south of the NHC track, although the 00Z
Non-NCEP suite did speed up a bit from the 12Z, and is better
matched now to the NHC forecast points. While a general blend is
sufficient to produce a best proxy to the track, for consistency a
GEFS/ECMWF/UKMET is still preferred for the best track compared to
the 03Z advisory positions.
 

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

Weiss