Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
235 PM EDT Thu Sep 13 2018

Valid Sep 13/1200 UTC thru Sep 17/0000 UTC

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

12Z model evaluation including preferences and confidence
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...Overall synoptic pattern across the CONUS...
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Preference: 12Z GFS/GEFS/ECMWF blend
Confidence: Average

17Z Update: 12Z CMC has come into better agreement with the
amplification and timing of the several shortwaves ejecting
through the mean trough across the Pacific NW, and is well
clustered within the ensemble envelope with the surface low
positions across the Great Plains Sunday morning. The UKMET is
also usable for the Pacific NW, but for consistency and agreement
with the ensemble means, the preferred blend will change only to
add the 12Z GEFS mean and incorporate the 12Z ECMWF.

Previous discussion:
Persistent longwave trough will deepen through a series of
shortwaves digging into the Pacific NW. This has the effect of
keeping the large ridge across the east in place through Sunday.
Mass fields are in generally good agreement with the exception of
the CMC which sheds a less amplified shortwave into the Pacific NW
early Saturday which is must faster than the rest of the guidance
suite. This faster/less amplified trough then persists through the
remainder of the period of interest causing the CMC to start to
become out of phase late in the weekend. The NAM is is in relative
agreement early, but by Sunday its shortwave energy sheds eastward
too quickly and is not phased with the sharpening mid-stream
trough as shown by the GFS/UKMET/ECMWF. A blend of the 12Z GFS /
00Z ECMWF is the best choice for the blend for consistency and
agreement.


...Western Gulf coast trough/low nearing south Texas Friday
morning...
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Preference: 12Z GFS/12Z ECMWF/12Z UKMET blend
Confidence: Slightly above average

17Z Update: 12Z ECMWF is aligned temporally with the
GFS/GEFS/UKMET so no changes needed for the preferred blend.
Please continue to refer to the latest NHC discussions and
products for more detailed information on this system.

Previous Discussion:
Note similar discrepancies in the guidance evolution of the
feature in the Gulf of Mexico approaching Texas. NHC has a 50
percent chance of development over the next few days as it lifts
NW towards the Texas coast. The CMC continues to be a fast outlier
with this feature while the NAM is several mb stronger than the
remaining operational models. The inherited blend is unchanged
other than to update for the 12Z GFS.

...Hurricane Florence...
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Preference:  12Z NAM/12Z GFS/12Z UKMET closet to 15Z NHC advisory

Most models are in good agreement before landfall but diverge
thereafter such that an initial best proxy to the latest NHC track
is a consensus of the 12Z GFS/NAM/UKMET which places the track
somewhere in the middle of those three. Please refer to the latest
NHC discussions and products for more detailed information on
Hurricane Florence.

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

Weiss