Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Tue Sep 11 2018

Valid Sep 11/1200 UTC thru Sep 15/0000 UTC

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

12Z Model Evaluation Including Preferences and Forecast Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Pacific Northwest...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS/ECMWF
Confidence: Slightly below Average

18Z Update: 12Z UKMET continues to be fast with several shortwaves
shedding into the Pac NW from the parent vortex across Canada.
While the previous CMC was reasonable with its agreement on the
trough development across the west, it is now the weakest and
least amplified with the 250mb jet dropping southward across
Alaska, causing its position of the longwave trough to be well
east of the rest of the guidance envelope. For this reason the
preferred blend has now removed the CMC for a GFS/ECMWF blend.

Persistent trough across the northwest will be reinforced by
shortwaves shedding from the main vortex across far northern
Canada. The NAM/UKMET continue to be strong and fast with the
shedding energy which leads to more rapid ejection of a surface
low through ND/MN into Manitoba on Thursday, The overall frontal
position which lags back into the NW is relatively unaffected
however, but still suggests a non NAM/UKMET blend for this portion
of the forecast. Late in the period, Friday, spread increases
significantly with the trough entering the NW. UKMET continues to
be faster than the other guidance due to its weaker phasing
attempt of the energy between the closed low across Canada and
mean trough near British Columbia. The blend for the approach of
this feature will remain unchanged despite the NAM agreement at 84
hours due to its differences earlier in the week.


...Trough approaching Texas...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS, 12Z ECWMF/UKMET blend
Confidence: Average

18Z Update: 12Z CMC remains fast and well west of the ensemble low
placement spread by Friday morning while the UKMET is still in
line with the rest of the guidance. No changes made to the
preferred blend.

A trough in the eastern Gulf beneath the large scale ridge across
the east will move into Texas on Friday. The CMC is much faster
than the remaining guidance suite, and with weak steering flow
appears to be the outlier. The NAM continues its trend of being
too strong with many of the features, this trough included, and is
also not preferred despite similar timing to the rest of the
suite. UKMET/GFS/ECMWF are well within a reasonable guidance
envelope, with the 06Z GEFS mean on top of the 12Z GFS operational
lending support to this solution.


...Hurricane Florence...
~~~~~~~~~~~~~~~~~~~~~~~~~
Preference:  See NHC forecast discussion
 Best Proxy: 06Z GFS Parallel, 00Z ECMWF, 12Z GFS blend

06Z GFS parallel (FV3) and 00Z ECMWF are now matched with the
recent 12Z GFS such that a consensus of these three produces a
track most closely resembling the official NHC forecast track for
Florence. Please refer to the latest NHC discussions and products
for more detailed information.

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

Weiss