Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1218 PM EST Wed Feb 21 2018

Valid Feb 21/1200 UTC thru Feb 25/0000 UTC

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

...12Z NAM/GFS evaluation including preliminary preferences...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--***Note that data flow issues at WPC/NCWCP continue to affect
the guidance available to be used as input into this discussion.
The preferences referenced in this preliminary discussion are
between the 12Z NAM and 12Z GFS***


...Frontal boundary current stretching from the Northeast to
Texas...
...Weak surface wave tracking through the upper MS valley and Ohio
valley on Friday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 70% 12Z GFS, 30% 12Z NAM blend
Confidence: Average

The 12Z NAM and GFS are rather similar to one another with the
evolution of the front, but the 12A NAM is a bit stronger than the
12Z GFS. The 12Z NAM is a little stronger with the surface wave
crossing the upper MS valley but otherwise no major differences
were noted.


...Shortwave crossing the Four Corners and ejecting into the
Plains Saturday morning...
...Surface cyclogenesis and related cold/warm fronts through the
Plains on Saturday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 70% 12Z GFS, 30% 12Z NAM blend
Confidence: Below Average

Similar to the above referenced system, the 12Z NAM and GFS are
similar to one another with the evolution of the front, but the
12Z NAM is a noticeably stronger than the 12Z GFS at 500 mb for
Saturday. The 12Z NAM is also little northward displaced with the
warm front extending across the Ohio valley on Saturday. Surface
low placement near Iowa/Missouri by 00Z/25 and cold frontal timing
through the MS valley into Texas was similar between the NAM/GFS
stronger with the surface wave crossing the upper MS valley. More
weight to the 12Z GFS is recommended overall with this system, but
both the 12Z NAM/GFS remain stronger with the surface low and 500
mb shortwave compared to older runs of the ECMWF/UKMET/CMC.


...Fast moving shortwave reaching the Pacific Northwest on
Friday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z NAM
Confidence: Average

Previous guidance along with the 12Z NAM and 12Z GFS show the 12Z
GFS as faster with the progression of the shortwave into the
Northwest on Friday. Trends in the NAM and GFS also show a flatter
wave compared to yesterday's runs, but still with more amplitude
than the previously flatter 00Z/20 ECMWF. Given an apparent
consensus for a slower shortwave than seen in the 12Z GFS, the
preference will be given to the 12Z NAM.


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