Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
135 PM EST Thu Dec 27 2018

Valid Dec 27/1200 UTC thru Dec 31/0000 UTC

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

12z Model Evaluation...with Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

19z update: No significant updates were noted with the rest of the
12z guidance coming (ECMWF, CMC, UKMET) with respect to the large
scale pattern and mass fields. The QPF differences seen earlier
across the southeast US have come into better agreement. As such,
a general model blend across the CONUS should suffice for the
forecast period.

---previous discussion---
A closed low over the central Plains will lift northeast today
followed by another shortwave trough digging into the southwest US
tonight into Friday. Through the weekend, the longwave trough over
the central US will remain in place taking on a positive tilt as
the subtropical ridge over Florida and the Caribbean holds or
builds further. For the Pacific Northwest, ridging initially will
then be replaced by a shortwave trough approaching the area
Saturday into Sunday. More amplification of the upper level
pattern is expected by the end of the weekend as a ridge
strengthens over the eastern Pacific.

Through 60 hours, forecast guidance mass fields are in very good
agreement and a general model blend will suffice. Beyond that,
more differences develop across the Pacific Northwest with respect
to the shortwave moving onshore Saturday/Saturday night. The GFS
was a bit flatter compared to the ECMWF and CMC solutions, while
the UKMET was more pronounced as the shortwave dug into the
Northwest. At the surface and with QPF fields, the models actually
show better agreement spatially and temporally.

Across the central/eastern US, much of the differences lie with
the QPF across the southeast US. The 06z GFS has its axis of
heaviest rainfall extending northward into the mid-Atlantic region
and is much higher compared to the ECMWF/CMC. It is worth noting
that the GEFS mean is also showing higher amounts and is not too
significantly different than its operational deterministic run.
The CMC looks too fast with the line of showers/storms tracking
east on Friday and also looks too light on amounts given the plume
of moisture expected. The UKMET/ECMWF and to some degree the 12z
NAM appear to be a good compromise (though the NAM might a tad too
slow).

By the end of the forecast period, another storm system is poised
to bring back moisture into the mid-MS River Valley and TN Valley.
There is a variety of QPF solutions, but the CMC appears the most
of an outlier. All told, for the WPC blend, a blend of the 00z
ECMWF/ECENS, 06z GFS/GEFS mean, and 12z NAM is preferred.


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

Taylor