Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1146 PM EST Thu Jan 31 2019

Valid Feb 01/0000 UTC thru Feb 04/1200 UTC

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

00Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
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...Overall Pattern across the CONUS...
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Preference: 00Z GFS, 12Z ECMWF blend
Confidence: Slightly Below Average

The start of the short range period shows deep troughing across
the northeastern quadrant of the U.S. while ridging generally
prevails across the western U.S. with an active southern stream
jet and weakening closed low over CA/AZ. Mid-level heights will
rise across the eastern third of the nation while a series of
upper troughs causes heights to fall along and west of the Rockies
through Monday.

There are numerous systems to track across the lower 48, with a
00Z GFS/12Z ECMWF blend working for all of them despite poorer
than average run to run consistency and modest ensemble spread.

A southern stream compact closed low currently near southern
California is expected to open up and track east into the southern
Plains Saturday morning and into the Southeast Sunday morning. Out
ahead, a less organized shortwave is evident over the western Gulf
of Mexico. As these upper level features track east over a coastal
front forecast to set up along the Southeast Coast, cyclogenesis
should occur. There is better support for a defined surface low in
the latest ensemble guidance compared to yesterday, but model
agreement is poor. The model solutions closer to the coast are on
the western edge of the latest ensemble scatter low plots, with a
00Z GFS/12Z ECMWF blend preferred which keeps the developing
surface low offshore...though the 12Z ECMWF is closer to the coast
compared to the flatter/weaker 00Z GFS.

Out West, the 12Z ECMWF and its ensemble mean are displaced a bit
to the north and end up faster with the closed low/upper trough
into California and the Intermountain West for Saturday into
Sunday. The 12Z UKMET shows slower compared to the available
ensemble guidance and is considered a low probability outcome. The
00Z NAM/GFS are rather similar with the track/strength of the
low-mid level system and have been trending slower. This is in
contrast to the ECMWF which has been steady or slightly trending
faster. A compromise between the 00Z GFS and 12Z ECMWF is best for
now including as the upper trough ejects into the Great Plains and
allows for the development of a surface low forecast to track into
the upper Midwest late Monday.

Another potent West Coast system is expected by Monday morning but
with this one farther north into the Pacific Northwest. Better
agreement exists for this system compared to the one to cross
California, and there is better ensemble agreement. However, there
are timing differences with the 12Z CMC fastest and considered
least likely to verify. A middle of the road approach is preferred
at this time, which is represented by a 00Z GFS/12Z ECMWF blend.

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

Otto