Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
234 PM EDT Wed Apr 18 2018

Valid Apr 18/1200 UTC thru Apr 22/0000 UTC

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

12Z Model Evaluation...With Preliminary Preferences and Confidence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...Deep occluded cyclone over the Northeast, gradually exiting
through the Canadian Maritimes Thu...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

Models continue to be in very good agreement with this system as
it departs the CONUS today, with very little spread. Therefore, a
general model blend is preferred with high confidence.


...Closed mid level low over Iowa today tracking across the Ohio
Valley and Northeast Thu and Thu night...
...Surface low moving across the Ohio Valley, with secondary
development off the New England coast Thu...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

Despite some small spread related to the interaction of the
mid-level low with a trough to the northeast, model forecasts are
similar enough that a general model blend is preferred. There is
no consistent bias that would favor one model over another, and
the 12Z NAM and GFS are reasonably close to the existing consensus.


...Long wave trough crossing the California coast Wed night, which
closes off over the Great Basin and Southwest Thu into Fri...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 00Z ECMWF; 00Z CMC; 12Z NAM
Confidence: Average

There has been a slight adjustment by the models over the past
couple model cycles to be a little faster with the progression of
the closed low over the Southwest. The distribution of model
spread has generally not changed, but all the models nudged the
low forward by a small amount. Therefore, the 12Z GFS remains on
the fast end of the spectrum, with the 00Z ECMWF and CMC remaining
on the slow end.

The overall pattern continues to favor a slow progression, and
thus the preference will continue to be given to the ECMWF and
CMC. All models agree that the low will attain multiple closed
height contours at 500mb (with a 60m interval), the downstream
ridge will amplify, and a mid-1030s MB surface high will be
situated downstream over the Mid Mississippi River Valley. None of
these factors favor a faster progression. Will also include the
12Z NAM in the preference as it is not as fast as the GFS, but
also not as slow as the ECMWF and CMC, in order to account for
some of the slight adjustments in the models over the past several
cycles.


...Strong shortwave approaching the Pacific Northwest and British
Columbia late Fri into early Sat...
...Associated surface cold front crossing the Pacific Northwest...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-UKMET Blend
Confidence: Average

The 00Z UKMET is exhibiting more downstream trough amplification
over the western U.S. as this shortwave approaches the Pacific
Northwest coast. This affects the trough as it moves into the
region, with a slightly flatter wave overall. Otherwise, minor
model differences exist and a general blend is preferred to
account for some uncertainty.


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

Lamers