Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
241 AM EDT Wed May 02 2018

Valid May 02/0000 UTC thru May 05/1200 UTC

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

00Z model with final model preferences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NAM/GFS initialization errors do not seem to degrade their short
range forecasts.


Shortwave moving into the Northwest Friday night
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Canadian/GFS/ECMWF blend
Confidence: Above average

The 00z UKMET was more amplified with this system than the other
guidance, while the 00z NAM was on the weak extreme.  As the
system is moving over the top of the ridging across the West, the
system should be fairly weak.  Even so, the NAM looks too weak.  A
compromise of the 12z Canadian, 00z GFS, and 12z ECMWF is
preferred with above average confidence.

07Z update: There were only minor adjustments seen in the 00Z
ECMWF/CMC relative to their previous runs. Therefore the previous
preference remains unchanged for the final preference.


Shortwave moving into the Lower MS Valley early Saturday
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: GFS/ECMWF/UKMET blend
Confidence: Slightly above average

The 00z NAM is stronger with this shortwave than the other
guidance, its usual bias.  Meanwhile, the 00z UKMET appears
weaker/more shallow at 500 hPa, though it is more comparable with
the other guidance at 700 hPa.  With a broad upstream ridge,
believe the shortwave should have some amplitude associated with
it.  Prefer a compromise of the 00z GFS/00z UKMET/12z ECMWF here
with above average confidence.

07Z update: Only minor adjustments were observed with the 00Z
ECMWF compared to its previous run, but the 00Z CMC trended
stronger like the 00Z NAM. The latest ensemble spaghetti heights
for 576 dam suggest a slightly weaker trough like the 00Z
GFS/ECMWF/UKMET is more likely.


Northern Plains system late Friday/early Saturday
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend
Confidence: Above average

The 00z NAM digs a shortwave more significantly than the other
guidance, which leads to a stronger low moving near the central
US/Canadian border Saturday morning, its usual bias.  The general
flow pattern favors shortwaves with minimal amplitude, thus favor
a compromise of the 00z GFS, 00z UKMET, 12z ECMWF, and 12z
Canadian with above average confidence.

07Z update: There were only minor adjustments seen in the 00Z
ECMWF/CMC relative to their previous runs. Therefore the 00Z NAM
continues to stand out as an outlier with this system and no
changes to the preference are needed for the final update.


Phasing systems near the central US/Canadian border Fri morning
Surface low moving from CO through MI by Friday morning
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Average

The 12z Canadian was north of the other guidance with the surface
wave sweeping across the Plains and Great Lakes due to a more
northerly track in the mid-levels.  A compromise of the 12z ECMWF,
00z GFS, 00z NAM, and 00z UKMET stays within the broader 12z
global ensemble clustering with the surface wave and is preferred
with average confidence.

07Z update: Some flip flopping has been observed between 00Z and
12Z cycles, but a compromise in the middle appears closest to the
latest ensemble spread. Given the 00Z Canadian adjusted near the
previous preference, a general model compromise can be used for
this system, but the run to run changes in the guidance lowers
confidence a bit.


Retrograding Upper Trough moving towards FL early Saturday
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: ECMWF/GFS/UKMET blend
Confidence: Average

The guidance has been advertising the development of this system
for about two weeks now, so feel fairly confident in its
formation.  An anticyclonic wave breaking event leads to the
forming of an upper low/TUTT cell which moves westward towards FL
under the base of northerly displaced subtropical ridge.  Of the
current guidance, the 12z Canadian is the slowest/deepest of the
guidance, with the 00z UKMET/00z GFS the quickest.  The 12z ECMWF
shows some convective feedback north of the Bahamas early Saturday
which leads to a sharper easterly wave that is more held back than
the 00z GFS/00z UKMET.  However, the guidance can sometimes
retrograde mid-level cyclones too much.  Prefer a compromise of
the 00z GFS, 00z UKMET, and 12z ECMWF to deal with the lingering
uncertainty with average confidence.

07Z update: The 00Z ECMWF sped up slightly with the approach of
this system toward FL early Saturday, but still appears reasonable
as part of a blend for the preference. Meanwhile, the 00Z Canadian
(like its 12Z run) remains rather deep with this system but it has
sped up to be near the remaining deterministic consensus. The
final preference will remain unchanged.

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

Roth/Otto