Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Thu Aug 13 2020

Valid Aug 13/1200 UTC thru Aug 17/0000 UTC

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

12Z Model Evaluation Including Preferences and Forecast Confidence

...Long wave trough moving from the Northwest to the Western Great
Lakes...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12z CMC/GFS/ECMWF (greater weight to CMC)       
Confidence: Average after 15.12z

19z update: A sizable shift toward the trended noted in the 12z
GFS was depicted in the 12z ECMWF/CMC and UKMET and GEFS.  The
UKMET still remains a bit south and therefore east while the 12z
ECMWF is now even slower than the lifting into Canada on Fri/Sat. 
The 12z CMC shows greatest run to run continuity but also is
slower to push the NAM to the fringe.  This change moves more into
line with initial meteorological thinking though the run to run
variation still hearkens some remaining uncertainty even if it was
more in the direction of preference.   As such will favor the CMC
in a blend with the GFS and lesser so the ECMWF.  As the remaining
elongated trof moves into the Great Lakes, the NAM appears slight
too strong and would favor its removal now given better agreement
into Day 3.   Confidence increases to average through the entire
short-range period.

---Prior Discussion---
A very strong closed low exists over NW Canada, with a stronger
than normal jet between it and the amplified ridge in the
Southwest US.  This will continue to transport shortwaves through
the northern tier, with a divergent/diffluent flow across the Red
River Valley into MN building by late Friday as the ridge
retrogrades west.   The UKMET is fast entering this region and
produces a very strong response but is also remains generally east
of the ensemble suite/solutions.  The 12z GFS has significantly
shifted slower and much stronger in response to this favorable
amplification environment; this may be too aggressive given the
remaining guidance, but think this may be the right direction. 
The 00z ECMWF remains very weak and generally flat (which there is
solid thunderstorm response), there is little surface development
and so ejects into Canada rapidly, which is increasingly
unfavorable.  The 12z NAM/00z CMC are middle ground overall though
are a tad west of the ensemble mean solutions.  Overall, trends
and global scale pattern would suggest a compromise very close to
the GEFS/ECENS but some inclusion of the GFS would be prudent
given the overall trend.  As such a 12z NAM/00z CMC with lower
weighted 12z GFS blend is preferred.  The spread remains quite
large and contingent on prior evening's convective events to have
lower confidence through Day 2 (Saturday morning)...so is slightly
below average

After Saturday, the next shortwave/jet streak will continue to
press the lingering trof energy eastward into the Midwest and
Upper Great Lakes, supporting some negative tilt orientation,
potentially increased by the evolution of the lingering shortwave
energy in the Ohio/TN Valleys (see section below).  The normally
negative bias of a slower ECMWF was counteracted by the weaker
evolution on Day 2, so the next wave appears to be amplifying
similar to the remaining guidance (including the CMC/NAM/GFS),
providing atypical increased confidence by Day 3 in the blend into
the Great Lakes.


...Base of weak Rex Block across Ohio Valley lingers through
Sunday before ejecting ahead of ...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-NAM blend
Confidence: Slightly below average

19z update: There was a solid increase in clustering toward the
12z GFS with respect to the later arriving, UKMET, ECMWF, CMC and
GEFS, particularity for the surface wave developing along the NC
coast.  This leaves the NAM a bit too deep in the Ohio Valley and
then a bit too slow/east with the developing coastal wave.  
Normal ECMWF/CMC lingering of the filling trof across the
Southeast by late Sunday is a more traditional/expected spread to
have some mild increase in confidence, even though mass
fields/flow remain contingent on convection and upscale chaotic
enhancements.  A non-NAM blend seems sufficient for now, with
increase in confidence to slightly below average

---Prior Discussion---
A very muddled weak flow environment exists over the eastern third
of the US with convectively induced MCVs driving additional
development initially.  With the greater approach of larger scale
energy in the PacNW and retrograding ridge, a weak Rex Block
pattern evolves by mid-afternoon Friday, though does get too
blocky in nature as the ridge in Canada slides east by Sat leading
to a larger positive tilt to the Rex pattern.  This is leading to
moderate overall spread due to the weak overall pattern.  However,
a shortwave entering the diffluent area today out of IA/MO, will
settle and amplify across the Lower Ohio Valley.  Only the 00z
UKMET is very weak and leads to a different convective evolution
(which feeds back on this weak wave), further east.  So a
Non-UKMET blend is supported initially.  The ensemble suite
(GEFS/ECENS/CMCE) has been trending toward a deeper and stronger
solution through the Ohio Valley, which supports the 00z ECMWF
which is solidly in the middle; and opposed by the remaining
NAM/GFS and CMC which show greater convective development eastward
and a trend to shift the trof that direction as well.  Given the
retrograding nature of the strong ridge, this seems counter
intuitive, and so would favor the slower evolution after 16.00z.  
Confidence remains below average given the overall weak flow and
contingency of convective/latent heat release and response to the
upscale growth which is too chaotic in nature to result in any
greater confidence.


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

This product will terminate August 15, 2020:
www.weather.gov/media/notification/pdf2/scn20-58pmdhmd_termination.
pdf

Gallina