Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1234 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 NAM/00z CMC blend with lower 12z GFS weighting
Confidence: Slightly below average through 15.12z
Average after 15.12z
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-UKMET thru 16.00z; 00z ECMWF/ECENS and 06z GEFS
blend after
Confidence: Below average
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