Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1249 AM EDT Fri Aug 14 2020

Valid Aug 14/0000 UTC thru Aug 17/1200 UTC

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

00Z Preliminary Model Evaluation Including Preferences and
Forecast Confidence

...Long wave trough moving from the Northwest to the Western Great
Lakes...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non GFS or UKMET      
Confidence: Average

Longwave trough moving across the northern tier will be reinforced
by a potent vorticity maxima rotating through the base of this
trough and into the Western Great Lakes Saturday, with a secondary
vort lobe driving eastward quickly on its heels Sunday. The
globals are overall in very good agreement both with position and
intensity of this lead shortwave, but the GFS continues to exhibit
a progressive bias and outruns the consensus early in the forecast
period. While the other guidance seems to have sped up a bit, the
GFS being on the fast edge of the envelope makes it an outlier and
the preference does not include it. The UKMET, while it is similar
to the others with the intensity of this lead impulse, its bias of
being too strong with ridges shows up with its 12Z run which
causes a latitudinal position north of the other guidance and is
also not preferred.

The second shortwave and associated sensible weather will be
highly dependent on the effects of the first, with the lingering
influence driving position and intensity changes in the associated
convection beneath the second shortwave. For consistency, a blend
which does not change from the first shortwave seems most
reasonable as small scale interactions from thunderstorms are
difficult to resolve even by D2.


...Longwave trough across the eastern CONUS ...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non-ECMWF/UKMET
Confidence: Slightly below average

Although the overall spread is pretty small in the amplitude of
the trough digging into the Gulf Coast this weekend, there is much
more variability in the secondary trough moving through the Ohio
Valley and how it impacts surface low development by Sunday moving
across the Mid-Atlantic. The ECMWF remains an outlier and is
completely out of phase (shortwave ridge vs. shortwave trough) in
the OH VLY Saturday aftn before eventually forming a lagging
trough. Otherwise, the deterministic guidance is in pretty close
agreement with the mid-level feature, although the GEFS mean is a
bit deep compared to the consensus, and the UKMET continues to
feature heights above the rest of the models even within the
trough. The GFS may be a bit fast to push the accompanying surface
low to the northeast, and a subtly lower blend near the NAM/CMC
could be a reasonable solution, but interaction with a wave off
North Carolina the morning, and potential MCVs diving into the
trough through the weekend will all impact the evolution of both
surface and mid-level features. This leaves lower than average
confidence, and something between the fastest GFS and slower
GEFS/ECMWF is probably most realistic at this time.



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

Weiss