Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
227 PM EDT Sat Aug 15 2020

Valid Aug 15/1200 UTC thru Aug 19/0000 UTC

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

***FINAL*** Short-term Model Evaluation Incl. Preferences and
Forecast Confidence

...Long wave trough expanding across the eastern CONUS into early
next week...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12z ECMWF/CMC and GFS (lower weight D3)    
Confidence: Slightly above average

19z update: The 12z UKMET still remains slightly weaker, while
trending toward the ECMWF/CMC...providing greater confidence
toward those solutions which remain fairly steadfast.  This puts
further uncertainty toward a stronger/faster GFS solution for the
end of Day 3.  As such will support a 12z ECMWF/CMC and GFS blend
but will taper weighting of GFS lower by the end of Day 3.

---Prior Discussion---
GOES-East WV suite depicts a strong block in mid-level flow across
SE Ontario, south of which the remains of a weak, elongated (N-S)
trof continues to press eastward across the Central/Southern
Appalachians.  Strong WNWly jet across the Northern Plains is
pressing an elongated base of a shortwave squeezed north and south
by the aforementioned block in Ontario.  So while the current weak
trof in the east will be replaced by the approaching wave, the
flow will remain weak but generally will reduce the higher
moisture associated with this slow moving trough over the
Southeast out to sea.  The 00z UKMET continues to be a weak
solution with the exiting trof and develops the weak surface wave
out of NC still south and flatter than the consensus.   The 12z
NAM has trended much stronger with the kicker shortwave across the
Great Lakes by late Monday, and adds increased energy to the
developing surface wave off the coast, eventually slowing/bending
it cyclonically westward toward Nova Scotia by Tuesday.  As such
will not favor either the UKMET/NAM.  The  12z GFS has slowed a
tad and gels well with the ECMWF/CMC through day 1 and 2 to
provide a slightly above average confidence in a blend of the
three through 00z Tuesday. There is some typical slightly
stronger, slightly faster bias noted in the later portions of Day
3 for the GFS, but this is likely to offset the typical slow bias
of the ECWMF/CMC. 


...Shortwave moving into the Pacific Northwest Sunday night...
...Shortwave clipping Pacific Northwest late Tuesday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend (No/limited QPF from UKMET)
Confidence: Slightly above average

19z update: No significant changes were noted with the closed low,
shearing into the Pacific Northwest by Sunday night.  So will stay
with general model blend, removing the UKMET for QPF in the West)

While the ECWMF trended a bit weaker/faster into the Pacific
northwest with very little sensible changes to the forecast, it
does make the NAM appear a bit slower/stronger than the other
solutions but not tremendously to break from the general model
blend preference and confidence slightly above average. 

---Prior Discussion---
The compact closed low west of 130W is already showing signs of
opening up toward the north and the approaching/digging shortwave
energy out of the S Gulf of AK.  The 12z NAM and 00z UKMET
continue to be the stronger solution in the overall cluster but
have tempered enough that blending with the much weaker CMC will
bring solid agreement to not wash out the signal.  The only
concern remains the slightly stronger moisture advection of the
UKMET in the Pacific NW.  This can be a bias of the UKMET
particularly when it is on the stronger side of the guidance
suite.  So would suggest tempering QPF from the UKMET in any
blend.

The aforementioned digging trough will continue to direct
moisture/flow into SE Canada, but toward the tail of the forecast
period (late Monday into early Tuesday) there is solid agreement
to suggest NW WA will be clipped by the base of the trof providing
a very slight chance of moisture reaching the far NW tip of the
Olympic Peninsula.  A general model blend will continue to be
favored with the evolution of this feature as well.

***********************************************************
This product will terminate today, August 15, 2020, with this
issuance:

WPC will continue to include forecast evaluations of model
guidance in other text discussions, which are described in the
Service Change Notice (SCN). In particular, note that the Extended
Forecast Discussion (PMDEPD) has a section specifically providing
a guidance and predictability assessment.

www.weather.gov/media/notification/pdf2/scn20-58pmdhmd_termination.
pdf
***********************************************************

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

Gallina