Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1229 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...

12Z Model Evaluation Including Preferences and Forecast Confidence

...Longwave trough expanding across the eastern CONUS into early
next week...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Non UKMET through 17.12Z
            Non UKMET/GFS D3    
Confidence: Slightly above average

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 si
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 Souteast
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

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 Candada, 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.


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