Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
244 PM EDT Tue Sep 25 2018
Valid Sep 25/1200 UTC thru Sep 29/0000 UTC
...See NOUS42 KWNO (ADMNFD) for the status of the upper air
12Z Model Evaluation...with Preferences and Forecast Confidence
...Large Scale Trof across much of Northern CONUS...
Preference: General model blend
Confidence: Slightly above average
19z update: The overall trends were toward a faster secondary
trof/surface "Clipper" low into the Great Lakes,... The 12z GFS
remains faster (further north closer to the mean center of the
large scale trof)...but the 12z CMC/ECMWF also trended faster
while the UKMET backed off the deeper amplification. While there
remains some modest timing/depth differences, there is much
stronger agreement overall and a general model blend should work
as a solid compromise. Confidence is slightly above average in
this better agreement.
Global scale pattern seems to be solid with large Rex Block just
off the Pacific West Coast, broad longwave trof across central
Canada into the Central Plains/Midwest, with subtropical ridge
across the Gulf to Western Atlantic. The spread of course,
remains in the timing and amplification of internal shortwaves/jet
streaks within the larger global trof. The shortwave and frontal
zone currently in the Midwest and Ohio Valley translates quickly
with good model agreement through the Northeast and Canada while
leaving the lingering front/tropical moisture connection across
the South into Western Gulf region.
A second shortwave settle into the mean "base" of the trof across
the Northern Tier into the Western Great Lakes by early Friday, it
is here that models start to divide. The 00z UKMET appears to be
most out of phase depicting a greater phasing of shortwave
"streams" into much stronger amplification of the wave and leading
shortwave ridging across Ontario, sticking out within the ensemble
spaghetti plots. The 12z GFS trended toward this greater
amplification but in the typical GFS fashion, it breaks down with
faster/progressive flow and the wave just becomes too fast
compared to the ensemble suite and run to run ensemble trend.
The 06z GFS is more solidly placed in the ensemble and remaining
deterministic guidance and would be preferred over the 12z run.
The 12z NAM is a nice compromise of the GFS solutions while not
being too amplified. The 00z ECMWF is ideal through day 2 within
the blend, but by day 3, it starts to break from its ensemble
suite suggesting a more compact cold pool of the Arctic airmass
over NW Canada, buckling the jet structure a bit slower and
eventually leading to southward push. This strengthens the low
level gradient across the Lee of the Northern US Rockies by Day 3
and falls into a known negative bias of the operational ECMWF,
back loading the trof over WY into the Dakotas by late Fri and
into NEB/IA...with increased QPF. This is less favored in the
overall pattern (large scale trof strong jet) that should be more
progressive across the Dakotas with the cold air/surface high.
The 00z CMC is slower than the GFS/NAM, but is a good compromise
with respect to ECMWF. All considered a general model blend
transitioning toward a blend of the 12z NAM, 00z CMC and 06z GFS
and 00z ECENS mean by Fri is preferred, above average initially
trending toward slightly below average by the end of day 3... or
perhaps average for the entire duration.
...Tail end shortwave across TX (frontal zone into Carolinas):
Preference: 12z ECMWF
Confidence: Slightly below average
19z update: The slower/compact solutions presented by the
CMC/ECMWF have adjusted favorably toward a more elongated trof.
The CMC dramatically shifted, including accelerating across the
south much faster, like the less preferred GFS. As such something
close to the 12z ECMWF may work out closest to initial thinking,
confidence is raised but given run to run inconsistency and
uncertainty remaining in timing...it is only slightly below
The tail end the initial trof will take up residence across TX in
the weaker flow, Thurs, into Friday. The placement/strength of
the frontal zone from NE TX across the south into the Southern
Appalachians/Carolinas is fairly well agreed upon, though timing
differences and intersection of this timing and moisture flux from
the Gulf leads to modest spread of QPF. The NAM/GFS maybe a bit
too weak in shearing out the lingering shortwave feature, issues
in the northern stream, make the UKMET a bit less favorable too.
The CMC/ECMWF appear a bit too compact, generating a spot low...
which given the overall flow regime (southwesterly flow out of the
Tropical East Pacific) seems unlikely as well. A compromise
between the GFS/NAM and the CMC/ECMWF seems to best represent
WPC's preference at this time, confidence is below average for
Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml