Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1238 AM EDT Sat Oct 06 2018

Valid Oct 06/0000 UTC thru Oct 09/1200 UTC

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

00Z Model Evaluation...with latest preferences and confidence
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...Overall Pattern Across the CONUS...
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Preference: Blend of the 12Z ECMWF and 12Z ECENS mean
Confidence: Slightly above average

The guidance is in good agreement with the details of a northern
stream shortwave crossing the upper MS Valley through Saturday
which will drive a wave of low pressure across the Midwest and
Great Lakes region and then across northern New England on Sunday
as the shortwave energy traverses southeast Canada. A cold front
will cross all of these areas, but the front will then stall out
by Sunday back to the southwest across the middle MS Valley and
the central Plains region.

In the wake of the initial northern stream shortwave, a secondary
shortwave currently dropping down across the Pacific Northwest
will amplify sharply down across the Great Basin during the
weekend, which will lead to a highly amplified large scale pattern
across the CONUS Sunday through Tuesday. This will feature a
strong longwave trough over the Western U.S. and a strong
subtropical ridge across the Eastern U.S., and this will be a
major reason for the aforementioned front becoming stalled out
over portions of the Plains and Midwest. Unfortunately, this will
also set the stage for a multi-day and high-impact rainfall event
across this part of the country.

The models agree very well with the details of the deep trough
evolving over the Four Corners region through Monday and Tuesday,
but there are some very meaningful differences with the placement
of the front across the Plains and Midwest, as the 00Z NAM
generally displaces its front farther south and east versus the
global model consensus. The global models are well clustered with
the portion of the front over the southern High Plains, but
farther north there is much more spread as the 00Z GFS distinctly
focuses its boundary northwest of the non-NCEP guidance along an
axis from eastern CO northeast up across the upper Midwest.
Despite there being some support for the GFS from the 18Z GEFS
mean, the 12Z ECMWF has the strongest overall support which would
include the 12Z CMC/UKMET solutions and the 12Z ECENS mean.

Elsewhere across the CONUS, there will likely be a vigorous trough
digging down across the Pacific Northwest from British Columbia by
early Tuesday. The 00Z NAM is a slow outlier, with the 00Z GFS the
fastest and weakest with the system. The 12Z UKMET is a deep
outlier, but actually does have timing agreement from the 12Z
CMC/ECMWF solutions. Overall, the ECMWF is closest to the model
consensus and has good support from the ECENS mean.

Based on the latest clustering of solutions across the CONUS for
this period, a blend of the 12Z ECMWF and 12Z ECENS mean will be
preferred, which will include the energy over the Northwest and
the deep system impacting the Four Corners region and adjacent
Plains/Midwest.


...Possible tropical cyclone over the Gulf of Mexico...
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Preference: Blend of the 00Z NAM, 12Z UKMET and 12Z ECMWF
Confidence: Average

The guidance continues to take a broad area of low pressure over
the western Caribbean Sea near the coast of Honduras gradually off
to the northwest over the next couple of days and toward the
northeast tip of the Yucatan peninsula. By Monday, the system is
expected to emerge over the southern Gulf of Mexico and turn
northward over the central Gulf of Mexico through early Tuesday.
All of the global models show development of this system into what
would appear to be a tropical cyclone as it enters the Gulf of
Mexico. The 00Z GFS is the strongest solution and is farthest
right with the track. The 12Z CMC is farthest to the left. The 00Z
NAM, 12Z UKMET and 12Z ECMWF are in between the GFS and CMC
tracks, with the ECMWF overall the weakest solution through this
period. The NAM and UKMET are also a little slower than the global
model consensus.

Overall, the GEFS low tracks are generally the to right of the
global model consensus, with the ECENS low tracks a bit more
evenly spread out in a fashion that could support either a track
farther left like the CMC or to the right like the GFS, however,
the consensus would be very close to the deterministic ECMWF
albeit a little slower. Even the GEFS members in general tended to
be a bit slower than the deterministic GFS. Thus, the relatively
slower NAM and UKMET have some support with timing. Based on the
deterministic spread and ensemble clustering, a blend of the 00Z
NAM, 12Z UKMET and 12Z ECMWF will be preferred. Please also
consult the latest NHC Tropical Weather Outlook concerning this
system.


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

Orrison