Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Sun Aug 18 2019

Valid Aug 18/0000 UTC thru Aug 21/1200 UTC

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

00Z Model Evaluation...Including Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: General model blend...FL Panhandle/Gulf Coast
            Blend of the 00Z GFS/ECMWF...Remainder of CONUS
Confidence: Average

A broad deep layer trough covers the northern half of the CONUS
with a few smaller scale shortwaves currently progressing through
the Midwest, Great Lakes and Northeast. The latest guidance favors
the deeper layer trough advancing east across the Midwest and
Great Lakes region through Sunday, before then brushing the
Northeast and retreating up across southeast Canada on Monday. In
general, the 00Z NAM again appears to be a tad more aggressive
than the global models with the details of the shortwave impulses.
On the larger scale, the guidance though appears to in reasonably
good agreement which includes the large scale trough evolution
over the northern U.S. and southern Canada. However, the height
falls will send a cold front from the Midwest through the
Northeast which is then expected to stall out across the Ohio
Valley and Mid-Atlantic region on Tuesday. There are some
differences with respect to a couple waves of low pressure riding
northeast up along the front. The 00Z NAM and to a greater extent
the 00Z UKMET appear to be too strong with their wave activity
impacting the Midwest and Great Lakes region.

In the wake of the trough, the flow will at least briefly flatten
out and allow a large 594+ dm ridge to expand across the Southwest
and much of the central and southern Plains. Once again though,
the UKMET has the strongest ridge axis out of all of the models
which has become a notable bias of the UKMET when it comes to
larger scale height rises/ridging in general. By the end of the
short range period, the large scale flow will become a bit more
amplified across the CONUS as a new upper trough digs southeast
across the upper Midwest and Great Lakes which will allow a new
cold front to settle southeast from Canada. The latest guidance is
in good agreement with the details of this outside of the 00Z CMC
which tends to be a bit deeper and farther south with the height
falls over the Midwest.

Meanwhile, there will be an upstream trough approaching the
Pacific Northwest by the end of the period, and this is where
there is more substantial model spread, as the 00Z CMC is somewhat
of a progressive outlier. Meanwhile, the 00Z NAM/GFS solutions
both have the core of their height falls digging a tad farther
south than the 00Z ECMWF solutions which allows a rather strong
surface low to track a tad farther south by comparison toward
coastal areas of British Columbia. The 00Z UKMET though is a weak
outlier aloft and even more so at the surface with its surface low
evolution. These height falls in any event will drive a cold front
to just offshore the Pacific Northwest by early Wednesday.

In the tropical confluence zone involving the central/eastern Gulf
Coast, Southeast and southern Mid-Atlantic region, there will be
multiple waves of low pressure that will need to be closely
monitored. One wave of low pressure just inland over far northeast
SC will be lifting northeast across the NC Outer Banks by early
Sunday afternoon and then well out to sea on Monday. The 00Z NAM
becomes the fastest solution with this, and the 00Z CMC the
slowest. Meanwhile, the 00Z UKMET is a tad stronger than the model
consensus. Meanwhile, there is evidence of a new compact area of
low pressure south of the FL Panhandle this evening which the
global models and hires guidance take gradually northward and
inland by Monday. The guidance is trending stronger with this
feature as the 00Z NAM/UKMET and 00Z ECMWF all have a fairly
well-defined low center. The 00Z GFS is the weakest solution.

Based on all of the model spread and latest trends, a blend of the
00Z GFS and 00Z ECMWF will be preferred across the CONUS for the
model mass fields, and accounting primarily for the outlier
solutions noted with the NAM and UKMET regionally. The latest
ensemble guidance strongly favors the GFS/ECMWF consensus.
However, one exception is with respect to the FL Panhandle low
center as a general model blend will be preferred with this
feature. This will support a reasonably well-defined low advancing
north in off the Gulf of Mexico by Monday.


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

Orrison