Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1256 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: Blend of the 00Z GFS and 12Z ECMWF
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 12Z 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 12Z ECMWF was a
tad slower than the model consensus with this next set of height
falls, but in general the model spread was relatively modest with
this and the next cold front.

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 12Z CMC is a
progressive outlier. Meanwhile, the 00Z NAM/GFS solutions both
have the core of their height falls digging a tad farther south
than the 12Z UKMET/ECMWF solutions which allows a rather strong
surface low to track a tad farther south by comparison toward
coastal areas of British Columbia. This energy 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 12Z CMC the
slowest. Meanwhile, the 12Z UKMET is a strong outlier solution.
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
00Z NAM is the strongest of all of the solutions with this feature.

Based on all of the model spread and latest trends, a blend of the
00Z GFS and 12Z ECMWF will be preferred across the CONUS, and
accounting primarily for the outlier solutions noted with the NAM
and UKMET for the time being. The latest ensemble guidance
strongly favors the GFS/ECMWF consensus.


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

Orrison