Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
246 PM EDT Sun Aug 11 2019

Valid Aug 11/1200 UTC thru Aug 15/0000 UTC

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

12Z Model Evaluation with Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: Compromise between the 12Z GFS and 12Z ECMWF
Confidence: Average

Through Wednesday, the synoptic flow will transition from a
middle-CONUS ridge, to nearly zonal flow, to a western
ridge/eastern trough by the end of the forecast period. While the
guidance is nearly uniform in developing this evolution, there is
still some temporal depth variance of the evolution among the
models.

Initially, a closed low moving across the Northwest and a
shortwave moving northeast through the Four Corners region will
work in tandem to flatten the ridge into a more zonal pattern. The
12Z NAM remains stronger than the reasonably well clustered global
models with the energy that traverses the northern Plains and near
the Canadian border, although it comes into somewhat better
agreement as the energy digs east-southeast across the Great Lakes
region and Northeast by later Tuesday and Wednesday. Similarly
during this time, the 12Z UKMET continues its summer-long trend of
being generally too strong with the mid-level ridge, in this case
keeping a more expansive axis of 594+dm heights through the
period. The UKMET is also the weakest of all the solutions with
the energy digging across the Upper Midwest, Great Lakes and the
broader eastern U.S. toward the end of the period. The UKMET is
also the weakest solution with a shortwave trough

The shortwave energy digging east-southeast across the Midwest and
into the East will drive a cold front eastward through the period
along with a well-defined wave of low pressure that initially
develops over the central High Plains in relation to the shortwave
exiting the Four Corners region. This low will cross through the
Midwest on Monday, and then advance progressively upper Ohio
Valley, northern Mid-Atlantic and southern New England through
Tuesday. As the low advances across southern New England, the 12Z
NAM becomes a stronger outlier, and accordingly has a much wetter
QPF solution over the northern Mid-Atlantic and southern New
England. The 12Z GFS tends to be a little stronger than the
multi-model consensus by the end of the period across the East
with the larger scale trough amplification, but it has trended in
this direction, and there is some ensemble support for this from
the GEFS suite. Consequently the GFS/GEFS guidance is also a bit
farther north with its axis of QPF across the Northeast, and
certainly relative to the ECMWF. All of the models favor a
trailing cold front crossing the Midwest and gradually slowing
down as it moves into the Mid-Atlantic and Southeast by the end of
the period. In the wake of the lead wave exiting southern New
England, additional waves of low pressure are likely to develop
along the front by the end of the period over the interior of the
Mid-Atlantic and the Southeast in response to the larger scale
trough amplification beginning to take shape.

Based on the latest ensemble guidance, trends, and deterministic
spread, a compromise between the GFS and ECMWF is preferred at
this time.

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

Orrison