Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Sat Sep 21 2019

Valid Sep 21/0000 UTC thru Sep 24/1200 UTC

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

00Z Model Evaluation with Preferences and Forecast Confidence
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...Synopsis...
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The synoptic pattern will feature a rather strong deep layer ridge
across a large portion of the eastern U.S. this weekend, as a pair
of fairly robust northern stream troughs traverse the western and
central U.S. The first trough will advance east across the
central/northern Plains and upper Midwest Saturday and Sunday
along with an attendant cold front and a multiple waves of low
pressure riding up along it. This energy will break down the ridge
over the East by later Sunday and will allow a cold front to
arrive across the OH Valley and lower Great Lakes region by
Monday. The height falls are expected to sharpen a bit over the
Northeast by Tuesday, and there is support for a somewhat stronger
surface low to develop over the interior of northern New England
as the cold front pushes off the East Coast. Meanwhile, a second
upstream trough will cross the West Coast and move into the Great
Basin by Monday before then cutting off from the westerlies and
dropping into the Southwest on Tuesday. This upper trough will be
a key player in also steering the moisture and energy associated
with Hurricane Lorena up across areas of northwest Mexico and into
parts of the Four Corners region on Monday and Tuesday. By the end
of the period, a new northern stream trough will begin to advance
across the northern High Plains along with a cold front.


...Deep trough crossing the Plains/Midwest this weekend...
...Energy closing off over the Northeast early next week...
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Preference: General model blend...through 60 hours
            Non-GFS blend...after 60 hours
Confidence: Slightly above average

Model differences with the first trough are rather small through
most of the weekend as the energy crosses the Intermountain West
and advances across the Plains and Midwest, but by later Monday
and Tuesday as the energy crosses the Great Lakes and moves into
the Northeast, the 00Z GFS becomes a bit more progressive than the
remaining guidance. The 00Z non-NCEP guidance is now all very well
clustered and close to that of the 00Z NAM. The 00Z GEFS mean is a
tad more progressive than the non-GFS suite of models toward the
end of the period, but not as progressive as the deterministic
GFS, which suggests the GFS is a bit of an outlier at least by the
end of the period. Will recommend a general model blend through
about 60 hours, and then a non-GFS blend thereafter.


...Upper trough digging into the West by Monday...
...Closed low evolution over the Southwest by Tuesday...
...Moisture/energy associated with Hurricane Lorena...
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Preference: Blend of the 00Z NAM/GFS and 00Z UKMET
Confidence: Average...becoming below average by Mon/Tues

Regarding the second trough, the model guidance is well clustered
and in good agreement with respect to timing and depth as the
system arrives on Monday in across the Intermountain West.
However, by Tuesday as the energy cuts off and drops south into
the Southwest, the 00Z ECMWF is positioned west of the 00Z NAM/GFS
and 00Z UKMET solutions which are now pretty well clustered. The
00Z CMC is now the farthest east solution by the end of the
period. The spread in the guidance does still play a role in how
the moisture and energy associated with Hurricane Lorena evolves
which is expected to lift north through the Gulf of CA and across
adjacent areas of northwest Mexico through the period. The 00Z
GEFS mean is farthest east with its trough/closed low at the end
of the period and is even a little east of the CMC solution. The
12Z ECENS mean tends to support the majority cluster of solutions,
but it does suggest the 00Z ECMWF may be a tad too far west. Based
on the latest clustering of solutions, a blend of the 00Z NAM/GFS
and 00Z UKMET will be preferred. Confidence becomes a bit limited
late in the period though based on the relatively large ensemble
spread seen with the height fall details.


...Upper trough over the northern High Plains by Tuesday...
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Preference: General model blend
Confidence: Above average

The models have good agreement overall with the details of a
northern stream trough and associated cold front crossing the
northern High Plains by Tuesday. The 00Z GFS is just a tad sharper
than the remaining guidance, but the model differences are modest
enough that a general model blend can be preferred.


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

Orrison