Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1255 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.


...Deep trough crossing the Plains/Midwest this weekend...
...Energy closing off over the Northeast early next week...
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Preference: Blend of the 00Z NAM/18Z GEFS mean/12Z ECENS mean
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 late Sunday
into Monday, the 12Z CMC becomes a bit slower than the overall
multi-model consensus as the 00Z GFS becomes a bit more
progressive. This especially becomes the case as the energy
crosses the Great Lakes and moves into the Northeast and is
reflected with the details of the surface low placement by the end
of the period near northern New England. The 18Z GEFS mean and 12Z
ECENS mean both lean toward the progressive side of the guidance
suite, and have support from the 00Z NAM. The non-NCEP guidance
overall appears to be a tad too slow by the end of the period.
Based on the latest clustering, a blend of the 00Z NAM and the
ensemble means will be preferred with this system's evolution.


...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 12Z ECENS mean
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 12Z CMC and 12Z ECMWF are both positioned west
of the 00Z NAM/GFS and 12Z UKMET. This is a critical difference
among guidance camps as this will influence the poleward advance
and placement of moisture and energy associated with Hurricane
Lorena which is expected to lift north through the Gulf of CA and
across adjacent areas of northwest Mexico through the period. The
18Z GEFS mean is farthest east like the 00Z NAM/GFS solutions. The
12Z ECENS mean is a little farther west, but not to the extent of
the 12Z CMC/ECMWF solutions. For the time being, a blend of the
00Z NAM/GFS and 12Z ECENS mean will be preferred to best
approximate the model consensus.


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

Orrison