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The Weather Prediction Center

Model Diagnostic Discussion

[Abbreviations and acronyms used in this product]
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Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
119 PM EDT Tue Apr 24 2018

Valid Apr 24/1200 UTC thru Apr 28/0000 UTC

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

12Z model evaluation including preferences and forecast confidence

...Closed low over the OH/TN Valley today, gradually opening into
a trough and lifting into the Mid Atlantic and Northeast, phasing
with northern-stream shortwave closing off through the Great Lakes
late Wednesday before lifting into the Northeast Thursday...
...Weak surface low over the interior Southeast transition to
Carolinas Wednesday/Thursday...
Preference: Blend of the 12Z GFS and 00Z ECMWF
Confidence: Average to slightly below average

The models continue to advertise the phasing of southern stream
troughing/closed low energy over the OH/TN Valley region with the
arrival of a northern stream shortwave dropping southeast across
the Great Lakes region. Differences with the southern stream
energy are quite small as the energy lifts off to the northeast
over the next 24 to 36 hours. However, the 12Z NAM and 12Z GFS are
a bit faster to advance the northern stream energy off to the east
through Wednesday, which allows for phasing to occur a bit farther
east over New England Wednesday night and Thursday. The 00Z CMC,
00Z ECMWF and especially the 00Z UKMET are all slower, and
consequently allow for phased low pressure to linger a bit farther
back to the west over the interior of the Northeast. The GFS and
ECMWF tend to show the strongest phasing and actually by Thursday
night tend to come into better spatial/timing agreement along with
the CMC solution, but the UKMET remains a slow outlier
post-phasing. For now, an attempt to compromise between camps will
be preferred given the ensemble spread, while also attempting to
approximate the model consensus. Thus a GFS/ECMWF blend will be
preferred for now.

...Elongated trough with embedded shortwaves crossing the
Rockies/Black Hills...
...Energy dropping southeast to the lower MS Valley Thursday
before weakening through the Southeast by Friday...
...Low pressure near the Mid-Atlantic by late Friday...
Preference: Blend of the 12Z GFS and 06Z GEFS mean
Confidence: Average...becoming below average

Energy crossing the northern Rockies and High Plains will drop
southeast over the next couple of days and dig in toward the
southeast U.S. by Friday. The 00Z UKMET becomes generally a fast
outlier in digging the energy across the South, with the best
model clustering generally residing with the 12Z NAM, 12Z GFS and
00Z ECMWF. All of this energy is expected to drive an area of low
pressure across the interior Southeast by Thursday and then
northeast up toward the Mid-Atlantic region by Friday. However,
there becomes some rather significant spread with respect to the
placement of low pressure at the end of the period as the guidance
differs with degree of possible northern stream interaction at
that time. The NAM and GFS show more northern stream interaction
and focus low pressure more over the interior of the Mid-Atlantic
on Friday. The UKMET, CMC and ECMWF are farther offshore and more
progressive in time. Multi-model cycle trends have been tending to
favor low pressure at least attempting to lift up across the
coastal plain, but the struggle is over how far inland. The
preference for now is to lean toward the GFS camp, with a blend
with the 06Z GEFS mean as the NAM may be just a tad too aggressive
with the northern stream phasing it supports late in the period.

...Northern stream portion of Gulf of Alaska trough breaking off
and sliding southeast through Northern Rockies early Thursday then
starting to anchor the Global scale trough from the Great Lakes to
the lower MS Valley late Friday...
Preference: General model blend
Confidence: Average

Energy emanating from the northern Gulf of Alaska will cross
through western Canada and drop southeast through the northern
Rockies and Plains states through Thursday and help lead the way
for more of a longer wave trough evolution over the Eastern U.S.
by late Friday. The guidance tends to agree on the larger scale
with the details of the height falls evolution and agree with
focusing the bulk of the energy farther north closer to the Great
Lakes on Friday along with an area of low pressure. However, the
models do show some degree of spread with how much energy may tend
to separate out toward the base of the trough closer to the Gulf
Coast states on Friday. The 00Z ECMWF in particular shows a
possible closed low evolution which is also at least hinted at by
the other global models including the NAM. Based on the latest
spread of guidance, a general model blend will be preferred with
all the details of this height fall evolution.

...Closed low evolution toward the West Coast by Friday...
Preference: General model blend
Confidence: Slightly above average

The southern portion of an elongated trough axis offshore the West
Coast and extending north through the Gulf of Alaska will separate
out and advance east toward the West Coast by Friday as a
well-defined closed low. On the larger scale the model differences
are rather small with respect to timing and depth, and so a
general model blend will be preferred for now.

Model trends at
500 mb forecasts at


Last Updated: 119 PM EDT Tue Apr 24 2018