Prognostic Meteorological Discussion
Issued by NWS

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5
FXUS10 KWNH 221836

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Sun Apr 22 2018

Valid Apr 22/1200 UTC thru Apr 26/0000 UTC

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

12Z Model Evaluation including Preferences and Forecast Confidence

...Strong closed low over the south-central US today, gradually
opening into a trough and lifting into the Mid Atlantic and
...Phasing with northern-stream shortwave digging through the
Great Lakes Tuesday Night into the Northeast-...
...Weak surface low over the interior Southeast, redeveloping
along the Atlantic coast by mid-week...
Preference: Blend of 12Z NAM, 12Z GFS, 12Z CMC
Confidence: Slightly below average

---18Z UPDATE---
The 12Z ECMWF continues to show a stronger shortwave digging into
the Great Lakes, and actually never seems to quite phase it with
the trough/wave lifting out of the Southeast. This continues to
lead to a surface low further inland, and a mean trough position
further west of the other models. The 12Z UKMET has trended in
this direction, but not quite to the degree of the ECMWF.
Meanwhile, the 12Z CMC has remained relatively consistent and
similar to the GFS and NAM. Again, the ECMWF scenario can`t be
ruled out, but will maintain continuity with the preliminary
preference and previous overnight preference.

Models continue to show some run-to-run variability with this
system, likely related to both the specifics of how the upper
level low opens up into a trough and begins to lift out of the
Southeast, as well as the interaction with the digging wave over
the Northeast. That being said, the 12Z NAM and 12Z GFS both seem
reasonably similar to the 00Z CMC and 00Z UKMET in terms of
general trough amplitude and trough axis position by 26.00Z
(Wednesday evening). The greatest difference continues to come
from the 00Z ECMWF which shows a stronger digging wave that also
digs more sharply into the Great Lakes, allowing the trough to
become more amplified with the mid-level vort max further
southwest as compared to other models. Ensemble cluster analysis
from the GEFS+ECMWF seems to give more support to the non-ECMWF
deterministic model consensus, although a couple clusters still
appear similar to the ECMWF. Therefore, although the ECMWF
scenario can`t be ruled out, the preference will be to lean toward
a non-ECMWF blend, which is consistent with the overnight
preference as well.

...Shortwave pushing into the Pacific Northwest today...
...Amplifying into the northern Rockies through Monday Night, and
then digging into the Plains...
Preference: Blend of 12Z GFS, 12Z ECMWF, 12Z CMC
Confidence: Slightly above average

---18Z UPDATE---
No change to the preliminary preference. The 12Z UKMET remains
slower with the trough, and the 12Z NAM continues to be more
amplified than the global model consensus.

Most of the global models show better agreement today as the wave
reaches the Plains. The 12Z GFS is very similar to the 06Z GEFS
Mean, 00Z ECMWF Ensemble Mean, 00Z ECMWF, and to a slightly lesser
extent the 00Z CMC by Tuesday afternoon (25.00Z). This continues
into Wednesday as well. The differences come from the 00Z UKMET
and 12Z NAM which have more amplitude to the trough, an upstream
ridge axis that is positioned to the west of the other global
models, and are slower with the timing of the trough axis. The
preference is to go with the consensus better supported by the

...Northern stream portion of Gulf of Alaska trough breaking off
and digging through the Canadian Rockies Tuesday...
...Continuing to dig into the northern Plains Wednesday, with
associated surface cold front pushing southeast...
Preference: Blend of 12Z CMC, 12Z UKMET, 12Z GEFS Mean
Confidence: Slightly below average

---18Z UPDATE---
No change to the preliminary preference.

The 00Z ECMWF also shows considerably more amplitude to this wave
than the other models as the wave digs into the Northern Plains.
That may be due to the synoptic pattern over the Western US and
eastern Pacific, with the ECMWF furthest to the west with its
position of the Pacific low, and also with the ridge axis in
between. The other models are reasonably similar with this digging
wave. The preference is to lean closest to the CMC, UKMET, and
GEFS Mean which show reasonable similarity with all three
components of the evolving synoptic pattern between 100W and 150W
(digging shortwave, western ridge, and east Pac low).

...Southern stream portion of Gulf of Alaska trough breaking off
and developing into a closed low around 35N/135W by Tuesday...
Preference: Blend of 12Z CMC, 12Z UKMET
Confidence: Slightly below average

---18Z UPDATE---
No change to the preliminary preference.

As stated for the previous system, the overall synoptic pattern
over the eastern Pacific and western US will be linked, so the
preference is largely the same as the previous system. The 00Z
ECMWF and its ensemble mean are on the western end of model spread
with the position of the upper level low, with the NCEP models
(NAM, GEFS, GFS) on the eastern end. Given the preference of the
CMC and UKMET for other components of the pattern over the western
US, and their approximation of a median scenario for the low
position over the Pacific, the preference here is to lean toward a
blend of the CMC and UKMET.

Model trends at
500 mb forecasts at


$$ is the U.S. government's official web portal to all federal, state and local government web resources and services.